The Natural Philosopher wrote:

I'm designing a dormer loft conversion. I omitted that off my original
post I think.
The room is to be open plan from front to back, i.e. to cantilever
support as in John Rumms conversion.
Simon.


So what have you removed to necessitate beefing up the structure?

Even punching a dormer out hardly weakens anything.

Its not so much weakening that is the problem. With a substantial dormer
(which basically amounted to one entire side of the roof in the case of
mine), you are removing several tones of weight of roof tiles. So the
ridge is no longer counter balanced by roughly equal loadings from
either side.

and surely you don't mean a cantilever..I have yet to see a cantilever
structure in any house..well a few Elizabethan ones where the top storey
overhangs the lower maybe.

My conversion did away with the need for a ridge beam, by having a
central wall dividing two rooms in the roof. The wall was not directly
under the ridge however but about a meter behind it, so the flat roof
joists of the dormer sailed over it to meet the slope of the remaining
front roof. These were hence supporting the front roof in cantilever:

http://www.internode.co.uk/loft/images/roofsupport.jpg

The supporting wall is just out of shot to the left of the above photo.

--
Cheers,

John.

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