Can someone describe the usual structural work required for a dormer
conversion from
a traditional rafter/purlin roof, i.e. the usual way of supporting the
ridge / dormer roof etc.
Thanks,
Simon.

Can someone describe the usual structural work required for a dormer
conversion from
a traditional rafter/purlin roof, i.e. the usual way of supporting the
ridge / dormer roof etc.
Thanks,
Simon.

Simon,
I have just built a Dormer myself. Basically the two vertical
side walls were built using 4" x 2" studs at 400mm. spacing. These
walls were held apart by 4" x 2" at 400mm spacing at a height of 2
metres. The ridge of the dormer can either 'T' into the existing main
ridge or be set down by say one foot. The angle formed by the new
ceiling 4" x 2"s and the new ridge timber should be about 35 degrees.
This angle is quite variable and can be set just so that the dormer
looks right. As mine will be finished using slates, the outside of the
4 x 2 walls were covered in 22mm x 150mm 'sarking' boards. The whatever
roofing felt ontop of that. Wait until the Spring.

I can send some digi photos if you send your e-mail to...


Chris.

I have just built a Dormer myself. Basically the two vertical
side walls were built using 4" x 2" studs at 400mm. spacing. These
walls were held apart by 4" x 2" at 400mm spacing at a height of 2
metres. The ridge of the dormer can either 'T' into the existing main
ridge or be set down by say one foot. The angle formed by the new
ceiling 4" x 2"s and the new ridge timber should be about 35 degrees.
This angle is quite variable and can be set just so that the dormer
looks right. As mine will be finished using slates, the outside of the
4 x 2 walls were covered in 22mm x 150mm 'sarking' boards. The whatever
roofing felt ontop of that. Wait until the Spring.
What about support of the house roof ridge, now that rafters and
purlins are
removed ? RSJs ?
35 degrees - are you saying your dormer does not have a flat roof ?

Cheers,
Simon.

wrote:

Can someone describe the usual structural work required for a dormer
conversion from
a traditional rafter/purlin roof, i.e. the usual way of supporting the
ridge / dormer roof etc.

Start with a structural floor...

What extra work you need to do on the roof with depend on the size and
number of the dormer(s) etc.

A small one with its own bonnet roof may have little or no impact on the
roof structure - the load of the dormer and its roof can be carried on
the floor and the quantity of tiles removed need not change the balance
of the roof.

However a maximum size one (i.e. basically removing one complete side of
a pitched roof) may require substantial work on the roof to counteract
the fact that you have removed the natural support and counterbalance
for the ridge - here RSJs may be required to support the ridge.

The one I built supported the ridge by cantilever the dormer roof joists
over a supporting wall about a meter from the ridge centre line. See
picture toward the bottom of this page:

http://www.internode.co.uk/loft/superstructure.htm

--
Cheers,

John.

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