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.