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where rafters cross purlins



 
 
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  #1  
Old April 10th 09, 01:29 AM posted to uk.d-i-y
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Posts: 2,322
Default where rafters cross purlins

On a traditional cut roof, when the rafter joins the wall plate you
cut a birdsmouth to give a good seat onto the wallplate. But where the
rafters cross purlins, what is "good form" for the joints here ? The
possibilities are
1. Cut birdsmouth at each purlin - gives good seat but very hard so
space correctly, and rafter cannot slide and this settle. No builder
would do this - too much work, I've never seen it.
2. Chamfer purlin to give a larger contact area - no locking in,
rafter can slide to settle
3. Just rest the rafter on the square-ish corner of the purlin. This
seems to be very common, but is this good form ? Very small contact
area / pressure point. Rafter may settle in and lock to some extent as
the corner of the purlin beds into the rafter
4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.

What should I do for my roof ? This is not specified on the plans.
Note - in my case roof is very low pitch (12.5 degrees - lowest
possible with tiles - redland regent), and the design has several
purlins. Sort of cross between a pitch and flat roof in design.
For some reason option 3 does not sit easy with my engineering
sensibilities, although this is most commonly seen on trad roofs of a
standard pitch.

Simon.
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  #2  
Old April 10th 09, 01:39 AM posted to uk.d-i-y
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Posts: 15,346
Default where rafters cross purlins

sm_jamieson wrote:

1. Cut birdsmouth at each purlin - gives good seat but very hard so
space correctly, and rafter cannot slide and this settle. No builder
would do this - too much work, I've never seen it.


not seen that

2. Chamfer purlin to give a larger contact area - no locking in,
rafter can slide to settle


or that

3. Just rest the rafter on the square-ish corner of the purlin. This
seems to be very common, but is this good form ? Very small contact
area / pressure point. Rafter may settle in and lock to some extent as
the corner of the purlin beds into the rafter


Have seen that - and a variation where a separate tilt fillet it cut to
sit on the purlin and match the slope on its top edge.

4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.


Probably seen this most often

What should I do for my roof ? This is not specified on the plans.
Note - in my case roof is very low pitch (12.5 degrees - lowest
possible with tiles - redland regent), and the design has several
purlins. Sort of cross between a pitch and flat roof in design.
For some reason option 3 does not sit easy with my engineering
sensibilities, although this is most commonly seen on trad roofs of a
standard pitch.


4 is easy enough if you have prop posts holding the purlins, and works
well enough on corbelled out brickwork.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #3  
Old April 10th 09, 10:59 AM posted to uk.d-i-y
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Posts: 82
Default where rafters cross purlins

On 10 Apr, 01:39, John Rumm wrote:
sm_jamieson wrote:
1. Cut birdsmouth at each purlin - gives good seat but very hard so
space correctly, and rafter cannot slide and this settle. No builder
would do this - too much work, I've never seen it.


not seen that

2. Chamfer purlin to give a larger contact area - no locking in,
rafter can slide to settle


or that

3. Just rest the rafter on the square-ish corner of the purlin. This
seems to be very common, but is this good form ? Very small contact
area / pressure point. Rafter may settle in and lock to some extent as
the corner of the purlin beds into the rafter


Have seen that - and a variation where a separate tilt fillet it cut to
sit on the purlin and match the slope on its top edge.

4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.


Probably seen this most often

What should I do for my roof ? This is not specified on the plans.
Note - in my case roof is very low pitch (12.5 degrees - lowest
possible with tiles - redland regent), and the design has several
purlins. Sort of cross between a pitch and flat roof in design.
For some reason option 3 does not sit easy with my engineering
sensibilities, although this is most commonly seen on trad roofs of a
standard pitch.


4 is easy enough if you have prop posts holding the purlins, and works
well enough on corbelled out brickwork.

--


Thanks for the reply.
I'll probably go for three (joist hangers mentioned on plans). A tilt
fillet at 12.5 degrees hardly seems worth it. But if using a tilt
fillet, how would it be fixed. A couple of nails ?

Do you know the maximum length you can get of rafters ("50mm x 47mm
counterbattens" on plans - note lots of purlins !) ?
It may be easier to join 2 lengths to make up the 5 metres. If I do
this, the joint will be over a purlin of course, but what is the best
way of joining them, and the overlap length ?

Also, any idea of the best way to line the purlins up with the
rafters ?
Seems to be 2 ways:
1. Fix rafters and then push purlins up underneath to align.
2. stretch a string and use this to position the purlins, then fit the
rafters over the top

Thanks,
Simon.



  #4  
Old April 10th 09, 11:01 AM posted to uk.d-i-y
external usenet poster
 
Posts: 9,577
Default where rafters cross purlins

On Thu, 9 Apr 2009 17:29:27 -0700 (PDT), sm_jamieson wrote:

4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.


This is the only version I have seen in old roofs. Can't really see the
problem with a few bits of cut brick and some mortar at the wall ends. In
fcat witha 12.5 deg pitch slope the purlins are not going to be that far
off vertical anyway. Couple of bits of broken tile to pack the space if
the mortar "look to thick"?

--
Cheers
Dave.



  #5  
Old April 10th 09, 12:02 PM posted to uk.d-i-y
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Posts: 2,541
Default where rafters cross purlins

John Rumm coughed up some electrons that declared:


Probably seen this most often


Mine are all like this...

If following the building practises in my house, simply place two bits of
wood to be joined in vague proximity and insert 4" nail

I'm really going to have to stop looking at my wood too hard - it's
depressing! OTOH, it hasn't fallen down in 30 years...

Cheers

Tim
  #6  
Old April 10th 09, 04:49 PM posted to uk.d-i-y
external usenet poster
 
Posts: 659
Default where rafters cross purlins

sm_jamieson wrote:
On a traditional cut roof, when the rafter joins the wall plate you
cut a birdsmouth to give a good seat onto the wallplate. But where the
rafters cross purlins, what is "good form" for the joints here ? The
possibilities are
1. Cut birdsmouth at each purlin - gives good seat but very hard so
space correctly, and rafter cannot slide and this settle. No builder
would do this - too much work, I've never seen it.
2. Chamfer purlin to give a larger contact area - no locking in,
rafter can slide to settle
3. Just rest the rafter on the square-ish corner of the purlin. This
seems to be very common, but is this good form ? Very small contact
area / pressure point. Rafter may settle in and lock to some extent as
the corner of the purlin beds into the rafter
4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.

What should I do for my roof ? This is not specified on the plans.
Note - in my case roof is very low pitch (12.5 degrees - lowest
possible with tiles - redland regent), and the design has several
purlins. Sort of cross between a pitch and flat roof in design.
For some reason option 3 does not sit easy with my engineering
sensibilities, although this is most commonly seen on trad roofs of a
standard pitch.

Simon.


Simon,

The purlin on a simple, single roof is there to stop the rafters bending -
and as such, the purlin rests at 90 degrees to the rafter pitch - and the
rafters simply rest on them and these are held by the plumb cut at the ridge
and the plumb and seat cuts on the wallplate. You could 'skew nail' the
rafters to the purlin as an extra fixing if you wish (normal).

if you are using them on a so-called double roof, then the rafters are
'cogged' (a type of housing joint) to the purlin.

Cash


  #7  
Old April 10th 09, 05:43 PM posted to uk.d-i-y
external usenet poster
 
Posts: 15,346
Default where rafters cross purlins

wrote:

3. Just rest the rafter on the square-ish corner of the purlin. This
seems to be very common, but is this good form ? Very small contact
area / pressure point. Rafter may settle in and lock to some extent as
the corner of the purlin beds into the rafter

Have seen that - and a variation where a separate tilt fillet it cut to
sit on the purlin and match the slope on its top edge.

4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.

Probably seen this most often

What should I do for my roof ? This is not specified on the plans.
Note - in my case roof is very low pitch (12.5 degrees - lowest
possible with tiles - redland regent), and the design has several
purlins. Sort of cross between a pitch and flat roof in design.
For some reason option 3 does not sit easy with my engineering
sensibilities, although this is most commonly seen on trad roofs of a
standard pitch.

4 is easy enough if you have prop posts holding the purlins, and works
well enough on corbelled out brickwork.

--


Thanks for the reply.
I'll probably go for three (joist hangers mentioned on plans). A tilt
fillet at 12.5 degrees hardly seems worth it. But if using a tilt
fillet, how would it be fixed. A couple of nails ?


Yup, nail it to the top of the pulin every so often

Do you know the maximum length you can get of rafters ("50mm x 47mm
counterbattens" on plans - note lots of purlins !) ?


50x47 seems a bit skimpy for a rafter 100x47 would be more common.

getting 100x47 in 5.4m is easy enough - and ripping one of those
lengthways would give you equivalent length 47mm square ish - not sure
if you will be able to buy it in that length as a standard size -
although a decent wood merchant could rip it to size for you.

It may be easier to join 2 lengths to make up the 5 metres. If I do
this, the joint will be over a purlin of course, but what is the best
way of joining them, and the overlap length ?


The downside of a join is the loss of the extra support you get in
cantilever. Perhaps longish overlaps nailed together would restore some
of this.

Also, any idea of the best way to line the purlins up with the
rafters ?
Seems to be 2 ways:
1. Fix rafters and then push purlins up underneath to align.
2. stretch a string and use this to position the purlins, then fit the
rafters over the top


Normally if you are starting from scratch, then you could cut a pattern
rafter and make as many copies as required, and erect those with
birdsmouth and ridge board (keeping the ridge in balance so it does not
get pushed out of line), and add the pulins later (but before felting
and battening the roof). Given you have a very low pitch, that is going
to encourage the rafters to sag more, and also you will have more
lateral thrust at the wall plate just from the weight of the rafters. So
having the purlins in place first may be advisable.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd -
http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/
  #8  
Old April 10th 09, 08:40 PM posted to uk.d-i-y
external usenet poster
 
Posts: 82
Default where rafters cross purlins

On 10 Apr, 17:43, John Rumm wrote:
wrote:
3. Just rest the rafter on the square-ish corner of the purlin. This
seems to be very common, but is this good form ? Very small contact
area / pressure point. Rafter may settle in and lock to some extent as
the corner of the purlin beds into the rafter
Have seen that - and a variation where a separate tilt fillet it cut to
sit on the purlin and match the slope on its top edge.


4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.
Probably seen this most often


What should I do for my roof ? This is not specified on the plans.
Note - in my case roof is very low pitch (12.5 degrees - lowest
possible with tiles - redland regent), and the design has several
purlins. Sort of cross between a pitch and flat roof in design.
For some reason option 3 does not sit easy with my engineering
sensibilities, although this is most commonly seen on trad roofs of a
standard pitch.
4 is easy enough if you have prop posts holding the purlins, and works
well enough on corbelled out brickwork.


--


Thanks for the reply.
I'll probably go for three (joist hangers mentioned on plans). A tilt
fillet at 12.5 degrees hardly seems worth it. But if using a tilt
fillet, how would it be fixed. A couple of nails ?


Yup, nail it to the top of the pulin every so often

Do you know the maximum length you can get of rafters ("50mm x 47mm
counterbattens" on plans - note lots of purlins !) ?


50x47 seems a bit skimpy for a rafter 100x47 would be more common.


They are more "counterbattens", at 400mm spacing over 225 x 75mm
purlins at 730mm spacing. Struct eng passed it, must be OK I guess.
The funny roof design is a monopitch with vaulted ceiling, and the
smaller dimension (4 metres) was parallel to the slope, thus lots of
"purlins", Requirement was to get the thinnest roof possible,
including the insulation, without using steel. Lost of glass fibre
between the purlins, 25mm celotex under the whole lot. I could have
got it thinner with steel, but that seemed a bit OTT.


getting 100x47 in 5.4m is easy enough - and ripping one of those
lengthways would give you equivalent length 47mm square ish - not sure
if you will be able to buy it in that length as a standard size -
although a decent wood merchant could rip it to size for you.

It may be easier to join 2 lengths to make up the 5 metres. If I do
this, the joint will be over a purlin of course, but what is the best
way of joining them, and the overlap length ?


The downside of a join is the loss of the extra support you get in
cantilever. Perhaps longish overlaps nailed together would restore some
of this.

Also, any idea of the best way to line the purlins up with the
rafters ?
Seems to be 2 ways:
1. Fix rafters and then push purlins up underneath to align.
2. stretch a string and use this to position the purlins, then fit the
rafters over the top


Normally if you are starting from scratch, then you could cut a pattern
rafter and make as many copies as required, and erect those with
birdsmouth and ridge board (keeping the ridge in balance so it does not
get pushed out of line), and add the pulins later (but before felting
and battening the roof). Given you have a very low pitch, that is going
to encourage the rafters to sag more, and also you will have more
lateral thrust at the wall plate just from the weight of the rafters. So
having the purlins in place first may be advisable.

Yep, I thought that. Its actually more like a flat roof in loadings
etc.

Simon.


  #9  
Old April 10th 09, 08:44 PM posted to uk.d-i-y
external usenet poster
 
Posts: 82
Default where rafters cross purlins

On 10 Apr, 16:49, "Cash"
wrote:
sm_jamieson wrote:
On a traditional cut roof, when the rafter joins the wall plate you
cut a birdsmouth to give a good seat onto the wallplate. But where the
rafters cross purlins, what is "good form" for the joints here ? The
possibilities are
1. Cut birdsmouth at each purlin - gives good seat but very hard so
space correctly, and rafter cannot slide and this settle. No builder
would do this - too much work, I've never seen it.
2. Chamfer purlin to give a larger contact area - no locking in,
rafter can slide to settle
3. Just rest the rafter on the square-ish corner of the purlin. This
seems to be very common, but is this good form ? Very small contact
area / pressure point. Rafter may settle in and lock to some extent as
the corner of the purlin beds into the rafter
4. Angled purlins. I've seen this, but a right pain to build in where
the purlins sit in the wall, or not good with joist hangers.


What should I do for my roof ? This is not specified on the plans.
Note - in my case roof is very low pitch (12.5 degrees - lowest
possible with tiles - redland regent), and the design has several
purlins. Sort of cross between a pitch and flat roof in design.
For some reason option 3 does not sit easy with my engineering
sensibilities, although this is most commonly seen on trad roofs of a
standard pitch.


Simon.


Simon,

The purlin on a simple, single roof is there to stop the rafters bending -
and as such, the purlin rests at 90 degrees to the rafter pitch - and the
rafters simply rest on them and these are held by the plumb cut at the ridge
and the plumb and seat cuts on the wallplate. *You could 'skew nail' the
rafters to the purlin as an extra fixing if you wish (normal).

if you are using them on a so-called double roof, then the rafters are
'cogged' (a type of housing joint) to the purlin.

Cash


Makes sense to have the purlin at the angle of the rafter, I agree.
But the last two houses I have lived in had the purlin vertical, and
the rafter bearing on the purlin "corner".
My parents house had them at the rafter pitch.
Simon.

 




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