Commonly, 15 pound roofing felt or 4 mil. polyethylene film are used to retard the passage of moisture to the underside of the hardwood floor in a plywood subfloor application.
In joist construction, this 15-pound saturated felt paper should be laid between the wood flooring and the wood subfloor. If the wood subfloor is laid over a concrete slab, the felt paper moisture retarder can be cemented to smooth, clean-swept concrete. Other vapor barriers such as underlayment with a vapor barrier attached may also be appropriate over concrete slabs.
15-pound roofing felt in asphalt mastic is applied with a notched trowel at the rate of 50 square feet per gallon. A second similar coating of mastic and asphalt felt should be added, with overlaps staggered to achieve a more even thickness.
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