Fire proof ratings of materials in extension

[Dopeydaz]

good afternoon, we have the ground floor of our extension done now. But I was thinking. Building control want triple layer of plasterboard on the external wall to give one hour fire resistance. The outer leaf is rendered concrete blocks. Surely they give one hours fire resistance and I could use a single layer of plasterboard or doesn't it work like that?

TIA

Darren

 

[^woody^]

It depends what the wall is made out of.

 

[John D v2.0]

what are they trying to stop the fire getting to? if it's because you're less than 1m from the boundary, then the outside wall should be fine if less than 1m2 of glazing facing that way(ish - for the exact rules see the ADs)
If it's something to stop the fire running up the cavity to the loft extension, or another dwelling, or getting to structural steel, then the outer skin is not relevant. Although in the simple cases it would normally be half hour.

 

[Dopeydaz]

Thanks.

It is a timber frame kit with a 7n block outer leaf which will be roughcast.

The build is right on the boundary with our neighbours which is why it's to be 1 hour fire resistance.

 

[John D v2.0]

Well on that basis, actually now I read the AD B and it seems he's right that it needs to be from both sides specifically in the <1m case.

 

[^woody^]

Two layers of 12.5 plasterboard with bonded joints and then skim, should give you the required fire resistance as long as there is rockwool or equivalent in the frame.