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B4 - Fire Protection beneath timber cladding


Visti

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Our Building Controller is insisting upon a fire resistant sheathing panel on the underside of our cladding and is citing B4 30 mins fire rating, detailed in green below.

 

Thing is, we can't for the scoobies find any requirement like it in the building regs. Anyone wiser, or faced a similar requirement?

 

It isn't related to cavity barriers (B3 6.5) as that is already addressed, nor is it B4 9.7 for unprotected areas as the cladding treatment below deals with that. 

 

Our cladding is Larch mounted vertically on cross battens and will be fireproofed with HR PROF, and the wall the standard MBC passive timbre frame. I believe the external panel of the MBC frame is Medite Vent that has a fire class of D-s2, d0, but nothing stating it meets the 30 mins rating.

 

Of everything it's our biggest concern that the controller has raised. No one seems to have thought of this over the past year (architect, QS, MBC) and the fix (Promat) looks to be £45/m2 adding a whooping +£12k to our budget.

 

Brexit already blew the contingency twice since we started, and this is going to eat half of the third contingency before we finish the foundation... we'd appreciate any input :)

 

image.png.284f938ac2e5c5d9460648c363bee090.png

 

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We have, just not getting a clear answer. We've asked for the exact paragraph this time around.

 

Mainly hoping to see others wall details with similar setups to see if this is overkill!

 

We're 4 and 5 meters from our neighbors on either side, so not that close.

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I think your architect may need to get involved.  Probably BC are agitated as you have a strongly ventilated cavity.  One simple idea may be to apply a fire resistant breather membrane in place of the standard one.  MBC could do this in the factory and I doubt it would be too expensive.  I would say no to Promat etc in your situation.  I have done several TF projects with cladding - timber, slate, render on carrier board, cement siding etc - and have never been asked to apply fire resistant sheathing.

 

In the event that you did need a FR sheathing, there are some fairly cheap MgO boards around.  The only time I have had it specified was for multi storey adjacent to other buildings and it was a during construction precaution (in case of arson etc) applied to the end walls and some of the party walls near the end of the block.  Not a building regs issue but a "best practice" H & S.

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Replying quickly so not sure if i'm off the mark but this is word for word what i put to my Building inspector and he approved it;

Hopefully a quick question, i’ve been phoning but never seem to catch you right.
Most northerly wall is the garage and the top sits roughly 1 to 2 feet below a 6ft fence on boundary with next doors garden (no nearby structure)….and less than a metre away from it. The fascia is planned to be cedar untreated. Likewise the garage east and west walls would have cedar fascia which would at their most northerly point get within a metre of that boundary fence.
My understanding is that the cedar, battening etc should be fire treated.
Is this stuff sufficient to meet the requirement?;
This stuff means I can fascia the rest of the house and not have it look different to the bits in this fire sensitive area as this spray doesn’t alter the visual appearance of the timber.
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