Ridge ventilation

[PeteWilliams]

My shed roof is going to have a ridge beam and will be shingled and I'm wondering how to ventilate it at the ridge.

Looking online, the Americans seem to use either…

A plastic ridge vent:

Or a foam-like material one like this:

I can't find either of these available in the UK, or at least anywhere locally.
Is there another option that's more readily available on this side of the pond?

I've found 'universal dry ridge' kits, but I've only seen these used with with tiled roofs, are there any reasons I can't use one of these on a bitumen shingled roof?

The other thing I was wondering was whether soffit vents are necessary on a cladded shed where there's an air gap behind the cladding that would lead air up from the ground, behind the walls and across the roof's ventilation gap.

Thanks

Pete

 

[^woody^]

Why is ventilation required?

 

[PeteWilliams]

Why is ventilation required?

The shed will also be used as a gym and so will be insulated. I'm no expert but my understanding was that you need a 50mm air gap between the insulation and the roof deck, and would need to be to ventilated in order to circulate that air.

 

[^woody^]

Surely you have not got a roof void between the rafters and a separate ceiling in this shed? If so why can't you just ventilate the shed as normal?

 

[PeteWilliams]

Surely you have not got a roof void between the rafters and a separate ceiling in this shed? If so why can't you just ventilate the shed as normal?

Not exactly. The roof has high collar ties rather than ceiling joists, as you can see:

The plan is to put insulation between the joists (supported on battens) and then put a vapour barrier over it, so the main body of the shed would be sealed from the air space between the insulation and the roof deck.

When you say 'ventilate as normal', what exactly would that be? Just static wall/soffit vents into an unsealed space?

 

[^woody^]

Your plan would need an air gap between the deck and insulation, and vents along the eaves to vent through to the ridge.

You could fully fill between the rafters, and tight up to the deck, fit your vapour barrier or tape joints if using Celotex or similar, and that won't need ventilating.

 

[PeteWilliams]

You could fully fill between the rafters, and tight up to the deck, fit your vapour barrier or tape joints if using Celotex or similar, and that won't need ventilating.

That sound preferable, but my understanding was that I would need the air gap/ventilation because my roof material (bitumen shingles) isn't breathable.
Would I not suffer from moisture/condensationslowly rotting my roof OSB if there's no ventilation?

 

[Scarlet Pimpernel]

If your roof is waterproof, and you're going to fit a membrane to the underside, where will the damp come from to rot the OSB?

If you're really worried, allow the OSB, and other roof materials to dry out (from any latent moisture during construction) before applying the membrane.

If the roof shingles are applied directly to the OSB, where is the gap that needs ventilating?

 

[cdbe]

I think I can see your shed through the doorway of your (impressive looking) garden building.

 

[PeteWilliams]

If your roof is waterproof, and you're going to fit a membrane to the underside, where will the damp come from to rot the OSB?

My understanding was that when it's cold one side and warm the other, you get condensation forming from the moisture in the air. I may have misunderstood this though.

If the roof shingles are applied directly to the OSB, where is the gap that needs ventilating?

I was planning on creating a gap between the roof deck and the roof insulation.

 

[^woody^]

My understanding was that when it's cold one side and warm the other, you get condensation forming from the moisture in the air. I may have misunderstood this though.

I was planning on creating a gap between the roof deck and the roof insulation.

Basically, condensation and associated rot can only occur if moist air hits a cold surface. If you remove the possiblity of moist air doing that via a vapour barrier and insualtion, then the risk of rot is removed. The important thing for this type of construction is to ensure that there are no air gaps or voids within the roof.

 

[PeteWilliams]

Basically, condensation and associated rot can only occur if moist air hits a cold surface. If you remove the possiblity of moist air doing that via a vapour barrier and insualtion, then the risk of rot is removed. The important thing for this type of construction is to ensure that there are no air gaps or voids within the roof.

OK, thanks. I was planning on using cheap loft insulation (supported by battens) between the rafters. Given that plenty of air sits inside it and its form means it won't fill every last gap, would that be a problem in terms of introducing air gaps, or is it just big voids that would be an issue?