1-hour rating vs sprinklers in upstairs/downstairs duplex remodel

[Bootspurs]

Context is a remodel of an existing 2-story single family home into an upstairs/downstairs duplex, in California. Code calls for 1 hour separation between dwellings, plus a 1 hour rating on all exterior walls on the lower floor walls that support the one-hour floor ceiling assembly. CRC section R302.3 provides an exception to the 1-hour fire requirement as follows:

A fire-resistance rating of 1/2 hour shall be permitted in buildings equipped throughout with an automatic sprinkler system installed in accordance with NFPA 13.

I’m the owner and am getting pushback from city contracted plan reviewed that this exception applies only to NFPA 13 fire sprinkler systems (which are used in commercial buildings), and therefore this exception doesn’t apply to NFPA 13R systems, which are what are used in most residential fire sprinkler systems.

Is there any basis for this? Wouldn’t any sprinkler system (NFPA 13/13R/13D) meet this intent of putting out a fire before the 1 hour rating would have been needed? I’ve found some AHJs that reference any sprinkler system that meets P2904, but nothing in California.

Fire sprinklers weren’t even required but I’d rather install them then take off all the siding to make the exterior walls 1-hour rated; I thought that exception would make that approach possible.

I’ll also note that this needs STC rating > 45 as well, and it has existing 2x6 ceiling joists. It’s an old house and would be best if I didn’t have to tear open ceilings and upstairs floors to get the STC rating.

City says both fire and STC rating must be listed assemblies.

I’ve found at least one similar cases described on this forum, but it wasn’t quite conclusive on what was finally accepted. Thanks for any ideas on how to proceed.

 

[Bootspurs]

I was able to talk to the chief building official. He agreed that the exception was only for NFPA 13, not 13R or 13D, and that it wouldn't make sense to install an NFPA 13 system here. He said that the local fire dept could make a decision on whether to allow the 13R or 13D as this exception, but even then the exception only applies to floor/ceiling ratings, not the supporting assembly. The exterior wall assembly is actually harder, so this doesn't help as much as I had hoped.

He also said I could cite R104.11, Alternative Materials and Methods, then use 703.3, which allows for a calculation. Referencing tables in 722.6 Wood Assemblies, I can provide a calculation that shows that the wall system would be rated at 1-hour.

 

[Jay Smith]

It seems like this wall would be rated for exposure on the interior side only, unless your fire separation distance is less than 5 feet.

 

[Bootspurs]

The distance from any exterior wall to the property line is greater than 10'.
Can you explain how an interior-only rating would work?

 

[Jay Smith]

Here’s an example - UL Design No. U346. It’s a rating as an exterior bearing wall, but not as a fire barrier or fire partition.

 

[Joe.B]

I had the same initial reaction as Jay. I'll point out that NFPA 13R sprinkler systems are not designed to put fires out or save buildings. They are intended to provide occupants with time to escape. An NFPA 13 system is designed to protect property, so the lower rating exception makes sense. All three NFPA systems are different and should not be considered equivalent. Check out this link, they have a good explanation of the differences.

Summary of Differences in NFPA 13, 13R, & 13D

This week's resource was a fun one to put together. Frequent Questions about NFPA 13 vs. 13R I've been asked a handful of times in early project planning phases on whether NFPA 13R would be allowed...

www.meyerfire.com

 

[Bootspurs]

That's a great link, thank you.
I spoke to another builder this morning about how they have handled this; he said it's easier to leave internal lath/plaster finished walls and ceilings, and remove siding to build the fire rating from the exterior. Added benefit is that many old windows were planned to be replaced with inserts, but this will allow flanged windows and better flashing.
So if I ran with that, my overall plan would be: don't install sprinklers, leave existing wall finishes, remove exterior siding on supporting walls, add material to achieve 1-hour rating from exterior.

 

[Joe.B]

An NFPA 13 system is designed to protect property, so the lower rating exception makes sense.

I should have said the exception would make sense with the inclusion of a better sprinkler system. The NFPA 13 system provides more protection.

It's a tough call, and one you'll have to pencil out and weigh the benefits. One thing to consider when you're weighing the costs is your home insurance rates. I'd be willing to bet your policy would be cheaper with sprinklers and that may help you make your decision. Obviously cost is everything, and it's hard to place a value on something that may never happen, but sprinkler systems do save lives. Accounting for savings on insurance may make the cost a little easier to justify.

 

[Bootspurs]

I should have said the exception would make sense with the inclusion of a better sprinkler system. The NFPA 13 system provides more protection.

It's a tough call, and one you'll have to pencil out and weigh the benefits. One thing to consider when you're weighing the costs is your home insurance rates. I'd be willing to bet your policy would be cheaper with sprinklers and that may help you make your decision. Obviously cost is everything, and it's hard to place a value on something that may never happen, but sprinkler systems do save lives. Accounting for savings on insurance may make the cost a little easier to justify.

Yes agree, it’s all about cost-benefit trade offs. Maybe lower insurance costs will make a sprinkler system pay off over time, versus higher first costs for install.

Still unsure what to do, but leaving heavily towards working from outside in. Depends on if my lead carpenter agrees.

Appreciate the discussion

 

[Jay Smith]

I would think there’s no choice about which side of the wall to protect. If the floor-ceiling assembly is required to be 1-hour rated and the supporting construction is required to be 1-hour rated, that applies to every bearing wall in the 1st story, including the interior side of the exterior wall.

We haven’t discussed the floor-ceiling assembly, yet. The solution there might be adding sound isolation clips [GenieClip] and gypsum board. That would require some work in the ceiling, but not tearing it out.

 

[Joe.B]

I think the OP was weighing the option of either stripping the siding to provide the 1-hour rating of installing sprinklers to avoid having to do that.

 

[Joe.B]

I think the OP was weighing the option of either stripping the siding to provide the 1-hour rating or installing sprinklers to avoid having to do that.

edit

 

[Jay Smith]

I think the OP was weighing the option of either stripping the siding to provide the 1-hour rating or installing sprinklers to avoid having to do that.

Yes, but what I meant to say is adding material to the exterior does not provide 1-hour rated supporting construction for the floor above. The exterior face is the only one without fire exposure. All the load bearing framing inside the house would require protection. NFPA 13 sprinklers might be the only option.

 

[Joe.B]

Good points. The OP mentioned they are in communication with the AHJ so hopefully these points are being considered.

 

[ICE]

The benefit of a sprinkler system is a 1/2 rated separation as opposed to a 1hr separation. That's akin to swatting flies with a shotgun.

 

[Robert]

It seems like this wall would be rated for exposure on the interior side only, unless your fire separation distance is less than 5 feet.

Jay, I went down this path once where I needed 2 hr interior (for supporting construction) but only one hour exterior (distance to property line). Plan checker came back with "unequal fire resistance is not allowed in the CBC...must use strictest on both faces". This was a big deal because the existing siding would need to be replaced to accommodate the 2 hour! I saw the unequal wall assemblies in the UL manual also but apparently not allowed by my plan checker.

 

[Inspector Gadget]

The benefit of a sprinkler system is a 1/2 rated separation as opposed to a 1hr separation. That's akin to swatting flies with a shotgun.

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You've never seen Canadian mosquitoes .... sometimes a 10-guage is a reasonable solution.

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