Dormer Flat Cold Roof Ventilation

[shbrooks]

I'm currently renovating my property and have two flat cold roof dormers which have been mostly constructed and whilst the cheeks and roofs have been battened out to form a ventilation cavity, there is no actual ventilator to the roof – the contractor is relying on the cheek ventilators which they say will allow air to go up one cheek, across the roof and down the other cheek.  Is this commonly accepted or should the roof also be ventilated independently of the cheeks?

 

The cheeks have a 50mm cavity ventilation zone and the entirety of the dormers will be clad in zinc (or aluminium).

[Redbeard]

It could work, but I have to say I have never heard of cheek ventilators. Might you perhaps be asking a lot of the air to follow a tortuous route? (I don't know the answer!). I do like the idea of a builder even thinking of venting the cheeks, though. I am not sure I have ever come across it before.

[Gus Potter]

2 hours ago, shbrooks said:

I'm currently renovating my property and have two flat cold roof dormers which have been mostly constructed and whilst the cheeks and roofs have been battened out to form a ventilation cavity, there is no actual ventilator to the roof – the contractor is relying on the cheek ventilators which they say will allow air to go up one cheek, across the roof and down the other cheek.  Is this commonly accepted or should the roof also be ventilated independently of the cheeks?

 

The cheeks have a 50mm cavity ventilation zone and the entirety of the dormers will be clad in zinc (or aluminium).

Good post and a good question.

 

To clarify. I think your contractor is talking about a ventilator at the soffit, (the bit under the gutter) the dormer cheeks (the sides) will also have a vent at the bottom to ventilate the vertical cladding of the dormer. 

 

Designs like this are generally taken with a big pinch of salt these days. The main reason is that folk started to do big flat roofs using the same principle and these lead to big rot and condensation problems. Basically folk started taking the pish. 

 

But on a small dormer the old rules are still applicable. I've counted your tiles and at worst you have 9 x 330mm = ~3m roof that needs ventilated. The dormer "sticks"out the roof so it is going to "catch the wind" and that will promote cross ventilation provided you have good soffit vents. So your builder is not talking out their hat, but check that BC will run with this.

 

Now the above will fly provided you are not installing an ensuite shower room in the dormer. 

 

 

[Mr Punter]

Not an answer but you could clad the dormer in lead which could be cheaper as more people can do it.

[shbrooks]

Thank you for your comments.  Building Control wanted ventilation across the roof so we've raised the roof and installed some eaves ventilators.  Better safe than sorry!

[ab12]

On 22/10/2025 at 00:20, Gus Potter said:

 

 

Now the above will fly provided you are not installing an ensuite shower room in the dormer. 

 

 

I would be grateful if you could explain the reasoning behind this please. Im about to do my roof and have a dormer like this but it finishes flush with the external gable wall rather than on the pitched rood part.

 

Now I would like to have toilet, shower and sink in the new room to be formed by a partition next to the dormer. Of course will have a good extractor fan in there. 

 

Is there a risk I will be introducing problems with moisture or condensation by having a shower room there?

 

How can I mitigate any risk?

 

Thanks

[Gus Potter]

The practical reasoning behinds this is that shower area in dormers are a big source of water gas.

 

You have to execute the work "perfectly" and it takes a lot of time to design and detail stuff like this. Few are willing to pay a designer to get it right.  BC and I know that few builders do this properly, hence my caution.

 

But if you are doing it yourself, are confident, dilligent in your workmanship and understand the theory, marry that up with your particular house (it needs to be detailed specifically, not generically)  then it's doable. 

[Nickfromwales]

On 16/05/2026 at 16:17, ab12 said:

Of course will have a good extractor fan in there. 

 

Is there a risk I will be introducing problems with moisture or condensation by having a shower room there?

 

How can I mitigate any risk?

Make sure the door to the bathroom has either a transfer grille, or an 8-10mm undercut on the bottom of the door, to let air in at the same volume that the extractor wants to remove it.

 

Overrun timer is a must, my bathroom fan runs for 30-40 mins after each activation, to completely remove any moisture; it was shite at first, then I realised I'd no air coming in to the room, so chopped 10mm off the bottom of the door and then the magic happened. I'm always rushed doing any work in my own house so often don't see the woods for the trees.