Roof with valleys against existing?

22 Aug 2006
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United Kingdom
We want to construct a roof over an extension, which will butt at 90deg against an existing trussed rafter roof, forming two valleys. The new roof will be hipped at the far end.
The main question is:
How do we handle the junction of new with old? Specifically, the old roof trussed rafters will have to remain in place, so there is no space to put 'typical' valley rafters. What is the most appropriate solution to take the feet of the new valley jack rafters?
Thanks for any advice.
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You put down what are called lay boards on your exsisting roof the feet of the (cripple)rafters are cut to these boards
Thanks Splinter
Yes.. I've seen that in small(e.g. dormer) roofs.
My concern is that I'm adding a roof that is as big as the existing, and I guess I'd be adding significantly to the load taken by each of the existing rafters if I simply land the ends of the new rafters in this way. A traditional, deep valley rafter would have carried these loads. Don't I need to allow for the added load on the old rafters?
Not technically adding that much loading. The exising rafters are still taking the same amount of tile load as they previously did. The lay boards spread the load out over the existing roof then a compound truss should start the new roof on the new walls.
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That seems reasonable... I guess we're taking some roofing away from the existing roof, and adding some by way of the new roof... so I can see how that more or less balances out... and if that's the way it's done in such cases I'm more than happy to take the advice.

Many thanks for the time.

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