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How to damp proof and ventilate flat cold roof?

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How to damp proof and ventilate flat cold roof?

Postby macleod92 » 14 Mar 2019, 13:37

Hi,

Sorry if this is the wrong place to be posting questions. I was hoping someone could help me with my workshop build plans.

My plan is to make a flat roof shed which will be insulated throughout with some form of mineral wool/ Rockwool. The reason for this is that sound is a big concern where I'm based.
I'm then going to have a vapour barrier on the inside of the studs followed by either plywood or OSB. The outside is going to go studs, OSB, breather membrane, 25mm batten and finally feather edge cladding.

As I'm going to be using mineral wool insulation I've opted to go for a cold roof as opposed to a warm one. From what I could tell a warm roof will only work with rigid insulation.

My question is about the best way to ventilate and damp proof the roof? I've done a bunch of research and understand that I need to leave at least a 50mm gap above the insulation to allow for ventilation. But where should I put my breathable membrane?
From what I can tell if I follow the examples (https://www.rubber4roofs.co.uk/epdm-col ... oof-design) I would have no protection from moisture if something is able to get through my roof (OSB and felt)?

I've read through MikeG's guides and know what I'd do if I had an apex roof, but the fact that I'm going with a flat roof design is confusing things.

I hope that makes sense, thanks for the help!
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Re: How to damp proof and ventilate flat cold roof?

Postby Mike G » 14 Mar 2019, 16:20

Welcome to the forum. Can I suggest you read the two sticky threads in this section under my name, discussing the principles of shed/ workshop building.

Let's go through the points you raise in order:

-there is no need for a vapour barrier on the inside of a wall or roof construction if there is also an OSB lining (or indeed, to a lesser extent, ply). OSB has so much glue in its construction that it is virtually impermeable to the passage of vapour, and thus its acts as a vapour barrier and obviates the necessity for plastic or foil.

-don't put OSB on the outside of the studs. That is the wrong side. Given what I just said about OSB, it should not be on the outside of insulation, but on the inside.

-on the outside of your studwork you should put a breather membrane, and then batten out to create a ventilated void between your framing and your cladding. No OSB or ply.

-the flat roof thing is a red herring. There is no difference between a flat roof and a pitched roof in terms of ventilation requirements. If you have an impermeable outer layer, such as OSB & EPDM, then you need to provide ventilation under it. This has to be done with vents at at the ends of the zones created by the joists or any counterbattens or purlins, such that air can flow freely from one side of the roof to the other. There is absolutely no need for a breather membrane.

-you may have read through my "how to build a shed" posts, but you've missed the basic principle of the walls!! Go back and have another look. :)
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Re: How to damp proof and ventilate flat cold roof?

Postby macleod92 » 14 Mar 2019, 19:34

Hi Mike,

Thanks for the quick and in depth response. Really appreciate your advice.

I must admit that while I read your posts, I saw what many others had done with their builds (using OSB on the outside) and assumed that I was going for a ‘belt and braces’ solution. Thanks for pointing out my mistake, I’ll definitely go and have another read through everything.

Thanks again!
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Re: How to damp proof and ventilate flat cold roof?

Postby Mike G » 15 Mar 2019, 08:43

It's really important to understand the difference between a workshop and a store shed, in as much as it affects the walls. In a workshop, which will be heated in the winter, there will be a moisture source (the occupant), an increased ability for the air to hold moisture (because it is heated) and a vapour differential (because outside is drier than inside). This leads to moisture levels trying to equalise (that's just physics), and that means potential moisture movement through the walls from inside to outside....leading to possible condensation within the wall. The way to combat this is to prevent moisture getting into the wall, and allowing any that gets in to vent to the outside.

With a store shed, there is no heating, unlikely to be any insulation, and there is no water source internally. Therefore there is no difference between the levels of water vapour in the air inside and outside the building........so you can put any sheathing (OSB, ply etc) anywhere you like in the wall structure.

So, when you look at other people's shed builds, take notice of what the use of the building is. But also understand that most people are actually getting it wrong. I even saw some clown with a large Youtube following putting a plastic membrane over the outside of his new workshop the other day, and then fixing his boards directly to that.
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Re: How to damp proof and ventilate flat cold roof?

Postby bluebirdnick » 15 Mar 2019, 13:53

Hi Mike

This is a bit of a tangential point, but is there any reason for favouring cold roofs over warm roofs, other than the depth?
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Re: How to damp proof and ventilate flat cold roof?

Postby Mike G » 15 Mar 2019, 14:17

If you can't get ventilation across the whole of the underside and you have a vapour- impermeable roofing material, then you are pretty much forced to have a warm roof.
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