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tommybanana
Junior Member
Joined: 02 Oct 2008
Posts: 2
Location: Scotland
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 Posted: Thu Oct 02, 2008 8:17 pm Post subject: breathable membrane and lead valley gutter |
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hi advice appreciated on this.
I am putting up a cold non vented roof - breathabe membrane draped over rafters with battens and counter battens and no boarding) in scotland (because I have an english architect!!). Noone is used to this type of roof up here cos in scotland roofs tended to have sarking board on .
anyhow.
when you cover a roof with breathable membrane I assumed you ran the membrane over the whole roof to make it waterproof - including valleys.
now i am doing the valley gutter ( i have a relative who is a plumber to do the leadwork) and running plywood boards with tilting fillets down the valleys to sit the lead on.
what we cant work out (cos all the manual show diagrams with felt removed for clarity) is if the breathable membrane should
a) be cut and overlapped into the lead gutter
b) should run over the plywood boards and thus directly under the lead
or
c) under the plywood boards.
The manufacturer of the membrane said to run the membrane either under the plywood or to cut it and have it overlapping into the lead gutter ( although that would mean making the roof leaky until the leadwork is finished).
He did say though that as condensation often forms on the top surface of breathable mebranes that I need to make sure that water can run off the roof on top of the felt and not pool under the plywood as it will eventually rot the plywood.
It would be really useful to get a feel for what people normally do in England with this type of roof and breathable membranes.
A tech guy from the lead manufacturer wasnt sure but said that the lead should really sit on building paper on top of the wood so that it can expand and contract in different temperatures. I wondered what the difference is between building paper and breathable membrane.
any advice thoughts greatly appreciated.
tommy banana
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thescruff
Senior Member
Joined: 10 Mar 2008
Posts: 4658
Location: Bath
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| Posted: Thu Oct 02, 2008 8:38 pm Post subject: |
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Builders felt in the valley and up the boards, lead starting at the lowest end, felt and batten or whatever you are using, tiles etc.
Always start at the bottom and each phase covers the one below.
http://www.leadsheetassociation.org.uk/html/1392.html
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tommybanana
Junior Member
Joined: 02 Oct 2008
Posts: 2
Location: Scotland
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| Posted: Thu Oct 02, 2008 10:03 pm Post subject: |
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hi scruff
thanks for the reply
do you mean felt the valley and then put the boards on top?
I have looked at the Lsa site. It says that with a roof with counterbattens that the plywood boards under the lead should sit on top of the rafters and be the same height as the counter battens. it doesnt clarify if the felt is above or below the board.
can you clarify whether you mean putting boards over or under the felt?
cheers
tommy b
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Tryanything
Senior Member
Joined: 18 Aug 2007
Posts: 1370
Location: Essex
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thescruff
Senior Member
Joined: 10 Mar 2008
Posts: 4658
Location: Bath
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| Posted: Thu Oct 02, 2008 10:11 pm Post subject: |
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You build the roof first including the boards, tilting fillets etc, everything then is the coverings.
The builders felt is to protect the lead and avoid condensation, so that goes in the valley and up the boards, the lead is laid in the valley and up the boards starting at the lowest end, in maximum 8ft sheets, at the top you will have the saddle or cover flashing.
The roofing felt then cover the lead to the tile line.
Depending on the tiles and situation, the soaker are placed as you tile, and finally a step flashing covers the soakers.
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Scruff |
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thescruff
Senior Member
Joined: 10 Mar 2008
Posts: 4658
Location: Bath
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| Posted: Thu Oct 02, 2008 10:13 pm Post subject: |
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| Tryanything wrote: |
Hi tb Maybe this will help
Lay your roofing felt over rafters and valley board
Fix tile batten to rafters and edge of valley board
Fit the lead valley over the roofing felt
Cut valley tiles, remove nibs where they sit on the valley board
Put a sand and cement fillet to cut edge of valley tiles
You wont get any condensation between the felt and the valley board as
the valley board will act as a thermal break |
NO NO NO.
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Scruff |
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Tryanything
Senior Member
Joined: 18 Aug 2007
Posts: 1370
Location: Essex
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| Posted: Thu Oct 02, 2008 10:50 pm Post subject: |
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What exactly are you saying NO NO NO to Scruff
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thescruff
Senior Member
Joined: 10 Mar 2008
Posts: 4658
Location: Bath
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| Posted: Thu Oct 02, 2008 11:10 pm Post subject: |
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| tryanything wrote: |
Lay your roofing felt over rafters and valley board
Fix tile batten to rafters and edge of valley board
Fit the lead valley over the roofing felt |
That is what I'm saying N0 NO NO too.
The felt has to go into the lead, or any penetration of water will get under the lead and cause problems, especially when it freezes.
In addition the moving lead will erode the felt.
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