extension roof design

I have to design a shallow mono-pitch roof (around 20 degrees) for my proposed extension. The roof is to have a 4m long ridge above an existing party wall, and a span of 5m across the back of the house. The roof space is to be completely open as a feature. May be cold or warm roof. It will be OSB/felt boarded under the tiles due to the low pitch. Roughly, my inclination is to have 2 purlins about 2x8, and rafters 2x6 at 400mm spacings. The tie to the party wall and house wall would be joist hangers on 2x8s fixed with expansion bolts. Does this sound reasonable, under/over specced, or am I totally ignorant ? BCO will proably want structural calc, I know. Simon.

Reply to
sm_jamieson
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I have to design a shallow mono-pitch roof (around 20 degrees) for my proposed extension. The roof is to have a 4m long ridge above an existing party wall, and a span of 5m across the back of the house. The roof space is to be completely open as a feature. May be cold or warm roof. It will be OSB/felt boarded under the tiles due to the low pitch. Roughly, my inclination is to have 2 purlins about 2x8, and rafters 2x6 at 400mm spacings. The tie to the party wall and house wall would be joist hangers on 2x8s fixed with expansion bolts. Does this sound reasonable, under/over specced, or am I totally ignorant ? BCO will proably want structural calc, I know. Simon.

Reply to
sm_jamieson

On 30 Sep 2005 02:12:52 -0700, a particular chimpanzee named sm snipped-for-privacy@hotmail.com randomly hit the keyboard and produced:

If I'm reading you right the rafters span 5m (on the flat), and the purlin will span 4m. If so, the rafters and fixings are OK, but the purlins will be way over spanned. I don't have the tables to hand, but IIRC 2/200x50 will span no more than 2.5m.

With any beam, bending is proportional to the square of the length, and deflection to the cube, meaning that timber members are rarely practical in a domestic situation over 3m. You'll have to get a steel section. You may be lucky and get a BCO who might say, 'a 7" or an 8" would piss that' (a technical term), but as you say be prepared to submit calcs to back it up.

20° is not that low a pitch for most concrete interlocking tiles, so unless you're in Scotland, the timber sarking probably isn't necessary. The warm roof (a _proper_ warm roof, with the insulation _over_ the rafters) is better than a cold roof IMHO. Look at the insulation manufacturer's websites for how it should be done.
Reply to
Hugo Nebula

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