Remote Area by Coverage Along Floor or Slope?

7/3/2020

I am working on a project with an attic. The pitch of the attic is 12:12 and the structure is combustible construction. The area is being used as a mechanical room so the fire sprinkler layout will be Ordinary Hazard Group 1 and a wet system will protect the space since the temperature will be maintained above 40ºF. Standard spray sprinklers are being utilized because Attic sprinklers are only listed for Light Hazard use.

Using a quick response area reduction, the remote area will be 1,500ft² (OH1) x 1.3 (30% pitch increase) x 0.6 (40% quick response decrease) = 1,170ft².

Since the sprinklers are spaced along the 12:12 pitch, the floor area in plan view is substantially less than the square footage being protected along this steep slope which picks up a ton of sprinklers when I draw my remote area based on the floor. I can't find any guidance in NFPA 13 about remote area on a slope besides the typical 1.2 x √RA split between sprinklers.

Should the 1,170ft² remote area be measured based on the coverage of the sprinkler along the slope (as the protection is spaced) or should the remote area be based on the floor even though it picks up way more sprinklers than 1,170ft² of actual coverage based on the slope?

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8 Comments

Jeremiah Holguin

7/3/2020 10:20:12 am

The 40% reduction seems a little high for a roof pitch of 12:12. Remember the height for design area reduction must be taken at the highest point. Also I assume this isnt in California. The way I approach this is to apply my remote area to the floor. Yes it will include a lot more sprinklers. Look at it from a fires perspective. The fire is growing in your remote area and doesnt grow based on the pitch of the roof. Our system responds based on the pitch of the roof. So apply remote area to the floor however for hydraulics I use the coverage area of the sprinkler based on the spacing along the slope.

M.New.

7/3/2020 11:24:16 am

NFPA tells you to space your sprinklers based on the slope it then tells you for discharge criteria to base it horizontal to the floor. So in the instance of the very strong pitch you are likely going to have to pick up more sprinklers in your remote area.

2019- 13 (9.5.3.1.2) - measure along the slope

I am working on finding the exact code that states the horizontal to floor for calculations excerpt.

Franck

7/3/2020 11:32:38 am

As indicated by Jeremiah, the area is to be calculated on the ground floor as the fire is located there and not at the roof.
I remember having some troubles with some contractors in the past trying to explain this...
But hydraulic calculations and space limitations are done at the ceiling level (actual distance between sprinklers...)
Regarding the 40% reduction, I normally don’t use it for new project to have more flexibility in the future (you never know what may happen). I only use it for existing situations to check the adequacy of systems that may have been poorly designed or to retrofit unacceptable situations by changing the type of heads to have an acceptable situation. But I have to admit that this is a very conservative standpoint (I work for an Insurance company, this may explain my point of view... 😃)

7/3/2020 12:21:04 pm

You cannot use the QResp 40% reduction in an attic- that is for areas with a ceiling. Not for attics or exposed areas. Also the SF area measures the floor space, not the slope. The required area to calculate will be 1500 +30% for roof slope and if this system is dry, add another 30%.

Dan Wilder

7/3/2020 06:00:02 pm

For the 2019 Ed. Definition of a "Ceiling Height" in 3.3.24:

The distance between the floor and the underside of the ceiling above (or roof deck) within the area.

Exposed areas are allowed al long as the highest point is used for the design area reduction.

7/7/2020 07:14:56 pm

Ty!

Gary

12/16/2021 03:56:04 pm

Not sure why you are saying you cant use reduction rule in a exposed situation. As long as it is a wet system / QR heads being used/ and it a light or ord hazard situation with a roof / ceiling no more than 20'-0" you can use reduction rule. Never been questioned on it anywhere by any engineer or AHJ. I actually had a engineer for the Army tell me about using this for exposed situations on a project.

Mike

7/3/2020 01:53:20 pm

Just curious what your state building code amendments are to NFPA 13.

I am in California and the state building code has adopted NFPA 13 but has amended the area reduction to exclude ordinary hazard.

Also, in my experience, the area reduction has never resulted in a significant reduction of pipe diameters except in a very few cases. It's usually irrelevant if it is a long line. I might save one piece one diameter.

Remote Area by Coverage Along Floor or Slope?

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