Cloud Ceiling & Sprinkler Protection Clarification?

6/27/2019

I have other smaller cloud ceilings that do not exceed 4 ft., however I do not think the sprinklers above have the 18” above the cloud to develop their sprinkler spray, so my gut tells me that means sprinklers are required below the cloud. I think I have seen an NFSA informal interpretation that agrees.

However, what about 8.6.5.3.2 (2016)?

NFPA 13 - 2016
8.6.5.3 Obstructions that Prevent Sprinkler Discharge from Reaching Hazard
8.6.5.3.2 The requirements of 8.6.5.3 shall also apply to obstructions 18 in. (450 mm) or less below the sprinkler for light and ordinary hazard occupancies.

8.6.5.3.3 Sprinklers shall be installed under fixed obstructions over 4 ft (1.2 m) wide.

Does that mean it does NOT really need the 18” in order for it to qualify for the following 8.6.5.3.3 rule for obstructions over 4 ft. wide? In other words, my cloud is not over 4 ft. wide, so doesn’t need sprinklers, even if the sprinklers above do not reach full spray development?

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Jonathan Joseph

6/27/2019 10:31:08 am

I believe that if the sprinklers that are less than 18" from the obstruction, you would have to use the "three times rule" 8.6.5.2.1.3 (2016).

Colin L

6/27/2019 10:41:46 am

Exactly the opposite....8.6.5.3.2 that you reference specifically says that the obstruction rules of 8.6.5.3 ALSO apply to obstructions less than 18" below the sprinkler for LH and OH....therefore, if your clouds are less than 18" below the sprinklers and are obstructing the spray pattern, 8.6.5.3 applies, and you must have a sprinkler in the cloud....in other words, you must treat a small could within 18" from the sprinkler the same as if it were a 4-ft cloud more than 18" below the sprinkler.

COLIN L

6/27/2019 10:53:20 am

And I agree with Jonathan, in order to determine whether or not the obstruction within 18" of the ceiling is going to affect the sprinkler spray pattern, you could either use the 3x rule of 8.6.5.2.1.3, or you might treat the cloud like a soffit and use 8.6.5.1.2 (2016) to ensure that the horizontal distance from the sprinkler to the edge of the cloud is sufficient so as not to obstruct the spray pattern.

Dave

6/27/2019 11:14:14 am

Thanks, that is the exact struggle I am having - Like Jonathan, I believe that by being within 18 inches, I would not be able to meet the 3x rule, and with no room for that spray development, I'd better get sprinklers below the clouds. The Annex commentary appears to reinforce this. But parsing-out the section numbers ("the requirements of the [entire] 8.6.5.3 shall apply"), it appears to be what I think Colin is saying - A small cloud does NOT require protection beneath, even if the cloud is within 18" of the sprinklers below. Because 8.6.5.3 says to do so only if it is over 4 ft wide (8.6.5.3.3). Further thoughts?

6/27/2019 11:28:45 am

Dave, where did I say sprinklers are not required in the cloud? I'm not sure where you're getting that, as that is the exact opposite of what I said. To quote from my earlier response, "8.6.5.3 applies, and you must have a sprinkler in the cloud...." not sure how I could be much clearer

6/27/2019 11:51:42 am

Colin, I think I merely mis-read / misattributed your line "Exactly the opposite." So consensus appears to be that if spray pattern is not developed (and one does not meet the 3x rule, partition rule, etc.) then sprinklers shall be provided beneath the cloud(s). That is logical.

6/27/2019 12:01:51 pm

No worries....Yes sir, that seems to be the consensus!

6/27/2019 11:57:51 am

It seems to me the clouds are not dense enough to reference 8.15.24 Cloud Ceilings and eliminate the above sprinklers. So you would have sprinklers above and below the clouds if you cannot meet the "three times rule" Use a strategic method of spacing the sprinklers. Good luck!!

6/27/2019 12:03:26 pm

Yes, I have rarely been able to eliminate the sprinkler above the clouds, although that appears to be the emphasis in the code sections.

...Allow me to take it one step further, because I run into this a lot when reviewing plans. Once it is deemed that sprinklers are required in the clouds, if one has an array of clouds, I find designers try to save sprinklers by spacing the cloud sprinklers out to the max, "skipping" clouds in-between. I would think the only way one could justify this would be if the gaps were smaller than the maximum 1 in. per 1 ft. in height threshold, derived by the FPRF cloud ceiling study, and implied by the 2016 Table 8.15.24.1 for maximum spacing.

6/27/2019 12:09:40 pm

P.S. - Thanks all, for taking the time to reply. Sometimes when it feels like a brick wall, all it takes is some sleep, a new day, and a fresh perspective from you guys.

Cloud Ceiling & Sprinkler Protection Clarification?

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