A.Mike Sloggatt, a remodeling contractor in Levittown, N.Y., responds: While wood trim expands and contracts across its width with changes in moisture content, PVC trim expands and contracts along its length with changes in temperature. An 18-foot-long PVC board can change seasonally by as much as 3/16 inch — easily enough to create the large gaps you describe.

So it’s important to account for your climate’s overall temperature range and the temperature of the material during installation. In cold weather, for example, I make an allowance for summer expansion so that the boards don’t buckle when they warm up. In hot weather, a little extra spring in the boards helps keep the joints intact when the...

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