10 Tips for Choosing and Buying Deck Lumber
Updated: May 01, 2024
A new deck is only as good as the materials you use to construct it. These tips from a lumber expert will help you choose your materials wisely.
When I first migrated west from my hometown in the Great Lakes region at the end of the previous century (which sounds like a longer time ago than it is), redwood was the material of choice for decks along the coast. It’s naturally weather-resistant, attractive and relatively abundant in the western states, and at the time, it was relatively inexpensive. These days, two-by-six-inch redwood decking retails for $3.50 per linear foot, a lot compared to pressure-treated pine or fir. But it’s still less expensive than Western red cedar, exotic hardwoods and most wood alternatives like PVC and composites such as Trex and Kebony, so it’s considered affordable.
The number of available decking materials has increased dramatically since the 1990s, owing to the growing numbers of engineered and artificial wood products on the market. But as far as I’m concerned, untreated redwood and cedar are still king.
Real softwoods do have some disadvantages compared to plastics and composites. Boards can warp and split due to weather exposure, and they may not even be perfectly straight when you bring them home. Lumber expert Josh Kou also notes that softwoods need maintenance to stay looking their best. Over time, softwoods “will lose their color, fading to gray within about a year or two, so they will require regular refurbishment with deck oil or other proprietary treatment,” says Kou.
Choosing the wood for your deck wisely can mitigate some of the problems of real wood decking. In this article, Kou offers some tips to help you make the perfect selection for your home.
Buy Pressure-Treated Lumber With the Right Amount of Preservative for the Job
Use pressure-treated lumber for the structural part of your deck — the posts, joists, beams and other members you normally don’t see. It’s the logical choice; PT lumber can support more weight and span longer distances than cedar, redwood, or other woods commonly used for building decks. It’s also much less expensive.
The lumber industry rates PT lumber according to the pounds of preservative retained per cubic foot of wood; the higher the number, the better the protection against fungi and insect attacks. Select decking boards with the preservative concentration suitable for their use. The wood used in situations where it’s more likely to rot more preservatives.
The three common ratings, which correspond to preservative concentration, are:
- Above-grade, use .25 (sometimes .15). This includes decking, fence and railing material.
- Ground-contact, use .40. This includes posts, beams, joists and decking.
- Below-grade, use .60. This includes support posts that are partially buried below grade and for permanent wood foundations and planters.
Note: Decking boards are tagged with the concentration and treating solution used. CCA (chromated copper arsenate) was once the most common preservative, but it was phased out in 2004 in favor of ACQ (alkaline copper quat) and other preservatives because of health concerns. “Modern preservatives are certified safe for plants, animals, people, ponds, and waterways,” says Kou.
Avoid Treated Lumber That Has a Lot of Heartwood
When buying pressure-treated lumber for decking boards or posts, Kou advises checking the end grain to make sure you don’t select decking boards with too much heartwood. Heartwood is from the center of the tree while sapwood is from the outer edges.
“Heartwood is denser than the outer sapwood, so pressure treatments find it harder to penetrate deeply into the structure of the wood,” he says. On the other hand, “heartwood contains more natural defensive oils, so it could be your first choice if you are skeptical of chemical treatments.”
Select Cedar or Redwood Decking Boards Cut From Heartwood
“If choosing untreated cedar or redwood lumber for your deck, ask for heartwood common boards,” says Kou. “These will last longer and be less likely to twist, cup, or otherwise distort.” Heartwood contains the natural preservative oils that give these woods their resistance to decay and insect attack. Heartwood in these species is the darker core of the tree. Sapwood — lighter in color and cut from the outer edges of the tree — has a lower concentration of these natural oils.
You may find two grades at the lumberyard: heartwood common and construction common. Kou recommends heartwood common because it has more heartwood.
When it comes to installation, Kou advises “bark side up” (with the end-grain pattern arching like a rainbow). His thinking, which is supported by the wood industry, is that the boards will gradually crown slightly and help rain run off.
Buy Wood that’s Dry
If you’re buying treated wood, buy boards that have had time to dry after they’ve been treated. “It is important to avoid buying boards that feel wet,” says Kou. “If they are recently treated these boards will be heavier, which will make them harder to handle, and the greater moisture content will make them harder to cut.”
Wet boards also shrink as they dry. This means joists can “rise up” from their hangers, making them bouncier and less well-supported. Fasteners can loosen, and your deck can wind up with uneven and unsightly wide gaps between the boards.
If you’re unsure whether a board is too wet, compare its weight with that of an untreated board the same size; if it’s twice as heavy and feels damp, it may need time to dry. You can “sticker” the wood (stack it in separated layers) and let it dry for a few weeks.
For decking boards, your best bet is to look for wood that’s labeled KDAT (kiln-dried after treatment).
Use Hefty Posts for High Decks
Tall decks look spindly and awkward perched on four-by-four posts. “Bigger is usually better if you want a deck that lasts longer and can handle a decent gathering,” says Kou. “Four-inch posts are fine if your deck is close to the ground, but if you are two feet or more above grade then trade up to six-inch.”
Bigger posts also last longer and can support extra weight if you add a structure to your deck somewhere down the road. Also, avoid posts that already have a twist or bow; chances are the defect will only get worse.
Build your own posts
If you can’t find decently treated support posts or you just want to save money, make your own by sandwiching three well-treated 2x4s or 2x6s together. The lumber in this image contains almost no heartwood and is almost 100 percent treated.
Research Deck Boards Before You Buy
Each of the four main deck board categories has its advantages and disadvantages.
Natural softwoods
Cedar and redwood decking boards are beautiful, have fantastic grain and natural coloring and are very stable. They lay flat and won’t move, cup, crack or split as much as treated boards.
However, they will lose their color, fading to gray within about a year or two, says Kou, so they will require regular refurbishment with deck oil or other proprietary treatment. And while they aren’t the most expensive option, they aren’t the cheapest either.
Natural hardwoods
Exotic hardwoods, such as ipé, cumaru and teak, are grown on plantations in the tropics, so they aren’t as environmentally unfriendly as they seem, but they are undoubtedly the priciest option. Plus, they’ll outlast most other decking materials by many years and continue looking great with an occasional oil treatment.
Their main downside is that their density makes cutting boards and driving screws hard on your tools and a general pain.
Treated lumber
“Pressure-treated decking boards cost the least,” says Kou, “so if you plan on building a big deck, then they are probably the best choice. PT boards will also last longer than untreated cedar or redwood, but they will shrink, split, crack and warp if they have not been properly kiln-dried.”
Composite deck boards
Composite boards are marketed as rot and vermin-resistant, and while they’re made from recycled plastic and timber material, they aren’t necessarily recyclable themselves and usually end up in landfills when they wear out. “The boards will be consistent in color, texture, and finish, so will have a nice uniform appearance right across a deck,” says Kou.
On the downside, he says composite decking is usually priced higher than pressure-treated and natural softwood boards, and it doesn’t perform well in areas that experience extreme weather conditions (long periods of below-freezing temperatures, or over-exposure to sunlight.)
Moreover, composite boards are prone to sagging and bending, so they require more support than PT or natural wood boards. Kou recommends washing composite boards regularly because moss or algae build-up can make them slippery.
Consider All Three Dimensions When Selecting Your Deck Boards
Some deck boards are thicker or wider and some have rounded edges. All perform a bit differently.
Width
“Be width-wise,” says Kou, who explains that even though four-inch boards are a little less expensive, stretching the budget to six inches will mean fewer cuts, fewer gaps, and fewer fasteners. “Don’t be tempted to use eight-inch,” he warns. “They will be more prone to splitting, cupping and crowning.”
Thickness
“Don’t skimp on thickness,” Kou continues. “Go for a minimum inch to inch-and-a-quarter thick boards. Anything less will require more joists to support it. Three-quarter-inch boards are too flimsy and will bend, even with 12-inch spacing on the deck joists.” If your joists are spaced at 24 inches, or you’re running your deck boards at a diagonal, you’ll probably need to use 2-by (1-1/2 in. thick) boards, which is a good default thickness.
Length
Purchase decking that can run the full length of your deck when possible. All lumberyards and home centers carry 16-ft. deck boards, but many also stock or can order 20- and 24-ft. boards, though they may cost more. “The advantage of working around supplied lengths will mean fewer cuts and fewer butt joints,” he says, and butt joints are potentially problematic. The ends of boards are more absorbent, slower to dry out and more susceptible to rotting, swelling and splintering. Fasteners driven close to the ends also tend to split the wood.
Avoid “Scalped Edges” and Uneven Widths
When you’re purchasing boards, check the crown by sighting along the edge of the board, then check that crowned edge for scalped edges, called “wane.” Wane on the bottom of a joist is OK, but wane on the top means there’s less wood for driving fasteners into—and you can’t butt two deck boards on an uneven joist.
It’s actually preferable to use joists with a slight crown, “slight” being an upward bow of 1/4 to 1/2 inches. Joists settle and sag slightly as they support weight and movement, and a perfectly straight joist will wind up with a dip while one with a slight crown will end up being straight.
The joists’ widths (or depths) can vary by as much as 3/8 inches. The 12-ft. long 2x10s at your lumberyard may measure 9-1/8 in. in width, while the 16-footers measure 9-1/2 in. Deck boards secured to uneven joists will flex more, and the fasteners will creak and pop, so use joists that vary by less than an eighth of an inch.
Don’t Bother Trying to Find Perfect Deck Lumber
Wood is a natural material, so there is bound to be some variation among boards. Over the 16, 20 or 24 feet of a standard decking board, there may be a little twist or bend. Most of the time, this will even out when you fix the decking board to the joists. “However, if you hold up the end of a board, look along its length and immediately recall the banana you ate the day before, put it to one side and select the next board,” says Kou.
- Deck boards: If a deck board isn’t perfectly straight, you can work the bow out as you nail it to the joists. And if there’s a foot or two of bad material, you can cut out the defect, then use the resulting two shorter pieces for a smaller area of the deck, stair treads or landings.
- Framing: Set aside your straightest joists and use them for the perimeter of your deck. If you have curved or twisted joists, straighten them with blocking or as you install the decking. Again, extremely bowed or knot-filled boards can be cut up and used as blocking or joists for smaller landings and deck sections.
Pro tip
- As you haul your material from the driveway to the backyard, stack it into piles of “pretty” and “ugly”, then use them accordingly.
Buy Straight, Solid 2x12s for Your Stair Jacks
Be fussy here. Look for straight boards with no splits or large knots. Stair jacks should be straight so that the stairs remain level and strong. You’ll cut deep triangles into each 2×12 to accommodate the treads, so avoid splits along the top edge and knots along the lower edge, which can weaken the already thinned body of the jack.
About the Expert
- Josh Kou is the shop manager for Chicago-based North Castle Hardwoods. A second-generation lumberman, Josh has been working with wood for his whole life.
Originally Published: May 01, 2024
