BS 476 Pt.6; Fire propagation test.

[JimK]

Does anyone out there know the fire propagation rating of European oak, or similar hardwood? I have trawled through several documents and test results but haven't found the answer.
The reason for the question? Can you have oak cladding on a large timber framed garage next to the boundary? The fire resistance must be provided when tested from inside and outside. Unprotected area will also be excessive.

[Fishy]

See Table A8 of the Approved Document B.  Timber is Class 3 spread of flame.  You also mention fire resistance - this will depend on the way that the walls are constructed.

[kurnal]

Theres plenty of info on the char rate of timber and hardwoods as we know are better than softwoods in this respect. It all depends on what evidence of performance you will need to provide to the Building Authority.
The chances are that there will be no test data for the construction you have in mind so before going any further I would ask about the standard of evidence. I guess you wont find any evidence of testing your assembly to BS476 so will the inspector accept a structured argument based on known char rates, structural strength, thickness of timbers used and the performance of any enhancements that you offer such as intumescent varnishes?
As it is not an element of structure (assuming single storey) you could use an internal lining or sandwich construction so long as you could show that the lining would stand up on its own  even if the timber wall burned away, theres plenty of plasterboard or supalux type solutions.

[saddlers]

Does anyone out there know the fire propagation rating of European oak, or similar hardwood? I have trawled through several documents and test results but haven't found the answer.
The reason for the question? Can you have oak cladding on a large timber framed garage next to the boundary? The fire resistance must be provided when tested from inside and outside. Unprotected area will also be excessive.

How large is large, and are we talking about a garage in association with a dwelling? Strictly speaking if it is detached and less than 30m² in floor area, it is exempt from Building Regulations and therefore there is no control on unprotected areas etc. However if it is less than 1m from the boundary it should be "constructed substantially of non-combustible material" (I just love that phrase!!!)

[JimK]

Thanks for previous replies; the garage is over 30 m2 and therefore needs Building Regs permission.
I know softwood has quite a high fire propagation rating but I have assumed that hardwood (oak) would have a much lower rating and I am trying to justify having excessive timber boarding close top the boundary. On the other side of the boundary is the next door neighbour's shed!

[kurnal]

It can be surface treated or pressure treated to class O and depending on thickness and slow char rate there may be sufficient strength left after 1 hours burning to maintain integrity (sacrificial timber concept). Or protect it with intumescent materials. You only have to cater for a single fire -one side exposed at any time

See the article in this months FEJ about the pagoda in Peasholm Park Scarboro. Makes an interesting read.

[saddlers]

Thanks for previous replies; the garage is over 30 m2 and therefore needs Building Regs permission.
I know softwood has quite a high fire propagation rating but I have assumed that hardwood (oak) would have a much lower rating and I am trying to justify having excessive timber boarding close top the boundary. On the other side of the boundary is the next door neighbour's shed!

The Building Regulations do not consider what is the other side of the boundary, due to the fact that it can constantly change. There are however other methods of calculating unprotected area, which can be used.

Firstly, if the hardwood is a cladding over a fire resistant wall beneath then only 50% of this area is considered as unprotected for the boundary calculations. Therefore by making the underlying wall FR, you can double the amount of unprotected area you are allowed.

Secondly the enclosing rectangles method of calculating fire spread issues tends to be less onerous in BR187.

Otherwise as Kurnal suggests investigate treatment methods.