How to Select the Coverage Area for a Sprinkler?

1/28/2026

We have an apartment, protected by a birdcage residential sprinkler system per NFPA 13. We need to calculate the most demanding 4 sprinklers, which end up being 3 in the room and another on the back side of one of the 3 (this apartment has the longest throw).

I need to calculate the most demanding of the two (1) flow rates per the approval chart in the sprinkler's listing, or (2) calculate based on 0.10 gpm/sqft over design area according to 8.5.2.1 or 8.6.2.1.2.

Can I choose 8.5.2.1 or 8.6.2.1.2? Or is it the more demanding of 8.5.2.1 or 8.6.2.1.2, and then compare that to the flow rates per the approval chart in the sprinkler's listing?

Thank you in advance.

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10 Comments

Pete H

1/28/2026 09:40:35 am

You're doing a residential 4 head calc and it's an NFPA 13 building not a 13R?

Brett

1/28/2026 10:45:38 am

In NFPA 13 it's still a four head calc but it's the four most demanding adjacent sprinklers regardless of the locations of walls. 13R tells you to break up the calcs by compartment.

Kevin Wilson

1/28/2026 01:54:13 pm

Yes. Using residential in the individual apts so calculating 4 most demanding

Shane

1/28/2026 10:13:59 am

I am not familiar with "birdcage" residential sprinkler system. Can you explain?

Moderator

1/28/2026 10:39:05 am

Great question. "Birdcage" is a zoning setup where the top-most floor of a building is served with sidewall sprinklers, each from the level below.

So say this is a 4-story building, the topmost 4th floor level are served from the floor below.

The runs of vertical pipe between the 3rd and 4th floor would give a "birdcage" look of vertical pipe runs if they were viewed as a whole system.

Technically, birdcage 'could' mean that 'all' floors are served in this manner (one zone, all sidewalls from vertical pipe runs), but we've generally taken it as meaning only the topmost floor is served in this way. That'd be 'whole-building birdcaged' versus 'top floor is birdcaged.'

Brett

1/28/2026 12:15:29 pm

We use the term for 13R buildings that are supplied by one system with multiple vertical risers tied into each floor's horizontal cross main. It resembles a bird cage if you sketch it out.

danefre

1/28/2026 10:48:49 am

See NFPA 13 (2013) Section 11.3.1.3 and then look at the clarification/addition on the NFPA 13 (2019) section 19.4.1.3. Even in 2013 you are permitted to do area of compartment divided by number of sprinklers for residential sprinklers. It was not very clear but always there.

Short answer, compartment over 800sqft (not meeting small room rule definition) you use the regular S x L per 8.5.2.1. That's a bit rare to have apartments with compartments over 800sqft in NFPA 13. If it's a corridor we usually do QR or Special application corridor sprinklers. If you do have this condition, this will probably the most demanding.

Compartment under 800sqft (meeting small room rule definition) you use the area of the compartment divided by the number of sprinklers for your demand, as per 8.6.2.1.2. This is the most common scenario.

You don't freely choose between 8.5.2.1 and 8.6.2.1.2, you pick whichever applies based on the compartment. Again the 2019 edition clarifies that better.

In practice, the manufacturer's data sheet (approval listing) is most demanding in most cases in NFPA 13 but you still have to prove it.

Kevin Wilson

1/28/2026 01:59:51 pm

Thank you for your reply!

James Art

1/28/2026 03:52:30 pm

Q. In the current 2025 editions, are FLOOR CONTROL valves required?
Perhaps this is a IFC requirement?

Brett

1/29/2026 06:38:11 am

They are required by NFPA 13 if the building is more than two stories although the top floor can be supplied by the piping on the floor below.

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How to Select the Coverage Area for a Sprinkler?

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