Roger wrote:

Or more relevantly when using the roof space as a room, you can add
another set of binders at 'loft ceiling' level bolted to the rafters.

Which doesn't do anywhere near as much as a purlin would to stop the
rafters bending under load.


Actually it does *more*.

Materials in bending are never as good as in compression. The binder
across a rafter pair effectively locates the end points of the binder
precisely. However it leaves a large trapezoid structure underneath. The
maximum sag will be about halfway between the tie point and the eaves.

Which is why the nasty modern trusses use W bracing at about halfway
down the rafters.

With full triangulation any bending stresses are limited to much shorter
lengths of timber.

The only reason I can see to use structural ridges and purlins is when
you have cheap wood no machinery and expensive labour. i.e. its easier
to square off a couple of tree trunks and support them on some masonry
and add a few bits of broomstick over, than make up a few 6x3 rafters.


Today there is no excuse for that waste of wood.