breathable roof membrane and valley gutters - what to do?

[tommybanana]

hi advice appreciated on this.

I am putting up a cold non vented roof in scotland (because I have an english architect!!) and 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 plywod 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 makimg the roof leak 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

 

[noseall]

lead takes the place of tiles, whereby tiles are not suitable of flexible enough to deal with shaping or 'bending' the tiles.

lead can be shaped whereas tiles can not.

with this in mind, and knowing that underslaters felt/sarking felt/tilers membrane, sits below the tiles, then i guess that all lead should have a membrane below.

so, it is sarking boards - counterbattens (if necessary) - membrane - battens- lead or tiles.

 

[^woody^]

I can't see why the membrane would be fitted any differently than felt would be - ie a piece up the whole valley, then the lead, then the sarking felt lapped over the edges of the lead?

You mention (c) fitting the membrane underneath the ply boards, which implies that you are fitting the ply on top of the rafters? The valley boards should be flush with the rafters so (c) would not be an option

 

[tommybanana]

thansk fotr the replies.

woody

according to the lead shet association rolled lead manual cos the roof has counter battens then the plywood should be the same thickness as the counter battens and sit on top of the rafters.

cheers

tommy b

 

[^woody^]

If this is a cold roof, then why is it counter-battened? You say you are not using sarking boards like the rest of the natives!

 

[Leadman]

The only thing under lead that should not be used is the old F1 bitumin felt which the lead would stick to and stop it expanding and contracting.
i see no reason why you cant use beather felt but would recomend using under lead fleece which allows the lead to expand and contract and cut the breath felt in to the valley.
Hope this helps
Dave

 

[tommybanana]

hi woody

the counter battens are there to provide a ventilation space above the membrane and below the slate ( so I'm told), and to stop any moisture running down the felt pooling against the battens and rotting them.

cheers

Tommy B.

 

[^woody^]

Its going a bit OT, but I cant see the need for counter-battens

The membrane would just be a straight replacement for felt. It will sag between rafters to allow any moisture to run down the slope, and there will be natural ventilation between the laths.

Counter-battens would only be used if there was a solid board and the laths needed to be lifted up off the board to allow the drainage you mention.

The counter battens are not doing anything, as you may as well have just used deeper laths

And this has caused the confusion on the valley board and membrane fitting, and the odd detail of the valley/membrane junction.

Really, your architect should have detailed this.

What you will have to do now is membrane the whole rafters, fit the counter-battens, fit the valleyboard flush with the counter-battens, lay some membrane on the boards to form a slip-plane (or fleece), then lay the lead, then lath and tile.

Its going to give an odd junction at the fascia, as you will need to allow for any water to get out from the membrane laid on the rafters beneath the vallyboards

The alternative is to mess about lapping the membrane up and over the lead, but this I feel, may create a potential for the membrane to tear where it comes up from the rafter level to the level of the lead

 

[tommybanana]

woody i hear what you are saying -

What I have done is the spec set down by the architect and fits with the picture of a cold unvented roof here

http://www.permavent.co.uk/pdf/brochure_web-11668.pdf

however he didnt specify what happens tothe felt at the valley gutter...

best

Tommy B.

 

[^woody^]

I had a flyer from Tyvek a while ago which demonstrated/stated that the practice of counter-battening for breather membranes was unnecessary as the natural air above the membrane/below the tiles is enough to evaporate any condensation moisture.

All my local councils are happy for counter-battens to be omitted on this type of cold roof

TBH, I would get the Architect to tell you exactly how his design should work and be built, and then do what he says and let him take the responsibility for it.

 

[tommybanana]

hi woody

I am sure you are right about this. there seems to be have been a ot of changes recently in how cold non vented roofs are rated and the attitude to the requirements for ventilation under the tile/slates..
oh well at least my roof will be very well ventilated!!

cheers

tommy b

 

[ali68]

Just out of interest really, seeing your are in Scotland are building control happy with the omission of sarking.

 

[tommybanana]

yes they have give a warrant to the plans. mind you many people just assume there are sarking boards on it - ity is only when you look closely that you realise there aren't.

I don't see why scottish building control would insist in it?

tommy b.

 

[BethesdaSlate]

Any felt MUST lap over the lead at the valley. Do not allow breather felt to sit under the lead.
This will allow any condensation coming up through the felt to become trapped under the lead and cause it to corrode prematurely.

Run a batten up the side of the valley(parallel with the centre of the valley) which will assist for fixing of slates/tiles close to the valley and also prevent the felt from being torn by the ends of battens.

Only use purpose made underlay or building paper beneath the lead.

Its easy enough to keep the valley waterproof by laying the felt across the valley until you come to put the lead in, then just cut it back allowing it to lap back over the lead.

Regards
Jon

 

[datarebal]

Any felt MUST lap over the lead at the valley. Do not allow breather felt to sit under the lead.
This will allow any condensation coming up through the felt to become trapped under the lead and cause it to corrode prematurely.

Jon

So what about condensation "coming up "and forming on the underside of the lead anyway?

Whats wrong with chalk slurry?