Synthetic Slate Lean To

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Hi all,

I'm installing a lean to roof in my back garden...

https://preview.ibb.co/dun9Xq/20181027-165756.jpg

https://preview.ibb.co/gqBxJV/IMG-20181029-WA0075.jpg

I've installed the ring beam and rafters and overboarded with 18mm WBP ply and stapled some breathable membrane down.

I've got a couple of questions though if someone could help that would be great -

The membrane is water tight (ish!) It's bone dry during heavy downpours and high winds but in the mornings the underside of the plywood is damp. This soon dries out so I'm assuming this is condensation?

I've got some Tapco Slates to install, is it OK that the clout nails / screws will be penetating the membrane? According to the instructions you need to use 30mm fixings so I'm going to be left with 400ish holes in the membrane and through the plywood. I've spoken to the Tapco rep who said this is fine, but just wanted to check as it seems crazy to have so many holes in a membrane. The rep said the membrane should be "self sealing" meaning the penetrations will seal around the head of the clout nail...?

I'm hoping once the slates are on, the plywood will stop getting damp in the mornings or it will rot out in no time...

Finally ventilation - I'm going to leave an airgap between the first layer of insulation and the plywood and I'll also cut holes in the facia to allow air to get underneath the ply, but should there be ventilation at the top of the roof before it reaches the lead flashing? Seems like there needs to be a flow of air for any ventilation to work correctly.

Thanks for any advice, sorry if any of the questions sound daft!

Alex.
 
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Your membrane will not self seal around the nails.

Ideally you should fix counter batten on the ply then membrane and slating batten.
The facia should have a vent on the top and you should fit an abutment vent at the top of the slope with a lead flashing over it.
 
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Thanks palaceray - turns out I need a vapour barrier ontop of the ply but I've decided I'm going to just remove the plywood entirely then felt and baton it so that the clouts going through the "slates" don't penetrate the membrane
 
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Cheers guys, I'm conscious (and confused) by the manufacturer's instructions regarding the slate's "air permeability". Apparently because they are synthetic, they are classed as "insufficiently air-open" compared to traditional slate.

"TapcoSlate Classic and Aledora roofing slates are classed as insufficiently air-open products and therefore, following the guidance within BS5250, a vapour impermeable (HR) roof membrane/underlay should be used. Tapco Roofing Products recommends Protect Wunderlay for TapcoSlate Classic and TapcoSlate Aledora and offers the following guidance"

The fitting guide suggests a Vapour Permeable Membrane should be fitted first, then counter batons, then slating batons -

https://imgur.com/a/MAKjYbj

I've just lifted up the felt on the ply and it's wet with mould growing on it and it's only been on for 3 weeks. The existing felt is a Vapour Permeable Membrane (breather?).

On page 10 of the fitting guide (https://www.tapcoroofingproducts.com/downloads/slate_fixing_guide.pdf), it suggests you can go straight onto the ply / OSB if you use a Vapour Impermeable Membrane which seems like the easiest route for me given the roof is already plyd out -

https://imgur.com/a/9uHEXOn

But then I'm back to the conundrum of having all the clout nails going through the membrane.

So confused!
 
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Thanks Palaceray - does it matter that the manufacturer says it should be membrane, then counter baton then slating baton? Would you use a vapour impermeable or permeable membrane? Can you explain why your method is better?
 
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The advantage of doing the counter battens first is that you can ensure that they are directly above and fixed into the rafters.

It also gives better detailing at the eave, if the membrane is direct on the board it will be 65mm below the facia height and would cause damming behind the facia.

We would normally use a breather membrane with vent top and bottom.
 
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That's great thank you for explaining. What purpose does the plywood serve in this example then could we just take it off? Just thinking of minimising trapping moisture and allowing the membrane to breath
 
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The ply is not needed.
You can fix the membrane directly on top of the rafters and then your 25x50 slating battens.
Fix a facia vent and underlay support trays, with an abutment vent at the top.
 
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