robgraham wrote:
On 14 Dec, 17:44, "Dave Plowman (News)" wrote:
In article ,
The Natural Philosopher wrote:

Both ridge and purlins only serve to stop the rafters moving sideways.
(Euler buckling etc,). They have very little structural effect beyond
that.
Perhaps you'd explain why my loft conversion involved the fitting of steel
purlins if they have so little work to do?

--
*When everything's coming your way, you're in the wrong lane *

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

I would tend to have to agree with the dissenters - a common 1950's
terraced council house roof structure involved heavyweight purlins
running from gable wall to gable wall supporting lightweight rafters.
This is equivalent to a long span roof with a number of trusses.

The essence is that the roof weight is transferred to the trusses/
gable walls by the purlins. We only have to look at the roof
structure of old buildings to see that the purlins were of significant
size, the actual roof timbers light in comparison and the structure
supporting the purlins massive too.


Mmm.

That is not strictly a trussed roof at all, then.

And shockingly bad design.

I guess you could get away with it over short roof spans. Spans that are
e.g. less than the cross span from front to back.

But you still need binders to hold the rafter ends in, unless you have a
massive ridge also, and then there is a distinct possibility of the
rafters sliding down the ridge board..you are relying on nails alone to
hold them up.

At this point it really isn't a truss roof at all. you are in fact using
the purlins and ridge as the main structural elements as simple beams.
and nailing rafters over the top simply to transfer load from the tiles
to the purlins and ridge.

It seems to me to be a very wasteful way to do it.




Rob