GRP dry valley - membrane detail

11 Apr 2006
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United Kingdom

I'm just fitting a filon dry GRP valley, and am seeing conflicting advice re: the detail on the membrane.

The main run of membrane that runs along the valley under the GRP is obvious. The question is how to lap in the runs that come in from the main roof. In the schematic below, which is correct:

Green - fit all membrane to the roof before laying the GRP side support battens.
Pink - lay side support battens and then membrane from the main roof is turned up against side of side battens.
Blue - lay roof membrane last and lap it into the valley.


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Does the trough sit on top or underneath the valley runner batten?
The grp valley has a lip on each edge so the valley side battens definitely need to go down first.
Assuming you've laid a valley sheet of felt and set battens to this your felt will terminate at the runners obviously as the runners will be obstructing the rafters.
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Not sure I follow. The rafters are below the level of all battens. Valley boards are set in between the rafters so the tops of the boards are flush with the top of the rafters.

I could run the roof membrane under the side battens, over them, or over them *and* over the sides of the grp valley... But which?
Run continuous valley sheet of felt. Align valley runner battens central to valley and if meeting another valley ensure they marry up. Felt and batten roof as normal cutting felt along the runner batten.
If you want to create an upstand by cutting felt past runner battens it's your preference. You won't be able to fix felt into a rafter past the runner batten without compromising drape and performance of the felt.
If you want to create an upstand by cutting felt past runner battens it's your preference

Thanks, I think that's what I'll do then. Upstand against the batten and then leave any extra lapped down into the valley to sit under the grp, without fixing for obvious reasons as you say.
There's a few variations of dry valleys ones that require battens on top you can trim the felt to the central upstand.
It's all preference I suppose I've seen some tilers just overlap felt in a valley without a continuous valley felt but I suppose they're not tilers thinking about it

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