A.Preferably none. The U.S. Forest Products Laboratory does not recommend any adhesive between the decking and the joists. The swelling and shrinking of the two members at right angles to each other will break the bond of any rigid adhesive. Many of the more flexible mastic-type products on the market will stick to treated wood, but the differential movement will cause failure in a few years. Neither type of adhesive is strong enough to prevent deck board cupping.
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