Roof ventilation/ breathable membrane

[Judith Birkett]

Hello, my architect has designed the following extension for me. They have specified a cold pitched roof (with insulation between and below rafters). They have also specified a breathable sarking felt (my architect has said Nilvent is good to use). The architect has also specified that sufficient ventilation should be provided at the ridge. (monopitch roof flashing vent or high level vent tile) and sufficient ventilation at the eaves (25mm continuous ventilation over at eaves) .

My builder is saying that because I am using a breathable membrane, which lets air in anyway, there is no need for ventilation gaps at the eaves or ridge. My architect disagrees with him. (one thing my architect said is that you need both, but the ventilation gap over insulation does not have to be as big with a breathable membrane. Also said that in cold, water in breathable membrane could freeze, so no longer breathable at times, hence need for ventilation.


who is correct?

 

[Notch7]

I cant see the drawings on my ipad, but my best source for construction details is tyvek. Its possible to do both warm and cold pitched tiled roofs, with supported and unsupported options.

For cold and warm pitched roofs, tyvek have a very good download that shows contstruction details
The download is 'view supported and unsupported applications'

http://www.dupont.co.uk/products-an...lications/warm-pitched-roof-construction.html

 

[freddiemercurystwin]

You only need 100mm kingspan between, the rafters are overspecified, you don't need additional ventilation if a breathable membrane is used though it won't do any harm.

 

[^woody^]

If you read the membrane manufacturer's blurb, or if a BBA Certificate for the membrane is available, this will state whether ventilation is required or not.

 

[freddiemercurystwin]

It is not.

 

[Judith Birkett]

oh, just another quick question as you guys have been so helpful. As per drawings (see above), my builder said that I need a new cavity tray in the main wall of the house (the wall being opened up). It needs to be above where the roof of the extension attached to the wall of the main house. To channel water out so it does not just run down and accumulate and drip through ceiling.

Is that correct, and is there a certain height above the roof of the extension it needs to be?

 

[freddiemercurystwin]

That old chestnut! Some Building Control inspectors will insist some won't care, which may make it a moot point, it's a lot of work to do but even more if it's not not done now and you do get damp ingress problems in the future as there will be internal repair work to be done too. You need to consider the condition of the existing wall and the exposure. When the new beams are inserted you can have a look up the cavity and see if there's any evidence of the outer skin having let water through in the past. It would go in at the same level as the roof lead flashing.

 

[Notch7]

oh, just another quick question as you guys have been so helpful. As per drawings (see above), my builder said that I need a new cavity tray in the main wall of the house (the wall being opened up). It needs to be above where the roof of the extension attached to the wall of the main house. To channel water out so it does not just run down and accumulate and drip through ceiling.

Is that correct, and is there a certain height above the roof of the extension it needs to be?

Yes. Although as above, the risk depends on the situation.

It needs to be set so the weep holes from the trays exit at the same mortar line as the lead flashing.

 

[John D v2.0]

If your existing wall is fully rendered then there no need to install a cavity tray as the render shouldi protect it. If not then the whole point of a cavity wall is to drain water to outside, so it would be only ok in sheltered situations.
Also there's no point in ventilating the warm side of a breathable membrane. But you do need to make sure water can drain freely down the outside so you would need a small drape between rafters or counter battens.
Your pdf is too slow to open on my phone so i can't check the details, but it looks like the guys who do this day in day out have helped you above

 

[^woody^]

Any cavity wall requires a cavity tray above an opening, unless it's immediately below the eaves.

It's always a good sign when a builder mentions it, and a bad sign if they don't. And a worse sign if the plan drawer misses it completely.

 

[Judith Birkett]

Thanks, you guys a bril!