Wicking membrane? Information, help and tips on many areas of roofing including trusses, tiling, venting, insulation etc....

[burt12345]

Hi, our bedroom ceiling has been starting to develop an issue in that it’s damp below the valley rafters (although actually was dripping a couple of months ago, but I found the gutter the valley was draining into was overflowing at the wrong end!, and I assumed back into the wall - but that all feels dry now I’ve been able to access the bottom of the valley.

However we’ve still got an issue, and I took the top 4 slates off the roof and the membrane below was all dry - this is lower than the issue inside.

Some of this blue membrane overlaps (found this off cut) the valley lead and so as it feels as tho it’s capable of absorbing moisture, therefore could it be wicking from the valley, and into and up the timber?

I’m sure this is a very simple question but I can’t find what is correct, and if the company who replaced our roof many years ago didn’t follow good practice. So before I take the slates from bottom up, I’d appreciate a bit of guidance so I can put / see what should be correct.

[stoneyboy]

Hi burt12345,
Starting from the lowest component you should have a valley rafter with roof rafters mitred onto it from both sides, there should then be a layer of underslaters felt, this can be the felt from both sides of the valley lapped right over the valley to provide a watertight covering or a length of felt can be taken up the valley with the felt from the side roofs lapped onto it. On this there should be valley boards both sides with battens coming in from both sides. On this should be the valley gutter itself with tiles taken over it from both sides with a tilting fillet to support the tile edges. The tile edge is usually mortared. Detail may vary depending on the type of valley gutter and type of tiles fitted.
The roof tiles and valley gutter should be the first waterproofing element of the roof, the underslaters felt layer is the second line of defence.
Regards S

[burt12345]

Thanks. That isn’t how it is. I’ve got battens butting up and that blue menbrane was on top of the rafter lead, although trimmed to the slate line.

I’ve pushed the membrane futher under spares, and have ordered some expanding weather tape to create the same as what you say being mortared either side of the valley.

Sound ok?

[stoneyboy]

Hi burt12345,
It’s difficult to say whether your proposal will solve the problem. It is possible that the blue membrane lapped too far into the valley was allowing rainwater to ingress by capillary action.
It sounds like you have a lead valley gutter and slate roofs either side, it could be that the top end of the lead has not been weathered correctly, there is also the possibility that the lead has perforated. Suggest you try your expanding tape and see if this solves the problem.
Regards S