More Design Criteria for a Slope Exceeding 4:12?

5/14/2025

I understand that sloped ceilings are those that exceed a 2:12 pitch.

NFPA 13, 2025 edition, Chapter 19.2.3.2.4 (3) & (4) mention ceiling slopes that do not exceed 4:12. This leads me to believe that there are additional criteria for ceilings exceeding 4:12 pitch.

Is anyone aware of additional criteria regarding design criteria where ceilings exceed 4:12?

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5 Comments

Pete H

5/14/2025 06:26:31 am

It's detailed in your reference. They updated what you can do within sloped ceilings in 2025 (or 2022... or sometime after 2016).

If it exceeds 2 in 12, but has a horizontal false ceiling capable of withstanding an uplift force of 3 pounds per square foot installed below the sloped ceiling and is supplemented with ceiling sprinklers (19.2.3.4.(1)), you no longer have to do a 30% design area increase.

Or you take on the requirement of (19.2.3.2.4(2)) and apply said 30% design area increase.

However if the slope does not exceed 4 in 12 and has purlins or beams supporting the roof deck run across the roof slope, purlins or beams do not exceed 18" deep, purlins or beams do not exceed 5'-0" center on center, Bays created by the solid structural members are not more than 40 feet center on center, and purlin or beam channels are provided with blocking above each structural member... then you don't have to apply (2) which is the 30% increase, as you applied (3).

Similarly, you can avoid the 30% increase if the slope does not exceed 4 in 12 if it is obstructed construction where sprinklers are installed in every channel. (4)

If it is obstructed construction that does not meet the requirements of (3) or (4), then you need to apply the 30% increase (5).

I don't know why they have (5) telling you you need to do the 30% increase in the same "one of the following requirements" lists with (2) when both have the requirement of: Apply the 30% design area increase (Okay, I do know why. (2) refers to unobstructed construction whereas (5) refers to obstructed construction).

But it's not additional criteria, it's the 30% design increase for sloped ceilings. It just has new ways to avoid the requirement in obstructed construction when ceilings don't exceed 4 in 12.

Brett

5/14/2025 06:49:51 am

That section was edited because of testing performed for a report titled "Protection of Storage Under Sloped Ceilings" by the Fire Protection Research Foundation. Report is here:

https://www.nfpa.org/education-and-research/research/fire-protection-research-foundation/projects-and-reports/protection-of-storage-under-sloped-ceilings

4 in 12 is the maximum slope that they tested with full scale burns at FM Global's testing facility.

Think of sections (3) and (4) as additional criteria that can be utilized when all the conditions listed are met. If you meet those conditions then you don't have to increase the design area by 30 percent. If slope is greater than 4 in 12 then you have to increase the design area by 30 percent. If you read the report you'll find that those conditions in sections (3) and (4) align with testing conditions.

Franck

5/14/2025 08:13:39 am

If it exceeds 4:12, the only option is to provide a flat false ceiling (but this only apply for storage, for occupancy you just need to increase the design area by 30% as soon as you exceed 2:12, but with no limit)

[WSF] Dan Wilder

5/14/2025 08:33:30 am

The design criteria for slopes greater than 4:12 hasn't changed.

This section creates different allowances not previously available to avoid increasing the design area 30% for things like unobstructed (adding a ceiling) or obstructed construction (specifies construction criteria or requires sprinklers within every channel).

Jose R Figueroa

5/14/2025 09:10:36 am

A false ceiling is required as described by Pete. Is this an industrial roof here in the US? I recall seeing roofs with a high pitch in Europe, where they typically install a flat false ceiling to accommodate the sprinkler system. I would recommend consulting with the sprinkler manufacturer for any special applications.

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More Design Criteria for a Slope Exceeding 4:12?

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