SIPS Part B Fire

[SGVB]

Calling on any builders, self-builders, architects, building reg gurus!

This is my scenario.
I have a SIPS building which is over 1m from any boundary.
The outside of the building is clad in a combination of corten steel and timber.
Building Inspector is happy it meets the surface spread of fire under the regs.
However!
The question is now raised about the resistance to fire of the structure itself.
Internally, it is simple (pink board etc) and the inspector is happy, but it is the outside which is raising the question.
Every SIPS supplier/designer I have spoken to tells me that if you are more than 1m from the boundary, the external face of the SIPS does not need any additional treatment and you just have to be concerned that the cladding meets surface spread regs.
The structural timber association also states that beyond a meter, only the inside needs to meet the relevant minutes for fire.

In part B there is a section which lists out the different parts of a building (roof, load-bearing walls, internal walls etc etc). Under "exterior walls" and in my situation it does state the protection is only required from the inside but if you consider the SIPS panels under the "Structural" section it states protection is required on exposed faces (which is weird terminology compared to other parts of this secton and I think because it is more to do with steel columns and beams).

Anyone had experience with this? Hoping to find someone that has SIPS and clad it in timber and has first hand knowledge.

Cheers in advance.

 

[Deluks]

The question is now raised about the resistance to fire of the structure itself.

Who raised the question? If the building inspector is happy...

 

[SGVB]

Hi Deluks, the building inspector raised it and is happy with the external spread of fire and the internal protection but not the external protection of fire for the structure (the SIPS).

 

[Deluks]

Hi Deluks, the building inspector raised it and is happy with the external spread of fire and the internal protection but not the external protection of fire for the structure (the SIPS).

That's hard to make sense of, are any part of the sips panels exposed? Inside or out?

 

[freddiemercurystwin]

Have you asked your BCO to state which section of Part B he is implying is relevant?

 

[SGVB]

That's hard to make sense of, are any part of the sips panels exposed? Inside or out?

No, external is Corten steel and timber cladding all mounted on timber battens. So this is the rain screen. This meets the regs for B4: External Spread of fire. I. The building is under 11m and 1.3m from the boundary at the closest point. The issue is focused around B19 Fire Resitance and the interpretation of SIPS with regard to tables B3 and then B4.

 

[SGVB]

Have you asked your BCO to state which section of Part B he is implying is relevant?

The issue is focused around B19 Fire Resitance and the interpretation of SIPS with regard to tables B3 and then B4.

 

[Deluks]

The issue is focused around B19 Fire Resitance and the interpretation of SIPS with regard to tables B3 and then B4.

Tell him you're having trouble interpreting and requite clarity.

 

[^woody^]

Get the details from the SIP manufacturer/supplier

 

[JuddlyFire]

Your BCO is right to question it, but your suppliers are also correct in certain circumstances. Here's the hard facts from the regs:

• Loadbearing external walls (which SIPS panels absolutely are) must have minimum fire resistance as specified in Table B3 ↗ (Section 5, paragraph 5.1)

• The key is understanding what "fire resistance" means here - it's about structural integrity during fire, not just surface spread

• For dwellings over 1m from boundary, Table B3 requires 30 minutes fire resistance ↗ (Section 6, paragraph 6.1)

• Crucially, Approved Document B doesn't specify protection must be on BOTH sides - it's about the assembly achieving the rating

• At your distance of 1.3m from boundary, Diagram 11.7 allows for some unprotected areas ↗ (Section 11, paragraph 11.18)

Here's what matters: Your SIPS manufacturer should provide fire test evidence showing their panel assembly (with your specific cladding system) achieves 30 minutes fire resistance from the inside only when >1m from boundary. This is standard practice.

The inspector needs to see:

1. The fire test certificate for your exact SIPS panel configuration
2. How it complies with Table B3 requirements
3. Confirmation it meets the boundary distance provisions in Section 11

Don't let them pull the "structural elements need protection on all exposed faces" line without showing you exactly where in the regs it says that for your specific boundary distance scenario. The "exposed faces" language in some sections refers primarily to steel elements, not timber frame/SIPS construction.

Get your SIPS supplier to provide written confirmation with reference to Part B Section 5 and Table B3 specifically addressing boundary distances over 1m. Should sort it.

Seen this trip people up before - BCOs sometimes apply commercial building standards to dwellings by mistake. Your situation is definitely covered under the dwellinghouse provisions, not the more stringent flats requirements.

 

[martygturner]

This is what god invented plasterboard for... to provide 30min fire protection when fixed using mechanical fixings ie screws.

 

[SGVB]

Your BCO is right to question it, but your suppliers are also correct in certain circumstances. Here's the hard facts from the regs:

• Loadbearing external walls (which SIPS panels absolutely are) must have minimum fire resistance as specified in Table B3 ↗ (Section 5, paragraph 5.1)

• The key is understanding what "fire resistance" means here - it's about structural integrity during fire, not just surface spread

• For dwellings over 1m from boundary, Table B3 requires 30 minutes fire resistance ↗ (Section 6, paragraph 6.1)

• Crucially, Approved Document B doesn't specify protection must be on BOTH sides - it's about the assembly achieving the rating

• At your distance of 1.3m from boundary, Diagram 11.7 allows for some unprotected areas ↗ (Section 11, paragraph 11.18)

Here's what matters: Your SIPS manufacturer should provide fire test evidence showing their panel assembly (with your specific cladding system) achieves 30 minutes fire resistance from the inside only when >1m from boundary. This is standard practice.

The inspector needs to see:

1. The fire test certificate for your exact SIPS panel configuration
2. How it complies with Table B3 requirements
3. Confirmation it meets the boundary distance provisions in Section 11

Don't let them pull the "structural elements need protection on all exposed faces" line without showing you exactly where in the regs it says that for your specific boundary distance scenario. The "exposed faces" language in some sections refers primarily to steel elements, not timber frame/SIPS construction.

Get your SIPS supplier to provide written confirmation with reference to Part B Section 5 and Table B3 specifically addressing boundary distances over 1m. Should sort it.

Seen this trip people up before - BCOs sometimes apply commercial building standards to dwellings by mistake. Your situation is definitely covered under the dwellinghouse provisions, not the more stringent flats requirements.

Hi @JuddlyFire , that is all very useful and I appreciate the detail you have gone to here. A couple of questions if I may as I am in the presence of an expert!!!

In table B3, Loadbearring Wall (which we are now clear SIPS are) the "type of exposure states "each side separately" so I assume from what you are saying is because B3 section 5 is about internal spread of fire the reference to each side separately is when you might have a load bearing wall inside the building with both sides accessible (like a dividing wall for rooms that is also load bearing) and not the side of a load bearing wall that is also external?

In section 11, 11.18, as you say, it advises on unprotected areas. When you click on the link in the document for "Unprotected Areas", it states:
Any part of the external wall constructed of material more than 1mm thick, if that material does not have a class B-s3, d2 rating or better, which is attached or applied, whether for cladding or any other purpose.
So this suggests to me that this now brings the cladding back into scrutiny unless of course this is referring to cladding on the inside of the building?? Maybe, but a strange way to phrase it.

I appreciate your comments about asking the SIPS manufacturer to provide test evidence of their panel with our exact cladding system but that would just never happen. Every SIPS supplier I have spoken to refers back to the 1m boundary and building height guidelines. And in reality, no SIPS supplier is going to test every single different combination of material that could be used inside and out each time a customer asked them.
I have all the test certificates for internal fire protection based on 30min and 60min combinations of plasterboard, this is what all SIPS suppliers have.

I am sure a lot of this is about interpretation and the use of certain language which creates some confusion within the regs.
Meeting with the BCO again this week to go through this and I am sure your comments will come in very useful.

 

[wessex101]

Not related to SIPs but similar timber framed walling I just have plasterboard or cement board internally and cement board externally fixed to the studs behind the cladding. Building Control have always accepted that.

 

[JuddlyFire]

@SGVB, I'll try to clarify some points.

Regarding your first question about Table B3, Loadbearing Wall, and the "type of exposure" stating "each side separately": This could be interpreted in different ways. However, looking at the context, Section 5 of Approved Document B deals with internal fire spread (structure). In this context, "each side separately" could imply the evaluation of fire resistance for internal walls with both sides accessible (such as a dividing wall between rooms that also serves as a load-bearing wall) [B1, Section 10]. It doesn't necessarily relate directly to the external side of a load-bearing wall that's also external.

Concerning your second question regarding unprotected areas in Section 11, paragraph 11.18, and the statement about "Any part of the external wall constructed of material more than 1mm thick, if that material does not have a class B-s3, d2 rating or better," you're correct that it brings the cladding into scrutiny. This part of the document defines what constitutes an "unprotected area" for the purposes of calculating the amount of unprotected area allowed within certain distances of boundaries [B1, Paragraph 11.12].

The document also highlights the need to consider cladding in the equation, specifically stating that cladding material not meeting the B-s3, d2 rating or better would be considered part of the "unprotected area." This definition applies to external walls and seems to relate directly to how external cladding affects the calculation of unprotected areas, especially in relation to boundary distances.

As for requesting test evidence from the SIPS manufacturer, it's understandable that suppliers refer back to the 1m boundary and building height guidelines. However, the specific scenario and the combination of materials (SIPS with your corten steel and timber cladding) do present a unique case that ideally should be backed by relevant fire test evidence, especially considering the lack of explicit guidance for every possible material combination in the Approved Documents.

It's indeed about interpretation and the specific language used in the regulations that can lead to confusion. Given the unique aspects of your project, further discussion with the Building Control Officer (BCO) and perhaps seeking specific advice from a more senior fire safety expert who can interpret the regulations more confidently with your particular details in mind would be advisable.

In conclusion, given the specifics of your situation and the details of the Approved Documents, the discussion seems to boil down to the interpretation of these documents in the context of your specific project details. A more detailed assessment by a senior fire safety expert who can weigh in on the project specifics might provide the clarity needed.

 

[SGVB]

The document also highlights the need to consider cladding in the equation, specifically stating that cladding material not meeting the B-s3, d2 rating or better would be considered part of the "unprotected area." This definition applies to external walls and seems to relate directly to how external cladding affects the calculation of unprotected areas, especially in relation to boundary distances.

The part I find confusing again is section 10, 10.5, table 10.1 provides the performance requirements relevant to the boundary, building height etc. In my case it is “no provisions” and in the notes, point 2 and 3 it’s specifically states timber cladding over 9mm and flat sheet steel over 0.5mm are also “acceptable”, it does not state those materials have to meet a certain test requirement.
The entire unprotected area aspect in relation to cladding does not make a lot of sense to me as the classic unprotected areas of a building (windows and doors) are surely primarily a risk to another building (on the other side of the boundary) if fire started INSIDE (I am sure most fires start inside a building) ? And we know the internal walls meet the requirement.
If a fire starts on the outside I appreciate the spread across the face of the building is the concern to other buildings but table 10.1 suggest timber cladding and sheet steel is ok. It is not a concern for the structure as we have already covered the fact the requirement for the structure is based on the inside. So why scrutinise the cladding on the outside of the building in Section 5 that is all about internal fire.

I know my cladding arrangement sounds unique but it is really not, I have found a lot of SIPS projects where entire buildings or sides of buildings are clad in timber with no additional fire treatment and seem to have been passed ok even when close to a border. Kingspans own tech documents even show a typical build up of their panel with timber cladding.