Fire socks

[Jammy5]

Hi there,

 

I've left it with the builder to fit fire socks at the top of the walls. Firstly it appears as though the socks aren't really a compression fit and more loose than anything. Will this be an issue in the future for damp and/or fire proofing. My understanding is they are intumescent and expand regardless so my concern is probably more about moisture ingress into the cavity.

 

Secondly, they were happy fitting the socks after the gable ladders had been fitted. What they appear to have done is cut them each side of the timber rather than fit them under the timber. Surely this isn't correct and leaves gaps between each that's filled with a flammable material?

 

Thoughts please all 

[DannyT]

We fit them under the ladder. As a full sock and cut at the top.

NHBC inspector states that socks can’t be bent around a corner. Should be butt jointed.

Sock should be a snug fit to either side of the cavity. If the cavity has spread a little over the height of the building we will either fit a wider sock or 2 thinner ones compressed together.

The thing is as a bricklayer, we are responsible for fitting them according to management. We are also told how vital they are since the disaster at Grenfell.

However we have never had any official training on fitting them nor do we get paid any extra for it.

 

[Jammy5]

Thanks for taking time to reply.

 

Does the NHBC inspector check for gaps on the socks or are they more concerned about the corner joints but butt jointed?

 

Are they not usually covered by the felt and battens at this point (my inspector wants to come once roof is about 80% complete with tiles).

 

Also can I buy 50mm socks to fill any gaps? Rather than use another 150mm wide sock? The only thing is the 50mm are blue - current ones are red, so will be noticeable. But both are 1 HR rated?

 

This still wouldn't resolve where they have been cut either side of the ladder rungs and my issue is the builders have already used 25 socks, probably not fitted to expectations in places, and likely now difficult to fit because of how high they have brought the PIR insulation in relation to the rungs on the ladders. 

 

 

[ADLIan]

Standard cavity stop socks (mineral wool encapsulated in polythene sleeve, perhaps colour coded) do not incorporate an intumescent strip. They must be sized correctly, installed in continuous runs and be a tight friction fit in the cavity in order to function correctly - check MIs carefully. Post Grenfell make sure install is correct. 

[Jammy5]

Are they purely for fire, or do they offer any other benefit by closing off the cavity?

 

 

[DannyT]

14 hours ago, Jammy5 said:

Thanks for taking time to reply.

 

Does the NHBC inspector check for gaps on the socks or are they more concerned about the corner joints but butt jointed?

 

 

 

NHBC will do random wall plate checks, say every 10th house. Looking for gaps in socks, etc. checking everything structurally before roof gets felted in also.

More about just closing off the cavity on a detached house and getting a box ticked.

The real fussiness with fire socks/barriers comes when you are on semi detached or apartments and fire needs to be contained to a single unit.

Edited December 31, 2025 by DannyT