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.