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.