Fire Safety Regs - Timber Cladding

[john0wingnut]

Afternoon all

 

I have just had an inspection visit for my timber frame rear extension. It is a single storey 6x2 timber frame, 400 centres, 11mm OSB on the outside, breather membrane and is to be finished with vertical thermo pine, on batten / counter batten.

 

The BCO has advised I need to put intumescent cavity closers in at the top / bottom and around the windows and doors, which is not something I thought was required for ventilated cladding; I've just got mesh top and bottom. Windows / doors  have a cladding cheek to finish, so are closed already.

 

Can anyone on here enlighten me?

 

John

[Alan Ambrose]

The intumescent strips seem to be accepted practice. Alternatives are 0.5mm steel / wood of a given thickness (I forget how much). There’s a fair amount of discussion on this on ‘t ‘ub.

 

My BC seemed to be more open-minded - I forget his exact words. We used the steel strip as it was cheaper, but it did take a while to put up.

[G and J]

OK, I can’t help feeling I should know this, but how does a steel strip stop the ventilation behind the cladding act like a chimney during a fire whilst still allowing the chimney effect to dry the cladding under normal conditions?

[john0wingnut]

Hi all

 

So having done some reading, document B refers to a cavity as a sealed entity, vs what I would call a ventilated void behind my cladding as it isn’t sealed.

 

I understand strips would be useful preventing fire spreading from first to second storey for example, but, if my cladding or the timber frame catches fire, no intumescent strip sealing the ventilated area is going to stop it; and given the soffit is 10mm above where the cladding finishes, it won’t stop that catching fire either.

 

My front extension is TF with brick veneer, and drawings for that clearly advise fire proof closures at the top of the cavity and around the windows / doors which makes sense.

 

 

 

 

 

[john0wingnut]

11 hours ago, john0wingnut said:

Hi all

 

So having done some reading, document B refers to a cavity as a sealed entity, vs what I would call a ventilated void behind my cladding as it isn’t sealed.

 

I understand strips would be useful preventing fire spreading from first to second storey for example, but, if my cladding or the timber frame catches fire, no intumescent strip sealing the ventilated area is going to stop it; and given the soffit is 10mm above where the cladding finishes, it won’t stop that catching fire either.

 

My front extension is TF with brick veneer, and drawings for that clearly advise fire proof closures at the top of the cavity and around the windows / doors which makes sense.

 

 

 

 

 

Cant seem to edit the above. I misread the regs. Cavity is a considered a concealed space. Not sealed!! So that statement above is incorrect.

[Alan Ambrose]

Following @G and J comments, I've been researching this again. It's not completely obvious - NHBC doesn't go into a lot of detail re timber cladding & cavities, nor the building regs. They say a bit but a picture or two would have been nice.

 

My current thinking is that 0.5mm steel is fine around windows & doors but intumescent might be required at the top of the cavity. I've not figured out the bottom yet.

[G and J]

I can imagine steel cavity closures round penetrations.  
 

I think the key thing is to close off the chimney (behind the cladding) when there is a fire.  

[Owain1602]

The regs say 0.8mm steel (or thicker) and 38mm wide timber (or thicker).

 

I agree, this can be quite a confusing topic. Whatever you do, don’t try and read the STA documents seeking clarity….it will confuse you even more!

 

The cavity barrier element of the regs doesn’t have anything to do with the fire rating of your cladding, that’s a separate topic. They want to see how you will stop a fire from using the external cavity behind the cladding as a means of bypassing the internal fire resisting elements. 
There are numerous arguments and nuances that can be put forward such that the guidance as read do not apply in certain sections. 
 

Obviously, a lot of this came out after Grenfell. My understanding is that the fire service couldn’t understand at the time how fire was spreading throughout the building, especially from floor to floor so quickly. They couldn’t see the fact that fire was spreading inside the cavity behind the cladding.

 

We had two different external claddings, one for the ground and another for the first floor, both had ventilated cavities. I specified a steel drip flashing between the two (which ended up 3mm thick). The ground floor cavity ventilates at the underside of the drip, and the start of the first floor cavity is above the drip. In the event of a fire, we wouldn’t fully close off either cavity, just that fire in the ground floor cavity would come up against a 3mm steel plate. It would have to come out of the ground floor cavity at the drip, come out 100mm to get around the drip then turn back on itself to get back to the first floor cavity. 
 

That is the detail around a lot of the house. 
at the heads of windows we have intumescent strips, side and bottom of windows is fully closed with 38mm or bigger timber. 
 

I had agreement from the BCO that the detail and argument I put forward for the eaves to roof was okay. But as I was doing the cladding, I had to change something to better attach the gutter. Rather than go through the whole approval loop again, I just ran intumescent strips around the whole of the eaves (not at the gables). Value my time and sanity more than about £500’s worth of strips.

 

If you can post a bit more detail, I’m sure that we can come up with more specific suggestions.

Edited 1 hour ago by Owain1602