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.