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.