Choosing Deck Boards to Prevent Cupping
Tom Silva explains why not to use 2x10 boards for decking
We were planning on using 2x6s for our new deck, but for some reason 2x10s turned out to be cheaper. The wider boards would have fewer joints, but would they be more likely to cup? —Keith Linderer, Bloomsdale, Mo.
You're right about 2x10s being prone to cupping. That's just one reason not to use them as decking. Wide boards are also more likely than narrow ones to check—show cracks—and to have defects like knots and sap pockets.
And while 2x10s do have fewer joints, the finished gaps would have to be fairly large—about ⅜-inch across—to allow the boards to expand and contract widthwise. Make those gaps any narrower and there's a risk that the boards will collide with each other and buckle when they swell.
So stick with your original plan to use 2x6s. Even better, check out the price and quality of 5/4x6 deck boards, the size that most builders use. They're ½ inch thinner than 2x stock, and therefore should be less expensive.
—This Old House general contractor Tom Silva