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What are your recommendations for dealing with expansion and contraction?
Jun 24, 2018


Use 8d stainless steel annular shank nails, or screws that are designed for wood trim, and are long enough to penetrate the solid substrate a minimum of 1 1/2 in. Simpson Strong-Tie makes an 8d nail with a 7d head in a 12-gauge thickness, available loose or collated, allowing it to be gun-nailed. The nail is called the “Trifecta.” It is half annular (tip) and half ring-shanked, and made from 316 stainless.

Screws are better for limiting the thermal movement of the trim. Allow PVC trim to acclimate to outside temperatures before installing. Bond PVC trim joints to prevent separation. Be sure to allow adequate expansion and contraction space at the end of long runs. If possible, decrease the on-center spacing between fasteners to 12 in. or less, and bond boards to substrate when practical.

Where you have an expansion joint, leave a full 3/16-in. gap when installing on a day where temperatures range from 30°F to 40°F. Leave a gap just large enough to accept a bead of sealant, or no gap at all (adhesive bond), when installing on a day when temperatures range from 80°F to 100°F. Shiplap joints are superior to scarf cut joints, especially on long runs.

If practical, install long runs of trim when the outside temperature and the temperature of the PVC trim board is 55°F to 65°F, in order to minimize thermal movement in the trim.