How to Layout Sprinklers Beneath Steep Slope?

11/3/2020

What design options do I have with a fire sprinkler design to address protection underneath a 45-degree pitched roof?

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8 Comments

Franck

11/3/2020 10:14:13 am

We may need more information on what design parameters you want some guidance. There are so many.

Here are just a few.

Normally, according to NFPA, you need to increase the area of application by 30%.
Regarding the sprinkler deflectors, there are 2 schools :
- NFPA recommend the deflector parallel to the roof
- FM Global recommend deflector parallel to the floor

If you have storage underneath, note that this is out of scope of NFPA 13 and you should normally provide a false ceiling to limit the slope.
BUT, if you have a sawtooth roof (high slope, but on a limited length), you may have storage underneath, you just need to adapt your remote area to the roof configuration, so that you will activate all sprinklers along one sloped area(or 2 adjacent, depending on your building configuration).

When you determine the area of operation (remote area), you have to take into consideration the area projected on the floor (2500 sq ft on the floor will be much more sq ft of sprinklers in operation at the roof ceiling). But the min./max. distances to consider are along the roof (on the branch lines or between adjacent branch lines).

Jonathan Sullivan

11/3/2020 10:46:27 am

Not doing a lot of storage design, can you explain the logic of a false ceiling? Is it a question of activation time?

Franck

11/3/2020 11:12:36 am

If you have a large peak roof with a slope >16.7%, you cannot protect it with NFPA (out of the scope of NFPA 13).
The 30% increase with sloped ceiling is only for occupancy hazards (LH, OH, EH), but not applicable to storage occupancies.
But if you have anyway storage in such a building, the only solution (when feasible, but most of the time not cost effective) is to provide a false ceiling to eliminate the peak roof and install sprinkler underneath.

This is also done with ESFR protection when you want to eliminate possible ceiling obstruction from construction or possible roof slope exceeding the ESFR requirements.

Another solution (sometimes accepted by some AHJ/Insurance companies) is to provide additional In Rack sprinklers at the top level of rack storage arrangement to compensate and have a 30% increase at the ceiling sprinkler density.
But this is not "according to the standards" and is subject to acceptability by AHJ and sense of judgement based on commodity classification... And is not applicable to pile storage, shelf storage... only rack storage...

Note that there are many studies at the moment done by FM Global and the NFPA Research foundation regarding the effect of slope to the water distribution pattern of sprinklers.

More info can be find here:

https://www.nfpa.org/News-and-Research/Data-research-and-tools/Suppression/Protection-of-Storage-Under-Sloped-Ceilings

Or more direct access:

https://nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Suppression/RFStorageUnderSlopedCeilingsPhase1.ashx

and

https://nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Suppression/rfslopedceilingsPhase2.ashx

Jonathan Sullivan

11/3/2020 01:02:57 pm

Thanks Franck. Is this an implied or explicit slope limit? I checked Ch. 1, 10, and 20 of 13-2019 and didn't see this 16.7% stated. I do doubt my reading comprehension sometimes.

Franck

11/3/2020 03:08:38 pm

NFPA 13 - 2019 Edition
20.6.1 Ceiling Slope.
The sprinkler system criteria specified in Chapters 20 through 25 are intended to apply to buildings with ceiling slopes not exceeding 2 in 12 (16.7 percent) unless modified by a specific section in Chapters 20 through 25.

It was 12.1.2 in 2016 Edition.

Jonathan Sullivan

11/3/2020 03:23:47 pm

Thank you for the reference. To conclude, as that section is for storage occupancies, NPFA 13 would still be applicable if the slope is greater than 2 in 12 in a non-storage occupancy.

Franck

11/3/2020 03:23:09 pm

It is one of the few "dark" areas of NFPA 13 where there is no proposed solution, either than just "out of the scope of NFPA 13".

I remember the good old time when I started this job and it was still allowed to have storage and sloped roofs. We just applied the 30% penalty on the remote area and that was it... And then, suddenly, it was not permitted anymore, with no solution !
Hard time !

For other occupancies, the same limit applies (16.7% or 1:6 or 2:12, or 9.4° which is all the same), but in that case you add 30%:

19.3.3.2.4 Sloped Ceilings.
The system area of operation shall be increased by 30 percent without revising the density when the following types of sprinklers are used on sloped ceilings with a pitch exceeding 1 in 6 (a rise of 2 units in a run of 12 units, a roof slope of 16.7 percent) in nonstorage applications:

(1)
Spray sprinklers, including extended coverage sprinklers listed in accordance with 11.2.1(4), and quick-response sprinklers

(2)
CMSA sprinklers

Chris

11/3/2020 10:29:39 am

There is some information still needed. You say you have a 12:12 pitch on your roof. What is the structure? Combustible or non-combustible? What is the distance from one eave to the other eave across the roof? Dry system or wet? If dry add an additional 30% onto your remote area. And due to the pitch of the roof, you will have an additional 30% area increase to your remote area.

The structure and construction will determine the head spacing. The distance is so you can see if can use a BB type head and avoid a lot of extra ssu's.

Good luck.

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