Should Design Area Go Beyond 2-hr Fire Barriers?

4/4/2025

I'm designing a system that has multiple, narrow tenant spaces divided by 2-hour rated fire walls.

When using density/area method, the square root × 1.2 design area doesn't include enough space to satisfy the 1500 square foot requirement.

In this case, would I extend the area further parallel along the branch lines, extend it along the main into the adjacent space, or call it good where it's at?

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11 Comments

Glenn Berger

4/4/2025 08:18:13 am

With the use of fire walls, there is no requirement to extend the design area further.

If you have fire barriers, you might have to extend the design area. It would depends on the current and adjacent occupancies and also what the authorities decide.

And if you have fire partitions, then yes you must extend the design area.

Chris

4/4/2025 08:29:38 am

Glenn,

Similar to the question above and your reply, I have a project with a long vestibule corridor with 2x rows of sprinklers, and has 2x huge firewalls on either side. Following your response, I would stay within the corridor with the design area, and not go into the adjacent rooms (gyms).

In this scenario, I am required to have a 1,500sqft area. If I did the required minimum length, I would end up with 1,221sqft without going into adjacent areas. If I extend it to go to 1,500sqft, then the design area will end up being 77' long. This seems unrealistic, but I see no other way to do it within code.

I have ran into this issue many times, mostly with dry systems. I would love to hear your interpretation and learn what others are doing.

Paul Pinigis link

4/4/2025 08:44:53 am

I am a bit confused by your description of the building. Firewalls are not just fire-rated walls, they separate buildings from one another and they must remain standing should the building on either side collapse. There is no requirement for sprinklers to be designed for the neighboring building, so you would have a long narrow design area for your long narrow building.

Jesse

4/4/2025 08:31:57 am

If these are indeed fire walls, you could call the area a compartment and use the room design method.

Mike Bergen

4/9/2025 02:57:41 pm

There seems to be some confusion
The original post said Fire BARRIER, Not Fire WALL
You are correct, Fire Walls define Buildings, hence independent structure Fire Barriers define SPACES / Areas Not Buildings

Does thaat help

Confucius say, first step in Wisdom is call things by same name

Brian Gerdwagen FPE

4/4/2025 08:32:50 am

If the wall rating is equal to or more than the design duration, stop your remote area at the wall.

Light Hazard density is 30 minutes, if the wall is one-hour, stop there. If the density requires two hours, with the same wall, pretend the wall is not there.

Matt

4/7/2025 11:43:03 am

Brian, this makes a lot of sense to me, but I'm not sure I've ever heard it before. Is this a rule of thumb or written in the standard somewhere?

Eric R

4/4/2025 08:37:51 am

I think there is quite a bit of "depends" here.

You are certainly allowed to extend past the wall to grab more heads in the adjacent tenant as nothing in NFPA 13 disallows that. This would be the most hydraulically advantageous approach.

Extending the design area length to include more heads is probably the more correct approach though, and I would point out that the relevant code section states "at least 1.2", so it isn't really giving you any leeway to just artificially stop there if you haven't met your minimum coverage area yet.

I think the correct next step is to check on what impact extending the heads along the branch-line has on your calculations. Does it increase the branch-line size substantially? I feel that designs sometimes get stuck in the "technically correct" mindset, when a much better frame is how much additional protection or safety factor can you provide without significant financial cost.

If the only different here is some branch-lines changing from 1.25" to 1.5" then it would feel wrong not to upsize them to insure you aren't cutting things close for basically no savings.

Jack G

4/4/2025 09:12:56 am

Fire walls. In order for the areas with firewalls to be considered separate buildings is for the walls to create parapits thru the roof like you see where owners limit areas between the parapits to less than 12000 sq.ft in lieu of protecting the B occupancy with fire sprinklers .
Generally i extend the MRA along the branch line, within the 2 hour walls. Get a beefier line which you need because of the firewalls. ( at least 1.2)
I then would calculate a " check area " in a not MRA
where the design area is normal --follows nfpa 13
( not interrupted by firewalls).
In this manner i have a beefer size line ( in the mra within the fire walls,) and a normal sized branch line for the rest of the system.
In " long " systems if it will save branch line sizes ( near beginnings of system ) i use the larger main sizes to reduce line sizing again and calculate additional check areas. ( or building broken up).
( calc "hydraulic pipe schedules ).

I

Dan Wilder

4/4/2025 10:12:36 am

This will be good because I have been apart of some very "passionate" discussions with other designers and AHJ's about this...

If you are using Density/Area and the design criteria is the same, walls are ignored, even rated ones (per NFPA as there is no specific allowance for or against). Walls play into if there is an extension of one design hazard or method needs to extend into the adjoining space or not and if "Phantom Flow" needs to be considered.

Walls, and sometimes the adjoining spaces, dictate hydraulic areas when used in Room Design which would seem to be a much more applicable design method in this instance.

As Glen noted, the use of "Fire Walls" (basically each side of the wall is treated as its own building) would negate the need to extend the remote area however that is a very costly construction method in most cases. This also may trigger additional requirements from the AHJ as typically they also want to have independent utility services for each area (some exceptions are out there).

Casey Milhorn

4/4/2025 03:52:08 pm

No need, use the room design method, as others have stated. The 1.2 goes out the window. This sounds like a prime example of why the room design method was created. HOWEVER, if the owner, EOR, or architect feel like this area is likely to be modified/expanded in the future, be careful as the owner may end up with an inadequate system. I wouldn't skimp on the underground and riser, just in case modifications happen later. Mains and branch lines can be modified/upsized a lot cheaper than taps, underground mains, and risers....

Should Design Area Go Beyond 2-hr Fire Barriers?

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