Q. When I installed the capped composite decking on my clients’ 30-foot-wide deck, I needed to use butt joints. Since then, my clients have noticed a slight swelling at the ends of the deck boards where the butt joints are located. Is this normal?

A. Kim Katwijk, a deck builder in Olympia, Wash., and a PDB contributing editor, responds: I recently encountered a similar situation with a project that I had installed a few years ago. Because I don’t believe that decking should swell after it has been installed, I contacted the decking manufacturer to file a warranty claim. Even though I’ve installed thousands of lineal feet of this particular product with virtually no problems (and good customer support the few times when there were), my conversation with the company’s tech department reminded me that it always pays to carefully read each deck manufacturer’s installation instructions.

For this particular capped composite decking, the instructions call for 1/16-inch-end-to-end spacing between boards for every 20°F increment, to account for shrinking and swelling caused by temperature changes. So, for example, if the decking is installed on a 50°F day, at least a 1/8-inch gap would be required in anticipation of the days here in the Pacific Northwest when temperatures soar into the 90s. Of course, temperatures here in the winter dip into the 30s and lower, in which case the gap could reasonably be expected to grow to 1/4 inch or more.

Butt joints in composite decking that aren’t properly gapped to allow for expansion can lead to swollen ends (left). When the break occurs over a single joist, it’s impossible to drive fasteners vertically into the joist so that they are perpendicular to the board face while maintaining proper clearance from the ends. Organic material that collects in the gaps between deck boards will trap moisture that can be absorbed by the decking (above).

These gaps—while perhaps unsightly—aren’t necessarily a bad thing, because they allow water to drain away from ends of the boards, which aren’t protected by the caps that protect the sides and tops. However, pine needles and other organic debris can become trapped in the gaps and hold moisture, which can then be absorbed into the ends of the boards. And make no mistake: Most composite decking absorbs moisture when exposed to enough of it, which is why the ends will swell.

Doubled deck joists with a gap between them allow for drainage between the butt ends of decking and proper face-fastener placement, but add labor and expense when framing a deck.

Another often-overlooked detail is the recommended placement of end fasteners. Again, for this particular decking, end fasteners aren’t permitted within 3/4 inch of the end of a board, and must be installed perpendicular to the board surface. So, even without the required gap, a 1 1/2-inch-wide single joist just doesn’t offer enough surface area to fasten the ends of two deck boards without angling the fasteners in toward the center of the joist. That means that a doubled joist should be installed wherever there will be a butt joint. That’s a costly measure for deck builders (like me) who like to randomly locate the butt joints, instead of lining them up along one or two joists. It also creates a wider platform for collecting and trapping moisture and debris.

Michael Walter You can avoid the need for butt joints by installing a seam board in the middle of a wide deck. Here, California deck builder Michael Walter used shims to create consistent gaps between the seam board and the ends of the deck boards.

Unfortunately, there’s no easy solution. My manufacturer suggested regular removal of the debris in the gaps to help reduce the swelling, but that’s impractical in my area—where trees are almost continuously shedding their needles—and it’s a maintenance burden for the homeowners. Another option is to install a seam board that interrupts the long run of decking and eliminates the need for butt joints. If you use this detail, you’ll still need to provide a proper gap per the manufacturer’s recommendations between the ends of the decking and the edge of the perpendicular seam board.