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Cash Cash is offline
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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