Dormer cheek fire resistance.

[tony1851]

Hi everyone;

Conventional dormer on 1930s semi; have been advised by Building Control that the dormer cheek adjacent to the party boundary on the roof must have 60 mins fire resistance from both sides.

1. Anyone know if this is this correct?
2. If it is, anyone know how to achieve this? - it will be clad externally by plain tiles/battens/
breather paper/counter battens/OSB/studs.

Thanks.

 

[wessex101]

Surely it only needs 60 minutes if it is a separating wall, ie. there would have to be another dormer joined to it.

If it is just close to party boundary then it is a surface spread of flame issue and needs to be within unprotected area limits or non-combustible. Down here if we have that situation we have to line the inside of the cheek with a layer of mineral fibre board plus an extra outer layer on the outside behind the cladding.

Edit: I might have got myself a bit confused there I was thinking of beach huts, oh yes one of my many talents is a beach hut designer. With a dormer cheek plasterboard internally would be OK with the mineral fibre board on the outside under the cladding.

 

[^woody^]

It's 30 minutes within 1 m of the boundary. That's in Part B and all the various council's BC guides. The concrete tiles satisfy this externally, and then 12.5mm plasterboard with all joints covered with a batten or double boards internally.

Otherwise for 60 minutes its a 12mm cement board on both sides and then the other stuff as you planned.

If the BCO is being pedantic about a 60 minute frame, then he should be equally stringent on the roof, close window frame and glazing, and the soffit/facia detailing.

 

[tony1851]

Thanks both; I thought it was 30 minutes, so will have a tactful word with the inspector.

 

[John D v2.0]

I think it depends on the angle between the surface and the boundary in terms of radiant heat angle of incidence. I am not really an expert but i reckon after Grenfell they are possibly more worried about that. We got away with a lot more at the side because our side is just over a metre from the actual boundary.