underlining felt

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underlayment, 1
1. A material such as plywood or hardboard placed on a subfloor to provide a smooth, even surface for applying the finish.
2. The material, usually No. 15 felt, used to cover a roof deck before shingles are applied; also called underfelt.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.