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#41
Posted to uk.d-i-y
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Installing a loft floor
On 21/10/2012 13:09, GMM wrote:
On Sunday, October 21, 2012 2:16:39 AM UTC+1, Tony Bryer wrote: On Sat, 20 Oct 2012 22:33:14 +0100 John Rumm wrote : Out of interest I had an experiment with superbeam to see what you can get away with on a 3x2 (well 72x47mm) and a typical floor load (uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about the limit - so you could probably still do a landing with it and comply with modern building regs. (having said that, its generally simpler to use one depth all over to save having to buy lots of timber sizes) Our old rule of thumb when I was a BCO which matched the tables pretty well was that for floor joists double the depth in inches and subtract two to get the permissible span in feet; flat roof joists, subtract one (2" joists). As you say, in most cases practicality requires all joists to be the same depth (you need tops of joists to be level and want them all to bear on wall at one level) so except for the largest span they are generally oversized. There's also more in reserve in that for virtually all joists, deflection governs the size, not bending stress and few floors are loaded to BR design loads (1.5kN/m2 30lb/ft2). So for a 14ft span (as here), the 8" joists I originally proposed would be about right? I wonder if the tables are constructed from that rule of thumb or from a complex calculation that gives the same result? I guess most of the discussion (now) is about what you might be able to get away with, rather than what should be done, but I'd prefer to over-engineer than under, for the sake of a couple inches. Of course, I'm equally concerned that they are mounted securely, as Mr R outlined...... If you can rule out that the space will ever be converted to habitable, then you can undersize a tad from the tabulated values. As Tony mentioned above, its normally the deflection limits that dictate the size rather than the shear or bending limits. (i.e the floor would be likely to damage decorative finishes, feel to bouncy, and upset inhabitants of rooms below, long before the timber is in danger of actually failing) For your application (i.e. with the new beams some distance above the existing ceiling, and not ceiling to be mounted on the underside of the new joists), deflection beyond normal limits is a non issue. So it reduces to a problem of what is adequate in terms of bending and shear loading on the timber (assuming you don't mind it feeling a little more bouncy than "normal" given that you know it is still structurally sound). Perhaps a play Tony's excellent bit of software might be in order (assuming there is still a demo version available for download?) -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
#42
Posted to uk.d-i-y
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Installing a loft floor
On Sunday, October 21, 2012 4:25:44 PM UTC+1, John Rumm wrote:
On 21/10/2012 13:09, GMM wrote: On Sunday, October 21, 2012 2:16:39 AM UTC+1, Tony Bryer wrote: On Sat, 20 Oct 2012 22:33:14 +0100 John Rumm wrote : Out of interest I had an experiment with superbeam to see what you can get away with on a 3x2 (well 72x47mm) and a typical floor load (uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about the limit - so you could probably still do a landing with it and comply with modern building regs. (having said that, its generally simpler to use one depth all over to save having to buy lots of timber sizes) Our old rule of thumb when I was a BCO which matched the tables pretty well was that for floor joists double the depth in inches and subtract two to get the permissible span in feet; flat roof joists, subtract one (2" joists). As you say, in most cases practicality requires all joists to be the same depth (you need tops of joists to be level and want them all to bear on wall at one level) so except for the largest span they are generally oversized. There's also more in reserve in that for virtually all joists, deflection governs the size, not bending stress and few floors are loaded to BR design loads (1.5kN/m2 30lb/ft2). So for a 14ft span (as here), the 8" joists I originally proposed would be about right? I wonder if the tables are constructed from that rule of thumb or from a complex calculation that gives the same result? I guess most of the discussion (now) is about what you might be able to get away with, rather than what should be done, but I'd prefer to over-engineer than under, for the sake of a couple inches. Of course, I'm equally concerned that they are mounted securely, as Mr R outlined...... If you can rule out that the space will ever be converted to habitable, then you can undersize a tad from the tabulated values. As Tony mentioned above, its normally the deflection limits that dictate the size rather than the shear or bending limits. (i.e the floor would be likely to damage decorative finishes, feel to bouncy, and upset inhabitants of rooms below, long before the timber is in danger of actually failing) For your application (i.e. with the new beams some distance above the existing ceiling, and not ceiling to be mounted on the underside of the new joists), deflection beyond normal limits is a non issue. So it reduces to a problem of what is adequate in terms of bending and shear loading on the timber (assuming you don't mind it feeling a little more bouncy than "normal" given that you know it is still structurally sound). Perhaps a play Tony's excellent bit of software might be in order (assuming there is still a demo version available for download?) -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ Yes John - it may potentially be that 2 x 6 joists (which are significantly cheaper per metre) could do an adequate job in this application, if they are available (my local timber yard couldn't supply that length for the living room when I was costing it: There, the lower spec was due to the wall running along the middle, effectively halving the span). TBH, I didn't spot Tony's software - I thought he just mentioned the rule of thumb(!). If I did go below spec (say 4m of 2 x 6), it would be great to have some idea of how wobbly such a floor would be. The joist tables just give maximum length for size, as far as I can see. I'd still feel a little uncomfortable that it would deny the option of making the space habitable in the future though, even though the rest of the house is big enough that it shouldn't be an issue. Although I get the point that BR specs change over time, they surely can't ask for joist that are much deeper than they require now, so I would have thought the current specs won't change much. (Apologies if my posts are hard to read. Someone told me a while ago they weren't wrapping, whilst in a recent thread someone else told me they had a lot of empty lines!) |
#43
Posted to uk.d-i-y
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Installing a loft floor
On 21/10/2012 17:47, GMM wrote:
On Sunday, October 21, 2012 4:25:44 PM UTC+1, John Rumm wrote: On 21/10/2012 13:09, GMM wrote: If you can rule out that the space will ever be converted to habitable, then you can undersize a tad from the tabulated values. As Tony mentioned above, its normally the deflection limits that dictate the size rather than the shear or bending limits. (i.e the floor would be likely to damage decorative finishes, feel to bouncy, and upset inhabitants of rooms below, long before the timber is in danger of actually failing) For your application (i.e. with the new beams some distance above the existing ceiling, and not ceiling to be mounted on the underside of the new joists), deflection beyond normal limits is a non issue. So it reduces to a problem of what is adequate in terms of bending and shear loading on the timber (assuming you don't mind it feeling a little more bouncy than "normal" given that you know it is still structurally sound). Perhaps a play Tony's excellent bit of software might be in order (assuming there is still a demo version available for download?) Yes John - it may potentially be that 2 x 6 joists (which are significantly cheaper per metre) could do an adequate job in this application, if they are available (my local timber yard couldn't supply that length for the living room when I was costing it: There, the lower spec was due to the wall running along the middle, effectively halving the span). A timber merchant ought to be able to get 6x2 in 5.3m lengths at least. TBH, I didn't spot Tony's software - I thought he just mentioned the rule of thumb(!). If I did go below spec (say 4m of 2 x 6), it would be great to have some idea of how wobbly such a floor would be. The joist tables just give maximum length for size, as far as I can see. I just checked, there is still a demo available. It has printing knobbled - but that won't be a handicap for your needs. http://www.superbeam.co.uk/sbwdemo.htm (the usual caveats about it letting you design unsafe structures faster apply, if you don't stick in sensible values!) It will show you the calculated deflection for whatever load you apply, and also tell you when you are exceeding the safe working limits on the timber. If you model your longest timber that should let you get a feel for the changes. The loadings to apply for a normal floor appear further up the thread (if you know what you are storing etc you may be able to use lower figures) I'd still feel a little uncomfortable that it would deny the option of making the space habitable in the future though, even though the rest of the house is big enough that it shouldn't be an issue. Might be worth working out what you could "get away with" and then comparing the cost difference to doing it to full spec. Although I get the point that BR specs change over time, they surely can't ask for joist that are much deeper than they require now, so I would have thought the current specs won't change much. I would not expect them to change in substance at all really. They may grow to include more on composite joists (i.e. man made beams with struts top and bottom and some sheet material webbing) (Apologies if my posts are hard to read. Someone told me a while ago they weren't wrapping, whilst in a recent thread someone else told me they had a lot of empty lines!) Its a combination of not wrapping and all the lines being double spaced. The former is easy to fix in a reply with a quick CTRL + R in thunderbird. That latter takes slightly more editing! -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
#45
Posted to uk.d-i-y
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Installing a loft floor
On Sunday, October 21, 2012 7:33:24 PM UTC+1, John Rumm wrote:
On 21/10/2012 17:47, GMM wrote: On Sunday, October 21, 2012 4:25:44 PM UTC+1, John Rumm wrote: On 21/10/2012 13:09, GMM wrote: If you can rule out that the space will ever be converted to habitable, then you can undersize a tad from the tabulated values. As Tony mentioned above, its normally the deflection limits that dictate the size rather than the shear or bending limits. (i.e the floor would be likely to damage decorative finishes, feel to bouncy, and upset inhabitants of rooms below, long before the timber is in danger of actually failing) For your application (i.e. with the new beams some distance above the existing ceiling, and not ceiling to be mounted on the underside of the new joists), deflection beyond normal limits is a non issue. So it reduces to a problem of what is adequate in terms of bending and shear loading on the timber (assuming you don't mind it feeling a little more bouncy than "normal" given that you know it is still structurally sound). Perhaps a play Tony's excellent bit of software might be in order (assuming there is still a demo version available for download?) Yes John - it may potentially be that 2 x 6 joists (which are significantly cheaper per metre) could do an adequate job in this application, if they are available (my local timber yard couldn't supply that length for the living room when I was costing it: There, the lower spec was due to the wall running along the middle, effectively halving the span). A timber merchant ought to be able to get 6x2 in 5.3m lengths at least. TBH, I didn't spot Tony's software - I thought he just mentioned the rule of thumb(!). If I did go below spec (say 4m of 2 x 6), it would be great to have some idea of how wobbly such a floor would be. The joist tables just give maximum length for size, as far as I can see. I just checked, there is still a demo available. It has printing knobbled - but that won't be a handicap for your needs. http://www.superbeam.co.uk/sbwdemo.htm (the usual caveats about it letting you design unsafe structures faster apply, if you don't stick in sensible values!) It will show you the calculated deflection for whatever load you apply, and also tell you when you are exceeding the safe working limits on the timber. If you model your longest timber that should let you get a feel for the changes. The loadings to apply for a normal floor appear further up the thread (if you know what you are storing etc you may be able to use lower figures) I'd still feel a little uncomfortable that it would deny the option of making the space habitable in the future though, even though the rest of the house is big enough that it shouldn't be an issue. Might be worth working out what you could "get away with" and then comparing the cost difference to doing it to full spec. Although I get the point that BR specs change over time, they surely can't ask for joist that are much deeper than they require now, so I would have thought the current specs won't change much. I would not expect them to change in substance at all really. They may grow to include more on composite joists (i.e. man made beams with struts top and bottom and some sheet material webbing) (Apologies if my posts are hard to read. Someone told me a while ago they weren't wrapping, whilst in a recent thread someone else told me they had a lot of empty lines!) Its a combination of not wrapping and all the lines being double spaced. The former is easy to fix in a reply with a quick CTRL + R in thunderbird. That latter takes slightly more editing! -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ Thanks for the link - Now I have to find a Windows PC I can spark up and run the software - should be interesting. I suspect a narrower joist could be significantly cheaper for these timbers, although it might not make a substantial impact on the job cost overall. I've rather taken a 'do it properly and do it once' approach to this house, rather than being cost-driven. I did toy at one stage with the prospects for making up composite joists in situ, given the access issues, but decided that would just add another variable (and potential disaster) to the equation (!) Must have another go at Thunderbird for this group. I had it working, then it wouldn't post for some reason. |
#46
Posted to uk.d-i-y
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Installing a loft floor
On 21/10/2012 21:42, wrote:
On Sunday, October 21, 2012 4:07:53 PM UTC+1, John Rumm wrote: On 21/10/2012 00:37, wrote: On Saturday, October 20, 2012 10:33:20 PM UTC+1, John Rumm wrote: On 20/10/2012 20:33, wrote: On Saturday, October 20, 2012 4:42:00 PM UTC+1, John Rumm wrote: On 20/10/2012 01:49, wrote: On Friday, October 19, 2012 3:53:36 PM UTC+1, John Rumm wrote: On 19/10/2012 13:38, wrote: Also building regs are not retrospective. So if a floor was designed as a floor, and was compliant with the standards of the time, you would be able to use as the basis of your room in the roof, it even if the standards applying had changed since it was built. There's no way a BCO will accept a loft conversion in a 1924 house on its original 3" loft floor joists. I doubt a loft with 3" joists would not have been deemed acceptable as a proper floor for a habitable space - even in 1924. However, my point was, that if you upgrade something now to the current standards of a floor in a habitable room, then there would be no need to upgrade it further if one later made the space habitable - even if the standards for a floor have changed by then. 3x3 was the smallest standard habitable flooring joist size in Victorian houses. It was much used for short spans, such as across corridors & landings. And it still might be acceptable now (for short lengths) IIRC the 1924 BR didn't specify joist sizes, so 3x3 would still be compliant for habitation then. It could be used in loft floors above corridors, where the span was short. A loft floor is not a floor in the accepted sense though - its not expected to carry significant load. I challenge you to find any BCO that would accept that in a loft conversion today. A BCO would be happy with a loft using 3x2 - its a good deal better than many a lofts built with modern trusses. However that is a very different thing from a loft floor which going to be used for a habitable room. If you are converting the loft, then the same spec as would apply to any other floor in the building will kick in. Out of interest I had an experiment with superbeam to see what you can get away with on a 3x2 (well 72x47mm) and a typical floor load (uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about the limit - so you could probably still do a landing with it and comply with modern building regs. (having said that, its generally simpler to use one depth all over to save having to buy lots of timber sizes) For clarity, lets take it a step further. Say the loft got 2x2s in 4' spans in 1924, hopelessly unsuitable for habitable rooms, but still compliant for them in 1924. I don't think 2x2 would have been used for the floor of a habitable room in 1924 or at any other time. Its seems to are engaging in a little reductio ad absurdum. That's exactly the point. All Victorian loft floor joists were compliant for habitable use at time of building, but no BCO is going to accept them in a conversion today. I don't quite follow the line of thought. A loft floor in 1924 was *not* designed for habitable use then, so its no surprise it would not be considered adequate now. Your point seems to be that the definition of "habitable" did not exist in 1924. However to cut through the confusion, look at what would have been installed in 1924 for a normal 1st floor set of joists, since that will have been designed for what we would today call habitable. If the loft had similar spans but thinner joists, (which which is a safe bet) then its not habitable - then or now. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
#47
Posted to uk.d-i-y
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Installing a loft floor
On 22/10/2012 10:19, GMM wrote:
Thanks for the link - Now I have to find a Windows PC I can spark up and run the software - should be interesting. Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. I suspect a narrower joist could be significantly cheaper for these timbers, although it might not make a substantial impact on the job cost overall. I've rather taken a 'do it properly and do it once' approach to this house, rather than being cost-driven. It depends a bit on the layout and how many joist hangers you need etc. Those will be the same price regardless. I did toy at one stage with the prospects for making up composite joists in situ, given the access issues, but decided that would just add another variable (and potential disaster) to the equation (!) Must have another go at Thunderbird for this group. I had it working, then it wouldn't post for some reason. It ought to be fairly painless for newsgroups if you are using your ISPs newsserver. If using a third party one, you may need to turn on the "request authentication" option in the account settings. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
#48
Posted to uk.d-i-y
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Installing a loft floor
On Monday, October 22, 2012 2:38:42 PM UTC+1, John Rumm wrote:
On 21/10/2012 21:42, wrote: On Sunday, October 21, 2012 4:07:53 PM UTC+1, John Rumm wrote: On 21/10/2012 00:37, wrote: On Saturday, October 20, 2012 10:33:20 PM UTC+1, John Rumm wrote: On 20/10/2012 20:33, wrote: On Saturday, October 20, 2012 4:42:00 PM UTC+1, John Rumm wrote: On 20/10/2012 01:49, wrote: On Friday, October 19, 2012 3:53:36 PM UTC+1, John Rumm wrote: On 19/10/2012 13:38, wrote: Also building regs are not retrospective. So if a floor was designed as a floor, and was compliant with the standards of the time, you would be able to use as the basis of your room in the roof, it even if the standards applying had changed since it was built. There's no way a BCO will accept a loft conversion in a 1924 house on its original 3" loft floor joists. I doubt a loft with 3" joists would not have been deemed acceptable as a proper floor for a habitable space - even in 1924. However, my point was, that if you upgrade something now to the current standards of a floor in a habitable room, then there would be no need to upgrade it further if one later made the space habitable - even if the standards for a floor have changed by then. 3x3 was the smallest standard habitable flooring joist size in Victorian houses. It was much used for short spans, such as across corridors & landings. And it still might be acceptable now (for short lengths) IIRC the 1924 BR didn't specify joist sizes, so 3x3 would still be compliant for habitation then. It could be used in loft floors above corridors, where the span was short. A loft floor is not a floor in the accepted sense though - its not expected to carry significant load. I challenge you to find any BCO that would accept that in a loft conversion today. A BCO would be happy with a loft using 3x2 - its a good deal better than many a lofts built with modern trusses. However that is a very different thing from a loft floor which going to be used for a habitable room. If you are converting the loft, then the same spec as would apply to any other floor in the building will kick in. Out of interest I had an experiment with superbeam to see what you can get away with on a 3x2 (well 72x47mm) and a typical floor load (uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about the limit - so you could probably still do a landing with it and comply with modern building regs. (having said that, its generally simpler to use one depth all over to save having to buy lots of timber sizes) For clarity, lets take it a step further. Say the loft got 2x2s in 4' spans in 1924, hopelessly unsuitable for habitable rooms, but still compliant for them in 1924. I don't think 2x2 would have been used for the floor of a habitable room in 1924 or at any other time. Its seems to are engaging in a little reductio ad absurdum. That's exactly the point. All Victorian loft floor joists were compliant for habitable use at time of building, but no BCO is going to accept them in a conversion today. I don't quite follow the line of thought. A loft floor in 1924 was *not* designed for habitable use then, so its no surprise it would not be considered adequate now. Your point seems to be that the definition of "habitable" did not exist in 1924. However to cut through the confusion, look at what would have been installed in 1924 for a normal 1st floor set of joists, since that will have been designed for what we would today call habitable. If the loft had similar spans but thinner joists, (which which is a safe bet) then its not habitable - then or now. I don't think you've followed what I'm saying at all. NT |
#49
Posted to uk.d-i-y
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Installing a loft floor
On 22/10/2012 14:43, John Rumm wrote:
On 22/10/2012 10:19, GMM wrote: Thanks for the link - Now I have to find a Windows PC I can spark up and run the software - should be interesting. Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. I suspect a narrower joist could be significantly cheaper for these timbers, although it might not make a substantial impact on the job cost overall. I've rather taken a 'do it properly and do it once' approach to this house, rather than being cost-driven. It depends a bit on the layout and how many joist hangers you need etc. Those will be the same price regardless. I did toy at one stage with the prospects for making up composite joists in situ, given the access issues, but decided that would just add another variable (and potential disaster) to the equation (!) Must have another go at Thunderbird for this group. I had it working, then it wouldn't post for some reason. It ought to be fairly painless for newsgroups if you are using your ISPs newsserver. If using a third party one, you may need to turn on the "request authentication" option in the account settings. I should take a browse through the phone apps (without wishing to turn the thread into another debate about which phone is best!) Your comments about ISP newservers made me realise that I set Thunderbird up for home, then might have failed to post over a different WiFi connection, so if this posts, problem solved (ish) |
#50
Posted to uk.d-i-y
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Installing a loft floor
On 22/10/2012 20:28, wrote:
On Monday, October 22, 2012 2:38:42 PM UTC+1, John Rumm wrote: On 21/10/2012 21:42, wrote: On Sunday, October 21, 2012 4:07:53 PM UTC+1, John Rumm wrote: On 21/10/2012 00:37, wrote: On Saturday, October 20, 2012 10:33:20 PM UTC+1, John Rumm wrote: On 20/10/2012 20:33, wrote: On Saturday, October 20, 2012 4:42:00 PM UTC+1, John Rumm wrote: On 20/10/2012 01:49, wrote: On Friday, October 19, 2012 3:53:36 PM UTC+1, John Rumm wrote: On 19/10/2012 13:38, wrote: Also building regs are not retrospective. So if a floor was designed as a floor, and was compliant with the standards of the time, you would be able to use as the basis of your room in the roof, it even if the standards applying had changed since it was built. There's no way a BCO will accept a loft conversion in a 1924 house on its original 3" loft floor joists. I doubt a loft with 3" joists would not have been deemed acceptable as a proper floor for a habitable space - even in 1924. However, my point was, that if you upgrade something now to the current standards of a floor in a habitable room, then there would be no need to upgrade it further if one later made the space habitable - even if the standards for a floor have changed by then. 3x3 was the smallest standard habitable flooring joist size in Victorian houses. It was much used for short spans, such as across corridors & landings. And it still might be acceptable now (for short lengths) IIRC the 1924 BR didn't specify joist sizes, so 3x3 would still be compliant for habitation then. It could be used in loft floors above corridors, where the span was short. A loft floor is not a floor in the accepted sense though - its not expected to carry significant load. I challenge you to find any BCO that would accept that in a loft conversion today. A BCO would be happy with a loft using 3x2 - its a good deal better than many a lofts built with modern trusses. However that is a very different thing from a loft floor which going to be used for a habitable room. If you are converting the loft, then the same spec as would apply to any other floor in the building will kick in. Out of interest I had an experiment with superbeam to see what you can get away with on a 3x2 (well 72x47mm) and a typical floor load (uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about the limit - so you could probably still do a landing with it and comply with modern building regs. (having said that, its generally simpler to use one depth all over to save having to buy lots of timber sizes) For clarity, lets take it a step further. Say the loft got 2x2s in 4' spans in 1924, hopelessly unsuitable for habitable rooms, but still compliant for them in 1924. I don't think 2x2 would have been used for the floor of a habitable room in 1924 or at any other time. Its seems to are engaging in a little reductio ad absurdum. That's exactly the point. All Victorian loft floor joists were compliant for habitable use at time of building, but no BCO is going to accept them in a conversion today. I don't quite follow the line of thought. A loft floor in 1924 was *not* designed for habitable use then, so its no surprise it would not be considered adequate now. Your point seems to be that the definition of "habitable" did not exist in 1924. However to cut through the confusion, look at what would have been installed in 1924 for a normal 1st floor set of joists, since that will have been designed for what we would today call habitable. If the loft had similar spans but thinner joists, (which which is a safe bet) then its not habitable - then or now. I don't think you've followed what I'm saying at all. So what is your point then? I thought we had covered: 1) Standards have edged up? Agreed. But not hugely. 2) Skimpy joists in a loft were acceptable as a habitable room floor in the past? Nope, don't buy it for a moment. They were acceptable as a loft floor and that is all - but there was no anticipation that was going to be used as a bedroom of office etc. They were also to a much lower standard than was deemed necessary for all the other "proper" floors in the same building. 3) Joists will survive loadings far beyond those specified in the building regs? Yup agreed. The specs are primarily intended to limit deflection, and excessive deflection occurs long before structural failure. 4) You can still use skimpy timbers if they are short enough? Yup agreed, nothing much has changed there. 5) Joist sizes are specced on sound transmission? I don't believe that really comes into it. Specs on sound transmission have got much stronger in recent times, but joist sizes themselves have not increased as a result. Joist support techniques have changed - reducing wall penetrations, ensuring adequate insulating material is included in floor construction to reduce noise. Eliminating gaps and air paths are all partly related to reducing noise transmission as well as improving thermal performance. Fire protection rules have also tightened, and that has had a knock on on ceiling coverings (i.e. 1/2" PB and skim, not 9mm etc, intumescent covers over ceiling penetrations). Have I missed any? -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
#51
Posted to uk.d-i-y
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Installing a loft floor
On 22/10/2012 21:48, GMM wrote:
On 22/10/2012 14:43, John Rumm wrote: On 22/10/2012 10:19, GMM wrote: Thanks for the link - Now I have to find a Windows PC I can spark up and run the software - should be interesting. Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. I suspect a narrower joist could be significantly cheaper for these timbers, although it might not make a substantial impact on the job cost overall. I've rather taken a 'do it properly and do it once' approach to this house, rather than being cost-driven. It depends a bit on the layout and how many joist hangers you need etc. Those will be the same price regardless. I did toy at one stage with the prospects for making up composite joists in situ, given the access issues, but decided that would just add another variable (and potential disaster) to the equation (!) Must have another go at Thunderbird for this group. I had it working, then it wouldn't post for some reason. It ought to be fairly painless for newsgroups if you are using your ISPs newsserver. If using a third party one, you may need to turn on the "request authentication" option in the account settings. I should take a browse through the phone apps (without wishing to turn the thread into another debate about which phone is best!) Hence why I did not mention a platform. I have seen them for mine, so I presume they exist for the others... I wonder if Tony is planning on an Android or iOS port of superbeam proper? Your comments about ISP newservers made me realise that I set Thunderbird up for home, then might have failed to post over a different WiFi connection, so if this posts, problem solved (ish) Normally if the newsserver accepts authentication then it will allow posts from any network. If it does not, then it will usually only accept them from its "own" network address block. BTW the quoting looks much better - no double spaced lines anymore ;-) -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
#52
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Installing a loft floor
On Tue, 23 Oct 2012 01:43:39 +0100 John Rumm wrote :
Hence why I did not mention a platform. I have seen them for mine, so I presume they exist for the others... I wonder if Tony is planning on an Android or iOS port of superbeam proper? No current plans. Would it be an interesting project? Probably. Would it make any money, given what people are used to paying for mobile apps? Probably not. If I was looking for an alternative platform, then the most likely route would be a web-based app, accessible via any browser. -- Tony Bryer, Greentram: 'Software to build on', Melbourne, Australia www.greentram.com |
#53
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Installing a loft floor
I should take a browse through the phone apps (without wishing to turn the thread into another debate about which phone is best!) Hence why I did not mention a platform. I have seen them for mine, so I presume they exist for the others... I wonder if Tony is planning on an Android or iOS port of superbeam proper? Your comments about ISP newservers made me realise that I set Thunderbird up for home, then might have failed to post over a different WiFi connection, so if this posts, problem solved (ish) Normally if the newsserver accepts authentication then it will allow posts from any network. If it does not, then it will usually only accept them from its "own" network address block. BTW the quoting looks much better - no double spaced lines anymore ;-) Excellent! Problem solved then (now to work on the typos....) |
#54
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Installing a loft floor
On 23/10/2012 01:43, John Rumm wrote:
Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. It's interesting that there seem to be more apps available than web-based calculators. Unfortunately most of these seem to be in American and I'm not sure how C16 compares with their specs but, putting a trial dimension of 4m in, using Redwood or Southern Pine (which seem to come out the same and probably equate broadly) gives a deflection of 69mm for a 6x2 for a light floor loading, which seems an awful lot. For some reason, the one I was using seems not to accept 2" beams after the first use so I can't go back and check but the deflection for 8x2 was about 20-something mm so about a 2" difference. So to keep the floor clear of the ceiling, either size would have to be mounted at the same height (roughly), which is interesting. So it might well be best to go for the bigger size and do it 'properly' after all. I suspect this is a worst case as adding boards and struts would stiffen the whole thing. On the other hand, there is bound to be a bit of gradual sag over time, as you saw, even in an unloaded joist, which might offset this. I suppose an alternative (to use narrower timbers) would be to change the joist pattern but the only way I can see to do this would be to put something very substantial across the middle of the 6m dimension (or split it into 3) and run the joists at right angles to this. This would make most joists around 3m, where 6x2 would probably be fine. I'm not sure this would be wise though as it would increase the loading on the walls to a great extent where this beam was mounted and would generate problems with actually getting everything into place. Thinking back to my last place, it's certainly true that we had a loft which I floored using chipboard panels onto 4x2 ceiling joists with no problems. I think the differences there were a) the spans were much smaller and b) I didn't give it any thought at all! |
#55
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Installing a loft floor
On 24/10/2012 09:55, GMM wrote:
On 23/10/2012 01:43, John Rumm wrote: Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. It's interesting that there seem to be more apps available than web-based calculators. Unfortunately most of these seem to be in American and I'm not sure how C16 compares with their specs but, putting a trial dimension of 4m in, using Redwood or Southern Pine (which seem to come out the same and probably equate broadly) gives a deflection of 69mm for a 6x2 for a light floor loading, which seems an awful lot. For some reason, the one I was using seems not to accept 2" beams after the first use so I can't go back and check but the deflection for 8x2 was about 20-something mm so about a 2" difference. Give me the specs of a beam if you like and I will see what numbers I get... So to keep the floor clear of the ceiling, either size would have to be mounted at the same height (roughly), which is interesting. So it might well be best to go for the bigger size and do it 'properly' after all. I suspect this is a worst case as adding boards and struts would stiffen the whole thing. On the other hand, there is bound to be a bit of gradual sag over time, as you saw, even in an unloaded joist, which might offset this. When you add floor boards, you can use load sharing in the calcs - since you tie a number of them together in effect, the statistical variation is likely to reduce, so you can assume stiffness's no so close to worst case. I suppose an alternative (to use narrower timbers) would be to change the joist pattern but the only way I can see to do this would be to put something very substantial across the middle of the 6m dimension (or split it into 3) and run the joists at right angles to this. This would make most joists around 3m, where 6x2 would probably be fine. I'm not sure this would be wise though as it would increase the loading on the walls to a great extent where this beam was mounted and would generate problems with actually getting everything into place. Thinking back to my last place, it's certainly true that we had a loft which I floored using chipboard panels onto 4x2 ceiling joists with no problems. I think the differences there were a) the spans were much smaller and b) I didn't give it any thought at all! Well indeed. Also a loft just used for storage will not be loaded to anything close to what one needs to allow for a room. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
#56
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Installing a loft floor
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#57
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Installing a loft floor
On Wednesday, October 17, 2012 11:49:06 AM UTC+1, GMM wrote:
I have to build a floor in a loft. I’d like it to be robust enough that it doesn’t all wind up in the bedroom below. At present, there is a lath and plaster ceiling, with rather wimpy-looking ceiling joists, So question 1: To work above the ceiling, I’m thinking of supporting it with boards on acro jacks, possibly moving these according to where I’m working as I go and putting some boards on the ceiling joists to spread the load (mostly me!). Does this sound like a sensible thing to do – ie any better suggestions? I'd put at a beam at right angles to the joists in the middle of the ceiling, supported by acro-props. This will halve the effective length of the joists, and make them plenty man enough for the job. You'll then need boards on top of the joists to stand on. Lastly, to get decent access into the loft space I need to create a new doorway. The only way I can approach this is from the inside of the loft. Normally, cutting a new opening would be best done using strongboys to support the triangle of brisk above until the lintel is installed but I’d hesitate to jack against the top of the ceiling and getting them through the hatch wouldn’t be easy. So question 3: Instead of using strongboys, would a board (4x1 or so) fixed to each brick (multimonti into the brick centre) above the lintel do an adequate job of stopping everything moving while I get a lintel in? Sounds like a reasonable plan. I'd probably just use a rawl-plug and No 10/ No 12 screw. It doesn't have to take much weight, and it doesn't have to take it for long. |
#58
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Installing a loft floor
On 24/10/2012 16:55, John Rumm wrote:
On 24/10/2012 09:55, GMM wrote: On 23/10/2012 01:43, John Rumm wrote: Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. It's interesting that there seem to be more apps available than web-based calculators. Unfortunately most of these seem to be in American and I'm not sure how C16 compares with their specs but, putting a trial dimension of 4m in, using Redwood or Southern Pine (which seem to come out the same and probably equate broadly) gives a deflection of 69mm for a 6x2 for a light floor loading, which seems an awful lot. For some reason, the one I was using seems not to accept 2" beams after the first use so I can't go back and check but the deflection for 8x2 was about 20-something mm so about a 2" difference. Give me the specs of a beam if you like and I will see what numbers I get... Thanks John, that would be very useful. All lengths are 4m (a little variation but no more than 100mm) and the TRADA tables say a 47 x 195 joist is good for that at 400 centres but of course give no further information. (A number of apps etc simply give the same data in their calculators - you put in the size and it tells you the max span.) Clearly very small ones would be inadequate at this length, but it would be interesting to know what the performance of 7" (ie 47 x 170) and 6" (47 x 145) would be at 4m for C16 timber. From what the tables indicate, the advantage of C24 is pretty minimal, so hardly worth the trouble of sourcing etc except in very marginal situations). I'll most likely cover with chipboard (8x2 t/g sheets), which comes in 18 or 22mm. I'm assuming that there's little difference here for 400mm joist centres (and would go for 18 as it's lighter) and the more important factor is to screw it down to every joist but I could easily be corrected on this (!) It's also pretty clear that a span of 4m will benefit from strutting at mid span, so that's on the agenda too. Friday night - must be time for a cold one (!) Cheers! |
#59
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Installing a loft floor
On 26/10/2012 23:45, John Rumm wrote:
On 26/10/2012 19:11, GMM wrote: On 24/10/2012 16:55, John Rumm wrote: On 24/10/2012 09:55, GMM wrote: On 23/10/2012 01:43, John Rumm wrote: Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. It's interesting that there seem to be more apps available than web-based calculators. Unfortunately most of these seem to be in American and I'm not sure how C16 compares with their specs but, putting a trial dimension of 4m in, using Redwood or Southern Pine (which seem to come out the same and probably equate broadly) gives a deflection of 69mm for a 6x2 for a light floor loading, which seems an awful lot. For some reason, the one I was using seems not to accept 2" beams after the first use so I can't go back and check but the deflection for 8x2 was about 20-something mm so about a 2" difference. Give me the specs of a beam if you like and I will see what numbers I get... Thanks John, that would be very useful. All lengths are 4m (a little variation but no more than 100mm) and the TRADA tables say a 47 x 195 joist is good for that at 400 centres but of course give no further information. (A number of apps etc simply give the same data in their calculators - you put in the size and it tells you the max span.) Well if I model that with a uniformly distributed load of 0.8kN/m (long term load), and treat them as a load sharing system, you get a deflection of just under 11mm. Go down to 175mm and the deflection goes up to just under 15mm. However its worth noting that it is also flagged as failing in maximum permitted bending stress at that point. 150mm and deflection is nearly 23mm. 100mm and deflection is approaching 80mm! That is a full floor load though - unless storing a large magazine or record collection its unlikely you would reach that. Clearly very small ones would be inadequate at this length, but it would be interesting to know what the performance of 7" (ie 47 x 170) and 6" (47 x 145) would be at 4m for C16 timber. From what the tables indicate, the advantage of C24 is pretty minimal, so hardly worth the trouble of sourcing etc except in very marginal situations). C24 at 175 for example would be a "pass" on 175mm (deflection 7.61mm) and on 150mm (11.98). (125mm and you are back to failing in bending stress) Note also you can push it firther with short term loads - so you don't necessarily need to make adjustments to factor in someone clod hoppering around up there shifting the boxes. I'll most likely cover with chipboard (8x2 t/g sheets), which comes in 18 or 22mm. I'm assuming that there's little difference here for 400mm joist centres (and would go for 18 as it's lighter) and the more important factor is to screw it down to every joist but I could easily be corrected on this (!) It interlocks, so only really needs enough screwing in this application to stop it sliding about. It's also pretty clear that a span of 4m will benefit from strutting at mid span, so that's on the agenda too. It will make it bounce less. How much that matters for storage space is debatable. Friday night - must be time for a cold one (!) Sounds like a good plan ;-) Thanks for doing that John, it's very interesting: It looks like 150 at C24 is pretty much comparable to 195 at C16 in terms of deflection: I didn't think the difference between the two would be anywhere near as much, from the other information I've seen. Of course, the C24 may be more expensive even in the smaller size (I'll have to get a quote) but if it's not too much more, the extra 2" might be a useful space saving: It would certainly reduce the step up onto this floor from the new access door, which might make it more user friendly. (It took me a while to get back - I've been away for a few days and it seems Firefox can't connect to the server from other networks) All the best |
#60
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Installing a loft floor
On 30/10/2012 12:45, GMM wrote:
On 26/10/2012 23:45, John Rumm wrote: On 26/10/2012 19:11, GMM wrote: On 24/10/2012 16:55, John Rumm wrote: On 24/10/2012 09:55, GMM wrote: On 23/10/2012 01:43, John Rumm wrote: Probably not as "industry standard" or trustworthy, but there are a number of smart phone apps about that will do calcs at least on a single beam. Might be worth trying one of those if you have platform for that. It's interesting that there seem to be more apps available than web-based calculators. Unfortunately most of these seem to be in American and I'm not sure how C16 compares with their specs but, putting a trial dimension of 4m in, using Redwood or Southern Pine (which seem to come out the same and probably equate broadly) gives a deflection of 69mm for a 6x2 for a light floor loading, which seems an awful lot. For some reason, the one I was using seems not to accept 2" beams after the first use so I can't go back and check but the deflection for 8x2 was about 20-something mm so about a 2" difference. Give me the specs of a beam if you like and I will see what numbers I get... Thanks John, that would be very useful. All lengths are 4m (a little variation but no more than 100mm) and the TRADA tables say a 47 x 195 joist is good for that at 400 centres but of course give no further information. (A number of apps etc simply give the same data in their calculators - you put in the size and it tells you the max span.) Well if I model that with a uniformly distributed load of 0.8kN/m (long term load), and treat them as a load sharing system, you get a deflection of just under 11mm. Go down to 175mm and the deflection goes up to just under 15mm. However its worth noting that it is also flagged as failing in maximum permitted bending stress at that point. 150mm and deflection is nearly 23mm. 100mm and deflection is approaching 80mm! That is a full floor load though - unless storing a large magazine or record collection its unlikely you would reach that. Clearly very small ones would be inadequate at this length, but it would be interesting to know what the performance of 7" (ie 47 x 170) and 6" (47 x 145) would be at 4m for C16 timber. From what the tables indicate, the advantage of C24 is pretty minimal, so hardly worth the trouble of sourcing etc except in very marginal situations). C24 at 175 for example would be a "pass" on 175mm (deflection 7.61mm) and on 150mm (11.98). (125mm and you are back to failing in bending stress) Note also you can push it firther with short term loads - so you don't necessarily need to make adjustments to factor in someone clod hoppering around up there shifting the boxes. I'll most likely cover with chipboard (8x2 t/g sheets), which comes in 18 or 22mm. I'm assuming that there's little difference here for 400mm joist centres (and would go for 18 as it's lighter) and the more important factor is to screw it down to every joist but I could easily be corrected on this (!) It interlocks, so only really needs enough screwing in this application to stop it sliding about. It's also pretty clear that a span of 4m will benefit from strutting at mid span, so that's on the agenda too. It will make it bounce less. How much that matters for storage space is debatable. Friday night - must be time for a cold one (!) Sounds like a good plan ;-) Thanks for doing that John, it's very interesting: It looks like 150 at C24 is pretty much comparable to 195 at C16 in terms of deflection: I didn't think the difference between the two would be anywhere near as much, from the other information I've seen. Of course, the C24 may be more expensive even in the smaller size (I'll have to get a quote) but if it's not too much more, the extra 2" might be a useful space saving: It would certainly reduce the step up onto this floor from the new access door, which might make it more user friendly. I remember when I did my loft conversion that there was some C24 specified (most was C16). It took the timber merchant about an extra week to order that in for me. Hence it may not be as readily stocked. (It took me a while to get back - I've been away for a few days and it seems Firefox can't connect to the server from other networks) Thunderbird perhaps? If your mail server supports authentication then you may be able to use it by turning on the "Always request authentication when connecting to this server" checkbox in the news server server settings dialogue. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
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