On 14 Dec, 14:54, The Natural Philosopher wrote:
sm_jamieson wrote:
On 14 Dec, 13:42, "Dave Plowman (News)" wrote:
In article
,
sm_jamieson wrote:

Trying to think outside the box.
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ? Loft
size 5.5m x 7.5m, normal cut roof with purlins at 1/3 and 2/3 heights,
V onto spine wall at purlin mid point, clay tiles etc.
It's more usual to install steel purlins - that's where the load is.

--
*Toilet stolen from police station. Cops have nothing to go on.

Dave Plowman London SW
To e-mail, change noise into sound.

Purlins need to be minimized (stairs running up parallel to the roof
slope), thus a ridge beam would be required to take some of the load
(why not, it's in dead space !), and the purlins only need to stop the
rafter sag. Roof pitch is quite steep at 40 degrees.
In the main section hopefully dwarf wall at 1/3 height and a ridge
beam will be enough. Purlins at 2/3 height are the awkward ones.
Simon.

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that. The rafters transfer all the loads to a downwards and outwards
force at the eaves level: That's where you need serous material.

Unless you go for warren braced (W braced) trusses, or some other method
of connecting the joist to the rafter in the roof truss, all the
strength is actually IN the rafter and the joist itself. Any other
bracing is for stability only and carries very little load, including
the ridge.

Howerver if the rafter to joist integrity is poor,so the rafters can
slide outwards at the base - a fairly common thing where upstairs
conversions under the eaves have been done and tie beams cut
through,leading to wall spreading - then a steel ridge running
gable-to-gable can act as a primary support. At this point teh roof
ceases to be a triangulated truss structure ands simply becomes a bunch
of poles leaned against a structural ride pole.The wight of the roof
instead of being carried as a downward an outward force on the rafter
bases, is transferred to a downward force on the ridge and a downward
and INWARD force on the rafter ends.

If you also add in *structural* steel purlins and carry those downwards
via posts to the joists, you can end up with a completely different
loading again.

That adds load to the joists, which are no longer in pure tension, but
also being subjected to a bending force. You might use such vertical
ties if the original ceiling was not strong enough to support rooms
above - you trasnfer loads to the roof timbers that way, which are
generally massive enough to take tiles or slates..they can cope.

In all cases, however, *unless* the purlins or the ridge is/are *very*
strong, *and* supported at the gable ends or by a central chimney etc,
they do not carry any major loads at all. They are there for stability.
Increasing their size is a waste of time.

Indeed.
So ...
I have plenty of headroom in the loft. What sized timber I beam would
I need to use instead of the typical RSJ to support the ridge ?
Cheers,
Simon.