When Is the 4-ft Sprinkler Obstruction Rule Met?

11/26/2019

NFPA 13 (2016 Edition) Section 8.5.5.3 requires sprinklers to be installed underneath fixed obstructions over 4ft (48") in width and provides no further information on how that 4ft width is determined to be an obstruction. For most situations this is rather straightforward, but the open ended 4ft wide requirement leaves quite a few obstructions up to interpretation.

With that being said, how is it determined that an obstruction such as a duct, can be considered unobstructed if it is 40" wide by 6ft long, but if it is 48" wide by 3ft long, it is considered an obstruction? How is the obstruction rule only applied to the width of a piece of equipment and in turn is not applied to the length?

Is there a ratio or rule of thumb that is widely used to determine whether an item hanging below the ceiling is considered to violate the 4ft rule?

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Matthew King

11/26/2019 10:41:17 am

If you read later editions of code it defines 4 ft aide and less than 2ft above the floo . When you look at definitions of obstructions it become clearer as to continuous and non-continuous. In speaking with a local fire marshal, he said it the best: " if that sprinkler discharges and covers differently than it was designed to, and its 2 for more off the floor, the you need to put sprinklers under it." Assuming he is correct and the way I read code is correct, 4 ft a number that many n loop non-FPEs banter as an exception to fire usually isn't the issue.

Bruce Seiler

11/26/2019 01:24:40 pm

The annex of the standard states "the width of an object is the lesser of the two horizontal dimensions (with the length being the longer horizontal dimension)."

Also, the object must be greater that 48" meaning the object you mentioned above of 48"X36" would not require protection under it.

Another note: These rules only apply when they are located more than 18" below the sprinkler deflector. The other obstruction rules apply when you get in that 18" space.

Alex H

11/26/2019 01:37:26 pm

In the NFPA 13 annex, section A.6.5.3.3 of the 2016 edition, it says, "The width of an object is the lesser of the two horizontal dimensions (with the length being the longer horizontal dimension). Sprinkler protection is not required under objects where the length is greater than 4 ft and the width is 4 ft or less."

One way of looking at it is that the length of an obstruction is extending the "width problem" to adjacent heads. The width (i.e. the smaller dimension) is the dimension that gets evaluated for obstructive influence. It does seem a bit odd, but you can kind of see how they came up with it, especially when you think about how long ducts are.

Glenn

11/26/2019 04:51:41 pm

An addition to the above, it is A 8.6.5.3.3.

Glad this annex section was added to the 2016 edition as this has come up in discussion many times. Thanks for providing the reference.

When Is the 4-ft Sprinkler Obstruction Rule Met?

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