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Q.We’re building a four-season addition over an existing raised deck framed with PT lumber. Is it okay to install OSB subfloor sheathing directly over the wood decking, or should the decking be removed first?

A.Bryan Readling, a senior engineer with APA/Engineered Wood Association, responds: If the deck footings, framing, and house connections are adequate for the anticipated loads, and the existing decking is sound and reasonably flat, you don’t necessarily have to remove it before laying down a new OSB subfloor and framing the addition. However, there are some advantages to doing so anyway.

For one, removing the decking would allow you to thoroughly inspect the framing and easily make any needed repairs or upgrades. You’ll also avoid squeaky floors, bumps in the floor sheathing, and other problems caused by loose, warped, or misaligned deck boards, or by wet joists drying out and causing decking fasteners to lose their grip. If you take off the decking, clean the tops of the joists with a scraper, and glue and nail the sheathing directly to the joists, you will end up with a solid floor system and be more likely to avoid callbacks.

If you still choose to install the sheathing over the existing decking, be sure to follow APA’s recommended nailing schedule for underlayment (the Engineered Wood Construction Guide, Form E30, is available at apawood.org). Fasteners — ring-shank nails are best — for 7/16-inch OSB should be 6 inches on-center around the panel perimeter and 8 inches on-center each way in a grid pattern on the interior of the panel. (If you’re using 2 3/32-inch T&G sheathing, the perimeter nailing schedule is the same, but the grid pattern can be increased to 12 inches on-center.)

To minimize squeaking and nail pops caused by shrinking lumber, use construction adhesive and nail into the decking rather than into the joists. Also, select fasteners that are long enough to protrude slightly below the bottom of the deck. Keep in mind, too, that the deck framing will now have to comply with local building-code guidelines for ground clearance, floor insulation, and ventilation.