Rough openings on the coast

I need to frame an opening for a patio door in a coastal house. What type of lumber is the best choice and the most weather-resistant? Can I use composite decking?

John Spier, a builder on Block Island, R.I., replies: From your question, I’m not sure whether you are asking about structural framing or framing the opening with trim. In either case, a compositedecking material is not a good choice. For structural framing— sills or plates, studs, trimmers, and headers—you need structurally rated dimensional lumber.

Only the sill plate needs to be rot resistant, either treated lumber or, under some codes, cedar or redwood. In a properly finished installation, none of the structural materials will ever be exposed to the weather, so the rest of the opening can be untreated framing lumber.

I’ve seen composite decking used for exterior trimwork, but it’s not an ideal material. Composite decking might not rot, and it takes paint. However, it’s not easy to work or fasten, and it has a high rate of thermal expansion and contraction. Rot-resistant woods, such as cedar or redwood, or synthetic trim materials such as Azek, cost about the same and are much better choices.