Open-Grid Ceilings?

[fpst]

The annex of NFPA 13 2007 defines Open-Grid Ceilings in A.3.7.2 (2): "The term open-grid ceilings as used in this standard refers to ceilings in which the openings are 1 ⁄ 4 in. (6.4 mm) or larger in the least dimension, the thickness of the ceiling material does not exceed the least dimension of the openings, and the openings constitute at least 70 percent of the ceiling area."

My question is what if you have a room which has 70% or more of it's ceiling area constituted of return and supply duct opening space (such as a small room with an unusually high number of supply/return ducts)? Obviously, heat could escape through these openings and delay sprinkler operation, but what about the 1/4" wording above? I'm having trouble deciphering its meaning.

 

[fpst]

To clarify, for example, the least dimension of a supply duct could be the length or width of the opening, which is much greater than the least dimension of the "Z" dimension of the opening of the duct, which could possibly make the duct not count towards an open-grid ceiling..?

 

[cdafd]

Know it is not accepted by some, but would not say the open grid applies

To me more of an obstruction issue, and sometimes sprinklers are installed under the duct as well as at roof level,,

Even though no one duct is over 4 feet

 

[skdesigner]

fpst,

A picture being worth 1000 words, see attached an example of an open-grid ceiling. The 1/4" dimension refers to the length of the sides of each square in the tile. As the "squares" in this particular tile measure 9/16"L x 9/16"W x 1/4" Deep, it would be permissible to install it beneath sprinklers, provided those sprinklers met the installation criteria of NFPA 13-2007 8.15.13 (1) (a) & (b). I realize this doesn't answer your duct question, but hopefully it gives you a better idea of what an open grid ceiling is.

skdesigner

R M Arsenault Engineering Inc.
www.rmae.ca