Roger wrote:

So where else does it go?

Onto the purlin. The rafter still has to function as a beam even though
it is by no means horizontal. Even if it wasn't nailed to the purlin it
would still load it and for the load to be in equilibrium at that point
the forces must balance.

Unless your purlins are actually PROPPED all the weight of the roof is
transmitted via the rafter ends. There *is* no other point of support.

No clear what you mean by propped in this context. Purlins are generally
simply supported at both ends and rely on their size to carry the load
transferred to them.

On mine, there were three purlins on each of three sides of the hipped
roof. The front and back ones were rested on a corbelled out section of
brickwork on the party wall, and nailed to third purlin at the other end
with a compound mitre. There were then three struts - each at mid span
of each purlin that were approximately at right angles to the pitch of
the rafters, that took the purlin load down to the spine wall across the
middle of the house.

Purlins are simply there to either stabilise the rafters, or if above
them, to transmit te roof weight TO the rafters.

You might think so but that doesn't make it a fact.

On my roof they carried a substantial proportion of the roof load.

If all that was needed to be done was to stabilise the rafters a dinky
bit of 4" x 2" (likely the same section as the rafters) nailed flat
against the rafters would do.

Thats geberally waht is used IME.

YMMV but I have never yet seen a proper roof with the horizontal members
fixed flat rather than upright (roofing laths excepted). To do so would
waste some of the strength of the beam. If the only purpose was to
stabilise the rafters using off-cuts for noggins would be more than
adequate.

This is the sort of thing you often see on trussed roofs - often wide
flatish timbers (4x1") are run (often at a jaunty angle) across the
rafters to add lateral restraint but not much else. I would hesitate to
call those purlins though.


--
Cheers,

John.

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