Roger wrote:

Sorry TNP but I don't buy that argument. Purlins are often very
substantial pieces of timber and with a heavy roof they need to be. Very
few roofs have a pitch greater than 45 degrees so if the load is
resolved into rightangle components only half or less is being
transmitted directly down the rafter.

I would go along with that... The purlins were about 7x4" on my roof.

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.

I am quite sure that if I nailed a few bits of 4 x 2 across my rafters
and then cut out the purlins most of the 20 or 30 tons of stone on my 30
degree roof would promptly join me inside the house. Rafters are only 4"
x 3" (or 4 x 2 where they have been replaced) but the clear span is
never more than about 4 feet (which is perhaps half what you might
expect for a lightweight slate roof).

Having actually done this I can tell you what happens! ;-)

On the hipped section of roof I was removing, we had de-tiled it when we
took the strut supporting the purlin out. The whole centre roof section
and purlin promptly dropped about 3 to 4" as the rafters sagged just
under their own load and that of the purlin. Had the tiles still been on
the roof, I expect the results would have been far more dramatic!

--
Cheers,

John.

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