Fire rated wall vs non-combustible covering

[flightcrazed]

Hi all, hoping you can help me.  I'm having a dispute with a contractor on whether a 1hr fire rated wall assembly or just a non-combustible covering is needed.  The project is type VB, R3 use in a WUI area and not located within 125' of another structure or property line. I am interpreting 2022 CRC R337.7.3 to mean that only the non-combustible covering is needed (metal in this case).  Am I missing anything?  

 

[richoffan]

Seriously? Argue about fire resistance assembly in a woo zee? In California? Now? I got two building inspector certifications in 1994, just to learn to read the code better. The #1 thing stressed in both courses was know how to read the code.in its entirety  Always check a definition as used in the code, especially if it is the determinate word. Better yet ask the AHJ

[flightcrazed]

3 minutes ago, richoffan said:

Seriously? Argue about fire resistance assembly in a woo zee? In California? Now? I got two building inspector certifications in 1994, just to learn to read the code better. The #1 thing stressed in both courses was know how to read the code.in its entirety  Always check a definition as used in the code, especially if it is the determinate word. Better yet ask the AHJ

I mean, if we want to get technical...exterior wall covering includes Siding. 

 

And your highlight refers to "finish" materials defined as:

 

So, there's that....

[Alaskan_Son]

6 minutes ago, richoffan said:

The #1 thing stressed in both courses was know how to read the code.in its entirety  Always check a definition as used in the code, especially if it is the determinate word. Better yet ask the AHJ

 

Pretty sure the highlighted section is very specifically referring to the finish coating or outer layer of a single composite material as opposed to the referenced "structural base" of that same material.  I'm thinking of the paint layer on metal siding, the foil face on rigid foam insulation, or the paper face on some fiberglass batts. 

[Gawdzira]

In WUI areas I have not been asked for a one hour wall assembly unless the fire code demanded it for buildings inside of 10' separation. It seems like the exterior siding material having a WUI approval should be sufficient. I am very quick to shoot these questions to the AHJ. 

 

3 things happen when I ask a question of the AHJ (building official)

1. I get the answer that covers my butt (and it almost always the correct answer)

2. I endear myself to the building official through being respectful of their authority and knowledge

3. I generally learn the thing and maybe some more via the answer

[Alaskan_Son]

1 hour ago, Gawdzira said:

3 things happen when I ask a question of the AHJ (building official)

1. I get the answer that covers my butt (and it almost always the correct answer)

2. I endear myself to the building official through being respectful of their authority and knowledge

3. I generally learn the thing and maybe some more via the answer

 

Well said good, sir, well said.

This advice is priceless and I could not agree more.  

[JKEdmo]

Very interesting post and it made me look at the code section again.  It's complicated and I'd be interested if others agree with my take.  I try to understand the purpose/spirit of the code instead of just the letter.

 

To me, the primary purpose of the WUI requirement is about preventing / reducing the ignition of exterior walls during wildfires.  That's a bit different from the intent of fire rated wall assemblies which is to delay the passage of fire so as to allow for evacuation and first responder response:

 

 

So, to prevent wall ignition, the focus in on the wall covering, which needs to be non-combustible, "ignition-resistant" or fire-retardant treated wood.  So, as long as the exterior wall covering complies, your non-rated wall complies.

 

But, say you decide to have a non-compliant (combustible) exterior wall covering.  In that case, you have additional options, including but not limited to 1-hour rated wall assemblies.  (This may be where the confusion comes from).  Note these options are only triggered if the exterior wall covering is combustible.

 

 

Options include:

• You can also stick 5/8" Type X exterior-rated drywall behind the non-compliant combustible siding (common approach but not exactly a tested 1-hour assembly).
• You can build a (solid wood) log cabin or cross-laminated timber building.
• You could also have a tested 1-hour rated wall assembly with any kind of combustible siding (e.g. wood shingles).  This is one option to comply with the WUI code but only needed in the case the exterior wall covering is non-compliant.  In a sense this is overkill for WUI.  But, your wall may already need to be 1-hour rated for other reasons such as proximity to property line.  So you could just use this to satisfy the WUI requirement.

 

Jim

 

(Excerpts from 2022 California Residential Code)

 

 

 

 

 

 

 

 

 

 

[flightcrazed]

On 3/15/2025 at 8:39 AM, Gawdzira said:

In WUI areas I have not been asked for a one hour wall assembly unless the fire code demanded it for buildings inside of 10' separation. It seems like the exterior siding material having a WUI approval should be sufficient. I am very quick to shoot these questions to the AHJ. 

 

3 things happen when I ask a question of the AHJ (building official)

1. I get the answer that covers my butt (and it almost always the correct answer)

2. I endear myself to the building official through being respectful of their authority and knowledge

3. I generally learn the thing and maybe some more via the answer

Thanks Alan.  I have asked the County (Plan Checker) to answer the question but he is either on vacation or worried about responding :).  I realize my mileage may vary depending on the inspector but feel like if my case is strong enough (plan check approved, code referenced, etc) then I should be good without having to tear down siding and adjust after the fact. 

 

Appreciate your inputs as always.

[flightcrazed]

On 3/15/2025 at 1:15 PM, JKEdmo said:

Very interesting post and it made me look at the code section again.  It's complicated and I'd be interested if others agree with my take.  I try to understand the purpose/spirit of the code instead of just the letter.

 

To me, the primary purpose of the WUI requirement is about preventing / reducing the ignition of exterior walls during wildfires.  That's a bit different from the intent of fire rated wall assemblies which is to delay the passage of fire so as to allow for evacuation and first responder response:

 

 

So, to prevent wall ignition, the focus in on the wall covering, which needs to be non-combustible, "ignition-resistant" or fire-retardant treated wood.  So, as long as the exterior wall covering complies, your non-rated wall complies.

 

But, say you decide to have a non-compliant (combustible) exterior wall covering.  In that case, you have additional options, including but not limited to 1-hour rated wall assemblies.  (This may be where the confusion comes from).  Note these options are only triggered if the exterior wall covering is combustible.

 

 

Options include:

• You can also stick 5/8" Type X exterior-rated drywall behind the non-compliant combustible siding (common approach but not exactly a tested 1-hour assembly).
• You can build a (solid wood) log cabin or cross-laminated timber building.
• You could also have a tested 1-hour rated wall assembly with any kind of combustible siding (e.g. wood shingles).  This is one option to comply with the WUI code but only needed in the case the exterior wall covering is non-compliant.  In a sense this is overkill for WUI.  But, your wall may already need to be 1-hour rated for other reasons such as proximity to property line.  So you could just use this to satisfy the WUI requirement.

 

Jim

 

(Excerpts from 2022 California Residential Code)

 

 

 

 

 

 

 

 

 

 

Thanks for the reply Jim.  This is my interpretation as well.  On this project specifically, County Fire has requirements for maintaining 100' defensible space, the first 5' away from home of which are "most important".  To me, that, combined with non-combustible/ignition resistant covering, should be sufficient for reducing the ignition likelihood of the wall during a wildfire (as you say, the intent) and meet the code requirements.  

[JKEdmo]

39 minutes ago, flightcrazed said:

This is my interpretation as well.

 

Be sure to give us an update on how this gets resolved.  It's an interesting circumstance.

 

Jim

 

[flightcrazed]

6 hours ago, JKEdmo said:

 

Be sure to give us an update on how this gets resolved.  It's an interesting circumstance.

 

Jim

 

You got it!