Roof valleys

13 May 2010
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United Kingdom
I'm currently building a house, and battening out my roof at the mo ready for slating. When installing support boards for the lead valleys can I lay 150mm(ish) wide 12mm ply over the rafters edged with batten then butt the 25 x 50 mm roof batten up to it rather than fitting board flush with the rafters.

A roofer who was helping us installed a valley board using 25mm gravel board but I can't see how the lead then fits in as there is no trough because the batten and board are the same thickness, surely the slate will kick up as it sails over the lead valley.

Also is the breather membrain over or under the valley board?

Hope this makes sense!!

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you can run a thin triangular fillet up the length of the valley both sides, that forms a check.

I always turn the edge back on its self for good measure
you can run a thin triangular fillet up the length of the valley both sides, that forms a check.

Sorry cant quite picture it... triangular fillet straight onto rather, then membrain, then lead with edges rolled over.
The best way is to cut the wood so that it fits in between the rafters and sits level with them. Doing it this way won't cause any uplift at the valley.
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not on slate its not

Any reason why? In Scotland that is how we do it and although we don't use battens it would still seem like the best way to me.

Have it flat and use some timber to create the upstand that would be level with the battens so the slate runs through level.
I think on the last roof i did, about 8 years ago we put thin ply over rafters then lead with edges rolled over on top of that but my reference pics are missing for that one and they guy 'helping' has confused the issue more.
Thanks for your replies, I've sussed out my valleys now! Charging onto the next problem some point soon no doubt!
I've always use rough sawn 6x1" its actually 3/4" and lay them over valley rafters. Lead turned back on itself as data say's. but that's in England where it never rains :D
I'm not a fan of plywood in valley's

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