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Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved
1. In building construction a material of which no part will ignite and burn when subjected to fire.
2. A building material having a structural base of noncombustible material, as defined above, with surfacing not over ? in. (0.32 cm) which has a flame-spread rating not higher than 50.
3. A material, other than those falling into the above two categories, which has a flame-spread rating not higher than 25 and which shows no evidence of continued progressive combustion.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.