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Testing my understanding of AD B1 section 4 for my build


dnb

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I am working out what I need to do to comply to the letter with part B volume 1 Req. B4 for external walls on my build. The question is deliberately specific so that I can be informed when I approach the experts I may need to enlist.  (I would prefer not to discuss the reasons here and now.) Sorry - it's a very dry discussion involving much quoting of regs!

 

The introduction (para 10.2) reminds us that there are other requirements (i.e. not just B4) that impact external walls. In my case of a single house, the only appropriate one is section 11 - fire spreading from one building to another.

 

Para 10.3 indicates I either follow the provisions in 10.5 to 10.8 or have full scale test data from particular BS documents (this is unlikely, so let's stick with the regs) because my house is not a building from regulation 7(4) as I understand it.

 

Para 10.5 - comply with table 10.1. My house is "Any other building" in this context. It is less than 18m in height and more than 1000mm from any boundary. This means I DO NOT need to make provision against spread of fire as far as the letter of the regs is concerned.

 

I think this is a bit silly, so let's assume the house is really tall. I therefore need something of class C rating. But there is a subscript note - as long as I use timber that is more than 9mm thick I don't need a specific C rating for fire. So we are back to no particular provision. It's not until anything gets really near the boundary that I need make special provision, and this to all intents and purposes inhibits the use of timber for cladding.  But none of this applies to my situation.

 

Para 10.6 doesn't apply. My house is shorter than 18m height.

 

Para 10.7 doesn't apply. I am using green paint but I don't think that's what they are getting at.

 

Para 10.8 is more interesting. It tells me to put wooden battens (47mm in my case) around all the windows and doors, and something around the top of the cladding cavity. This will will probably need to be intumescent material since I need this to ventillate in normal situations. With a SIPS roof as well as SIPS walls, I am not sure if I can to do anything different because the fire can't easily get into the roof structure, but it can get into the void over the soffit - the overhang is quite large to act as a summer solar gain limiter for the bedrooms - and the void itself is compartmentalised by the SIPS splines. (My BCO didn't want to give an immediate answer on this either because I am doing one of the first fully SIPS builds in the local area so the experience base isn't established yet.) I also need to close the top of the cavity where the brick plinth at the base of the house stops and gives way to the cladding. All of this is described in section 5.

 

Section 11 is concerned with fire resistance and should be considered for situation.

Para 11.5 is concerned with distance from boundaries. My house is a long way from all my plot boundaries, but would be 3m or so from a proposed detached garage. So I should assume a "notinal boundary" for this wall as per 10.5.c.i.  Measurements in my case from the plot boundary are all over 14 metres, and the distance from house to garage will be 3 metres.

 

Para 11.6 sets the protection requirement, in my case 30 minutes.

 

Para 11.7 is a definition, but is important because we've established my cladding can be much worse than class B and will be more than 1mm thick. So all walls must be considered as having unprotected areas, but at half size.

 

Para 11.8 does not therefore apply. All distances are greater than 1000mm.

 

Para 11.9 indicates that the unprotected area should be less than a particular size, but importantly the protected area only needs to resist fire from the INSIDE of the building.

 

Para 11.16 covers calculation of acceptable amounts of unprotected area. I will use method 1 and the site boundary in the first instance.

Wall 1 - 15 metres from boundary, area 50m2, unprotected area 60%. Meets requirement of unlimited unprotected area.

Wall 2 - 20 metres from boundary, area 80m2, unprotected area 60%. Meets requirement of unlimited unprotected area.

Wall 3 - 100 metres from boundary, area 65m2, unprotected area 55%. Meets requirement of unlimited unprotected area.

Wall 4 - 13 metres from boundary, area 80m2, unprotected area 50%. Meets requirement of unlimited unprotected area.

 

Now, if I include the proposed detached garage, wall 4 now doesn't comply because the unprotected area should be no more than approx 20% so this wall would require different (i.e. class B or better) cladding if/when the garage is built.

 

Para 11.21 allows for some wiggle room with sprinkers, but this doesn't make enough difference to bring wall 4 into compliance with the garage in consideration.

 

So all in all, it seems I could still build a flamable house within the rules. Provided it's sufficiently far away from anything else.

 

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Without the context of what you're trying to argue/prove its hard to have any discussion on this; yes it's entirely possible to build a house which burns down and complies with domestic regs, the regs related to fire are there to allow safe escape for occupants, not to stop the thing burning down - and the spread of fire ones are there to stop the fire spreading  to other properties...

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1 hour ago, the_r_sole said:

the regs related to fire are there to allow safe escape for occupants, not to stop the thing burning down

I understand this. I am trying to find out what the minimum standard is for a given property "a long way" from any boundary. It seems due to the distances involved there aren't many rules in this context. Section 2 (means of escape for houses) doesn't look like it adds requirements to external cladding, and for the purposes of this discussion we can assume this is covered unless I have missed something that materially affects the exterior. I might be misunderstanding the document so want to be sure of my story before I do anything "official".

 

1 hour ago, the_r_sole said:

yes it's entirely possible to build a house which burns down and complies with domestic regs

Sounds like I might have interpreted it correctly then.

 

For this discussion we can assume all other relevant requirements to the house are met and the thing I am worried about is what the external cladding and to a lesser extent battening and cavity closers need to achieve. To provide a bit more context, the wall build up can be assumed to be (inside to outside):

 

skimmed plasterboard, to the requirements set out in B2 section 4

25mm timber battens

membrane

SIPS panel

membrane

47mm battens, cavity closers etc as appropriate

interlocking timber/other cladding as the requirements dictate. (Figuring out the requirement here is the key thing.)

 

19 minutes ago, Temp said:

I guess similar to many barn conversions?

Not looked at barn conversions, but I guess so, except the cladding here is horizontal and interlocking, not a rain screen type with gaps.

 

Is any further context necessary? Remember this is about the minimum required standards, not "can I get away with doing..."

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Summary here appears to agree with interpretations above..

https://www.woodcampus.co.uk/approved-doc-b-fire-safety-2019/

 

"For any building other than a recreation or assembly building of 18m in height or lower:

No provision

Unless any wall is less than 1000 mm from a boundary,  when the cladding must be FR treated as a minimum to Euroclass B-s3, d2"

 

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And

https://www.google.com/url?sa=t&source=web&rct=j&url=https://cdn.buildresources.co.uk/sites/967/docs/Specification/WPA_FR/WPA_FR5.pdf&ved=2ahUKEwiNubL0-s_sAhXMiFwKHQyNA4cQFjABegQIAhAB&usg=AOvVaw2vqil7kXtdyEaWCPy-E-Ko&cshid=1603636566093

 

"Below 18 metres 

For any building where the upper floor level is less than 18m above external ground level:• The faces less than 1000mm from boundary must meet class Euroclass B-s3, d2 or better.• Where faces are more than 1000mm from boundary there is no requirement for flame retardant treatment, except for assembly or recreation buildings of more than one storey...continues."

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3 hours ago, Temp said:

Summary here appears to agree with interpretations above

Excellent. Thank you!

Looks like my understanding is fine but this leaves me with fewer options to get where I would like to go.

 

 

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