Sounds to me like the thickness of the material nor the depth of the material can exceed the least dimension of the opening. Are your sprinklers above the grating?
9.3.10*<https://link.nfpa.org/publications/13/2019/annexes/A/groups/9#ID000130005688> Open-Grid Ceilings. Open-grid ceilings shall only be installed beneath sprinklers where one of the following is met: * (1) Open-grid ceilings in which the openings are 1⁄4 in. (6 mm) or larger in the least dimension, where the thickness or depth of the material does not exceed the least dimension of the opening, and where such openings constitute 70 percent of the area of the ceiling material. The spacing of the sprinklers over the open-grid ceiling shall then comply with the following: * (a) In light hazard occupancies where sprinkler spacing (either spray or old-style sprinklers) is less than 10 ft × 10 ft (3 m × 3 m), a minimum clearance of at least 18 in. (450 mm) shall be provided between the sprinkler deflectors and the upper surface of the open-grid ceiling. Where spacing is greater than 10 ft × 10 ft (3 m × 3 m) but less than 10 ft × 12 ft (3 m × 3.7 m), a clearance of at least 24 in. (600 mm) shall be provided from spray sprinklers and at least 36 in. (900 mm) from old-style sprinklers. Where spacing is greater than 10 ft × 12 ft (3 m × 3.7 m), a clearance of at least 48 in. (1.2 m) shall be provided. * (b) In ordinary hazard occupancies, open-grid ceilings shall be permitted to be installed beneath spray sprinklers only. Where sprinkler spacing is less than 10 ft × 10 ft (3 m × 3 m), a minimum clearance of at least 24 in. (600 mm) shall be provided between the sprinkler deflectors and the upper surface of the open-grid ceiling. Where spacing is greater than 10 ft × 10 ft (3 m × 3 m), a clearance of at least 36 in. (900 m) shall be provided. * (2) Other types of open-grid ceilings shall be permitted to be installed beneath sprinklers where they are listed for such service and are installed in accordance with instructions contained in each package of ceiling material. ________________________________ From: Bob Knight <b...@firebyknight.com> Sent: Friday, April 12, 2024 6:41 PM To: sprinklerforum@lists.firesprinkler.org <sprinklerforum@lists.firesprinkler.org> Subject: [Sprinklerforum] Steel Grating I have an area that has Steel Grating that is 1/4" x 1-1/2". The openings are 15/16" x 3 3/4". When A.3.3.41.2(2) {2019 ED} states "the thickness of the ceiling material does not exceed the least dimension of the openings", is this referring to the depth of the member (1 1/2") or the width between openings (15/16") or the actual thickness on the material (1/4")? It's late on a Friday and I'm tired. Thanks for the help. -- Thank you, Bob Knight, CET III Fire by Knight, LLC 208-318-3057 [FIREBYKNIGHT] [https://s-install.avcdn.net/ipm/preview/icons/icon-envelope-tick-green-avg-v1.png]<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> Virus-free.www.avg.com<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient>
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