Clemens Jellema, a deck builder in Owings, Md., provides clear guidance on overcoming differential movement with composite exterior trim, one of the leading causes of annoying callbacks for deck builders and house carpenters alike:
The problem with composite and cellular PVC trim is that these synthetic materials react differently to humidity and temperature changes than does pressure-treated wood framing ... If it’s cool and moist when you build the deck, and then the weather becomes hot and dry, your fascia is going to expand lengthwise, while the pressure-treated wood framing is going to contract across its width.
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