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John Rumm John Rumm is offline
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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.

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