How much harder is it to make a flat roof with a small slope?
I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get built with a slight slope, maybe 1-2cms per metre? If that were done, the worst of the water would run off rather than pooling and the roof would dry out quickly once the rain stops. Of course, that's harder to build, but how much harder? Is the main issue packing out the joists so the roof slopes, or is there more to it than that? -- Murphy's ultimate law is that if something that could go wrong doesn't, it turns out that it would have been better if it had gone wrong. |
How much harder is it to make a flat roof with a small slope?
GB wrote:
I see them all the time - flat roofs that are completely level, and water pools on the roof just waiting to get into the building. Why don't they get built with a slight slope, maybe 1-2cms per metre? If that were done, the worst of the water would run off rather than pooling and the roof would dry out quickly once the rain stops. Of course, that's harder to build, but how much harder? Is the main issue packing out the joists so the roof slopes, or is there more to it than that? Hmmm. Is this a wind up? 'flat' rooves are not flat, they already have this slope built into them, and no, it's not hard to do, it's a taper - timber merchants will cut any sized taper you want, up to 6m in length - these are placed one upon each joist to achieve the fall. |
How much harder is it to make a flat roof with a small slope?
GB wrote:
I see them all the time - flat roofs that are completely level, and water pools on the roof just waiting to get into the building. Why don't they get built with a slight slope, maybe 1-2cms per metre? If that were done, the worst of the water would run off rather than pooling and the roof would dry out quickly once the rain stops. Of course, that's harder to build, but how much harder? Is the main issue packing out the joists so the roof slopes, or is there more to it than that? I suspect because like my carpenters, the principles of trigonometry escape them. They insisted on laying out the rafters for the 57 degree pitched roof on the ground, and not trusting to sins coses and tans... Bill: ere, how come this sheet of ply don't fit? Alf: I told you that it doesn't work if it ain't flat. Bill: so why dont we make it a bit bigger? Alf: oh YES, but how MUCH bigger eh? That's rocket science, that is. Trigernomettry. Not carpentry. |
How much harder is it to make a flat roof with a small slope?
GB wrote:
I see them all the time - flat roofs that are completely level, and water pools on the roof just waiting to get into the building. Why don't they get built with a slight slope, maybe 1-2cms per metre? If that were done, the worst of the water would run off rather than pooling and the roof would dry out quickly once the rain stops. Err they do ... Of course, that's harder to build, but how much harder? Is the main issue packing out the joists so the roof slopes, or is there more to it than that? Search for "firring strips" |
How much harder is it to make a flat roof with a small slope?
"GB" wrote in message ... I see them all the time - flat roofs that are completely level, and water pools on the roof just waiting to get into the building. Why don't they get built with a slight slope, maybe 1-2cms per metre? If that were done, the worst of the water would run off rather than pooling and the roof would dry out quickly once the rain stops. Of course, that's harder to build, but how much harder? Is the main issue packing out the joists so the roof slopes, or is there more to it than that? -- Murphy's ultimate law is that if something that could go wrong doesn't, it turns out that it would have been better if it had gone wrong. They usually are sloped. "Firring pieces" are attached to the roof joists to achieve this. The roof drain may be blocked. But flat roofs are bad news, you're right. |
How much harder is it to make a flat roof with a small slope?
The Natural Philosopher wrote:
I suspect because like my carpenters, the principles of trigonometry escape them. They insisted on laying out the rafters for the 57 degree pitched roof on the ground, and not trusting to sins coses and tans... IMO, a physical check is so much easier than trying to get the maths right. If I had a pound for every calculation where I try and be clever with the maths, and fail, I could probably afford to emply an emmeritus professor of trig! -- Scott Where are we going and why am I in this handbasket? |
How much harder is it to make a flat roof with a small slope?
On Jul 20, 9:15*pm, John Rumm wrote:
On 20/07/2011 19:49, GB wrote: I see them all the time - flat roofs that are completely level, and water pools on the roof just waiting to get into the building. Why don't they get built with a slight slope, maybe 1-2cms per metre? If that were done, the worst of the water would run off rather than pooling and the roof would dry out quickly once the rain stops. There's 2 sorts of 'flat roof': 1. Completely flat The roof sides, and the deck itself, are tanked ie completely waterproofed. The resulting ponding is deliberate part of the design used to control temperature of the roofing. There may even be an arrangement to flood the roof in drought conditions. There some like this in SE London which you can look down on from the west side of the line into Charing Cross from Tonbridge. Of course, that's harder to build, but how much harder? Is the main issue packing out the joists so the roof slopes, or is there more to it than that? The term "flat roof" really should me a misnomer in the sense that while they should be flat, they should not be level! The minimum fall on a roof should be about 1 in 80. Typically achieved by placing firings (i.e. tapered timbers) on top of the roof joists. 2. Standard 'flat' These have a slope of at least 1 in 80, but IIRC Ruberoid recommend (or certainly did 20 years back!) at least 1 in 40. Apart from having to resist snow, rain & tempest, a flat roof can become extremely hot in sunny weather with consequent risk of warping. Another factor is that a really heavy fall of snow really is *heavy*! My guess is that 1 in 80 is the minimum required when the roof is new to allow for resulting distortions and avoid ponding after a few years of the worst our weather can throw at it. A 1 in 40 (about 2.5deg) deck doesn't sound much, but shows a pronounced tilt when viewed sideways on. That can be difficult to make look 'level' if required for aesthetic reasons, for instance on a double garage roof sloping to the side rather than front to back, when viewed from the road. HTH |
How much harder is it to make a flat roof with a small slope?
In article ,
harryagain wrote: But flat roofs are bad news, you're right. They are, of course, sometimes unavoidable. Properly done, they are not such bad news as most say. -- *Speak softly and carry a cellular phone * Dave Plowman London SW To e-mail, change noise into sound. |
How much harder is it to make a flat roof with a small slope?
"Dave Plowman (News)" wrote in message ... In article , harryagain wrote: But flat roofs are bad news, you're right. They are, of course, sometimes unavoidable. Properly done, they are not such bad news as most say. They work OK when maintained. They just don't last forever like tiles/slates do. When they start to leak its often too late. And they always start to leak when the weather makes repair difficult. |
How much harder is it to make a flat roof with a small slope?
Dave Plowman (News) wrote:
In article , harryagain wrote: But flat roofs are bad news, you're right. They are, of course, sometimes unavoidable. Properly done, they are not such bad news as most say. Thanks all. The two I am thinking about are an extension roof done around 60 years ago and a garage roof done around 50 years ago. Maybe things were different then, or maybe they were just done badly, but they both pond. -- Murphy's ultimate law is that if something that could go wrong doesn't, it turns out that it would have been better if it had gone wrong. |
How much harder is it to make a flat roof with a small slope?
In article ,
dennis@home wrote: They are, of course, sometimes unavoidable. Properly done, they are not such bad news as most say. They work OK when maintained. They just don't last forever like tiles/slates do. Nothing lasts forever. Pretty well every house round here has had a new roof. Some were slate, some tiles. -- *If your feet smell and your nose runs, you're built upside down. Dave Plowman London SW To e-mail, change noise into sound. |
How much harder is it to make a flat roof with a small slope?
"Dave Plowman (News)" wrote in message ... In article , dennis@home wrote: They are, of course, sometimes unavoidable. Properly done, they are not such bad news as most say. They work OK when maintained. They just don't last forever like tiles/slates do. Nothing lasts forever. Pretty well every house round here has had a new roof. Some were slate, some tiles. While they would have had five new flat roofs. |
How much harder is it to make a flat roof with a small slope?
On Jul 21, 10:08*am, jim wrote:
A 1 in 40 (about 2.5deg) deck doesn't sound much, but shows a pronounced tilt when viewed sideways on. *That can be difficult to make look 'level' if required for aesthetic reasons, for instance on a double garage roof sloping to the side rather than front to back, when viewed from the road. That's just a poor job. It's not hard to keep the check kerb level and hide the slope of the roof. MBQ |
How much harder is it to make a flat roof with a small slope?
In article ,
dennis@home wrote: Nothing lasts forever. Pretty well every house round here has had a new roof. Some were slate, some tiles. While they would have had five new flat roofs. My flat roof is over 30 years old. The house isn't 150. And had been re-roofed before I bought it - and had it subsequently re-done. Plenty examples of traditional roofs being badly replaced. So in some ways the same as a flat one - good workmanship and materials matter. -- *Who are these kids and why are they calling me Mom? Dave Plowman London SW To e-mail, change noise into sound. |
How much harder is it to make a flat roof with a small slope?
On Wed, 20 Jul 2011 19:51:54 +0100, "Phil L"
wrote: 'flat' rooves are not flat, they already have this slope built into them, and no, it's not hard to do, it's a taper - timber merchants will cut any sized taper you want, up to 6m in length - these are placed one upon each joist to achieve the fall. What my builder did last year. But the flat roof built on my dad's extension c1963 was truly flat; with roofing felt hot tarred on and solar chippings it was not very good but probably typical of the time. |
How much harder is it to make a flat roof with a small slope?
On Jul 21, 1:39*pm, "Dave Plowman (News)"
wrote: In article , * *dennis@home wrote: Nothing lasts forever. Pretty well every house round here has had a new roof. Some were slate, some tiles. While they would have had five new flat roofs. My flat roof is over 30 years old. +1 same with our garage/car port roof. MBQ |
How much harder is it to make a flat roof with a small slope?
On Jul 21, 12:32*pm, "Man at B&Q" wrote:
On Jul 21, 10:08*am, jim wrote: A 1 in 40 (about 2.5deg) deck doesn't sound much, but shows a pronounced tilt when viewed sideways on. *That can be difficult to make look 'level' if required for aesthetic reasons, for instance on a double garage roof sloping to the side rather than front to back, when viewed from the road. That's just a poor job. It's not hard to keep the check kerb level and hide the slope of the roof. It would be most informative to see your design whch hides the slope from all sides and keeps the roof line looking slim all round. Yes you can hide it somewhat, but personally, I consider a step or kerb at the front of a garage looks a ghastly fudge when seen from the side. So often it just shows up flat roofing for what it is, a cheapo job. OTOH you have cut your suit to suit the cloth... |
How much harder is it to make a flat roof with a small slope?
In article
, jim wrote: Yes you can hide it somewhat, but personally, I consider a step or kerb at the front of a garage looks a ghastly fudge when seen from the side. So often it just shows up flat roofing for what it is, a cheapo job. OTOH you have cut your suit to suit the cloth... There are plenty of examples where a flat roof is the only option, planning wise. Of course if it is done as a 'cheap' job the results will be poor. Which is true of everything. Including a conventional roof. -- *In "Casablanca", Humphrey Bogart never said "Play it again, Sam" * Dave Plowman London SW To e-mail, change noise into sound. |
How much harder is it to make a flat roof with a small slope?
On Jul 21, 4:30*pm, jim wrote:
On Jul 21, 12:32*pm, "Man at B&Q" wrote: On Jul 21, 10:08*am, jim wrote: A 1 in 40 (about 2.5deg) deck doesn't sound much, but shows a pronounced tilt when viewed sideways on. *That can be difficult to make look 'level' if required for aesthetic reasons, for instance on a double garage roof sloping to the side rather than front to back, when viewed from the road. That's just a poor job. It's not hard to keep the check kerb level and hide the slope of the roof. It would be most informative to see your design whch hides the slope from all sides and keeps the roof line looking slim all round. See fig. 3 in http://www.wickes.co.uk/pcat/pcat/65roofing you see the drip edge which is no more than a couple of inches and the fascia which is as deep as it needs to be. Both are level when seen from all 3 sides of the roof except the side where the gutter is. If you want to get really fancy you could have an internal gutter and have a completely lavel appearance all round. Yes you can hide it somewhat, completely. but personally, I consider a step or kerb at the front of a garage looks a ghastly fudge when seen from the side. Where does the step come from? MBQ |
How much harder is it to make a flat roof with a small slope?
On 21/07/2011 10:08, jim wrote:
On Jul 20, 9:15 pm, John wrote: On 20/07/2011 19:49, GB wrote: I see them all the time - flat roofs that are completely level, and water pools on the roof just waiting to get into the building. Why don't they get built with a slight slope, maybe 1-2cms per metre? If that were done, the worst of the water would run off rather than pooling and the roof would dry out quickly once the rain stops. There's 2 sorts of 'flat roof': 1. Completely flat The roof sides, and the deck itself, are tanked ie completely waterproofed. The resulting ponding is deliberate part of the design used to control temperature of the roofing. There may even be an arrangement to flood the roof in drought conditions. There some like this in SE London which you can look down on from the west side of the line into Charing Cross from Tonbridge. Yup common elsewhere in europe as well. It does require the tanking to be "perfect" though. Of course, that's harder to build, but how much harder? Is the main issue packing out the joists so the roof slopes, or is there more to it than that? The term "flat roof" really should me a misnomer in the sense that while they should be flat, they should not be level! The minimum fall on a roof should be about 1 in 80. Typically achieved by placing firings (i.e. tapered timbers) on top of the roof joists. 2. Standard 'flat' These have a slope of at least 1 in 80, but IIRC Ruberoid recommend (or certainly did 20 years back!) at least 1 in 40. In in 80 is the minimum fall as required by the building regs. In practice a little steeper works better. Apart from having to resist snow, rain& tempest, a flat roof can become extremely hot in sunny weather with consequent risk of warping. Another factor is that a really heavy fall of snow really is *heavy*! My guess is that 1 in 80 is the minimum required when the roof is new to allow for resulting distortions and avoid ponding after a few years of the worst our weather can throw at it. A 1 in 40 (about 2.5deg) deck doesn't sound much, but shows a pronounced tilt when viewed sideways on. That can be difficult to make look 'level' if required for aesthetic reasons, for instance on a double garage roof sloping to the side rather than front to back, when viewed from the road. You can disguise the deck angle with the edge detail. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
How much harder is it to make a flat roof with a small slope?
On Jul 21, 10:50*am, "GB" wrote:
Dave Plowman (News) wrote: In article , * harryagain wrote: But flat roofs are bad news, you're right. They are, of course, sometimes unavoidable. Properly done, they are not such bad news as most say. Thanks all. The two I am thinking about are an extension roof done around 60 years ago and a garage roof done around 50 years ago. Maybe things were different then, or maybe they were just done badly, but they both pond. Firring stips already mentioned. The other way is to add an extra row of bricks along the back. Now if you want a long lasting flattish roof, add 3 or 4 rows of bricks, use asbestos replacement flat sheet to roof it, then add roof tiles on top. That really lasts. NT |
How much harder is it to make a flat roof with a small slope?
In article
, NT wrote: Firring stips already mentioned. The other way is to add an extra row of bricks along the back. Now if you want a long lasting flattish roof, add 3 or 4 rows of bricks, use asbestos replacement flat sheet to roof it, then add roof tiles on top. That really lasts. Can you get tiles for such a gentle slope? I'd have thought rain could get blown between them too easily. -- *No radio - Already stolen. Dave Plowman London SW To e-mail, change noise into sound. |
How much harder is it to make a flat roof with a small slope?
On Jul 22, 12:53*pm, "Dave Plowman (News)"
wrote: In article , * *NT wrote: Firring stips already mentioned. The other way is to add an extra row of bricks along the back. Now if you want a long lasting flattish roof, add 3 or 4 rows of bricks, use asbestos replacement flat sheet to roof it, then add roof tiles on top. That really lasts. Can you get tiles for such a gentle slope? I'd have thought rain could get blown between them too easily. The point is to use ordinary roofing tiles. Rain does blow in, hence the asbestos or similar underlayer. The asbestos (etc) solves the issue of the tiles (rain gets blown in), and the tiles solve the issue of the asbestos (fragility). Result is a shallow sloped roof that can last a lifetime and looks ok. NT |
How much harder is it to make a flat roof with a small slope?
In article
, NT wrote: Can you get tiles for such a gentle slope? I'd have thought rain could get blown between them too easily. The point is to use ordinary roofing tiles. Rain does blow in, hence the asbestos or similar underlayer. The asbestos (etc) solves the issue of the tiles (rain gets blown in), and the tiles solve the issue of the asbestos (fragility). Result is a shallow sloped roof that can last a lifetime and looks ok. But how do you waterproof the joints between the asbestos sheet - even if you could get hold of it these days? It's not normally the look of a flat roof that most object to - more its life, if badly made. -- *Eagles may soar, but weasels don't get sucked into jet engines * Dave Plowman London SW To e-mail, change noise into sound. |
How much harder is it to make a flat roof with a small slope?
On Jul 22, 3:26*pm, "Dave Plowman (News)"
wrote: In article , * *NT wrote: Can you get tiles for such a gentle slope? I'd have thought rain could get blown between them too easily. The point is to use ordinary roofing tiles. Rain does blow in, hence the asbestos or similar underlayer. The asbestos (etc) solves the issue of the tiles (rain gets blown in), and the tiles solve the issue of the asbestos (fragility). Result is a shallow sloped roof that can last a lifetime and looks ok. But how do you waterproof the joints between the asbestos sheet just overlap them. Underneath the tiles there's very little wind. - even if you could get hold of it these days? modern asbestos-like sheet is fiberglass in cement It's not normally the look of a flat roof that most object to - more its life, if badly made. hence this trick. Its not the cheapest way to do it in the short term, but it does last. NT |
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