How to Buy Deck Lumber

10 things you need to know about how to buy the good stuff

Tip 1: Buy pressure-treated lumber with the right amount of treatment for the job

Pressure-treated lumber is the logical choice for the structural part of your deck—the posts, joists, beams and other members you normally don't see. Pressure-treated 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.

Pressure-treated lumber is rated according to the pounds of preservative retained per cubic foot of wood; the higher the number, the better the protection against fungi and insect attack. Select boards with the preservative concentration suitable for their use.

The three common ratings:

  1. Above-ground use (.25, sometimes .15). Typically used for decking, fence and railing material.
  2. Ground-contact use (.40). Typically used for posts, beams, joists and, again, decking.
  3. Below-grade (.60). Typically used for support posts that are partially buried below grade and for permanent wood foundations and planters.

Your boards will be tagged with the concentration and treating solution used. Use .40 material if you can't find .25. CCA (chromated copper arsenate) is being phased out because of health concerns. ACQ (Alkaline Copper Quat) and other preservatives are replacing it.

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Tip 2: Avoid treated lumber that has a lot of heartwood

Since heartwood—wood from the center of the tree—is denser, it accepts pressure treatment less readily than sapwood—wood cut from the outer edges. This isn't as great an issue with dimensional lumber like 2x10s and deck boards; these boards are thin enough that the preservatives are usually driven throughout. But with 4x4 and 6x6 posts, the preservatives may not penetrate the dense heartwood. However, it can be difficult to find posts not cut from heartwood.

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Tip 3: When buying cedar or redwood decking, select boards cut from heartwood

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—lacks these natural oils. If you're purchasing redwood, you may find two grades. Look for lumber labeled “heartwood common,” which has more heartwood than “construction common.”

Install deck boards “good side up.” Some swear deck boards should only be installed “bark side up, ” the theory being, if a board cups, boards laid “bark side up” will warp into a hump that water will run off rather than a dip where water can settle. But tests have shown there are many reasons boards cup and they don't always cup according to “bark side. ”

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Tip 4: Buy wood that's dry

If you're buying treated wood, buy boards that have had time to dry after they've been treated. Boards still saturated with the water used to carry the wood preservative into the wood cells can be literally twice as heavy as dry wood. The extra weight makes wet boards harder to work and to cut, and they shrink when they dry. This means joists can “rise up” out of 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 and let it dry for a few weeks (see photo). For deck boards, look for wood that's labeled KDAT (kiln dried after treatment).

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Tip 5: Use posts with some heft, especially if your deck is more than 2 ft. off the ground

Tall decks look spindly and awkward perched on 4x4 posts. Use 6x6 posts instead; they look and are more solid and substantial. They'll also last longer and support extra weight if you add a structure to your deck somewhere down the road. And avoid posts that already have a twist or bow; chances are the defect will only get worse.

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Tip 6: Every type of deck board has its pros and cons

There are three main categories of deck boards, each with its own advantages and disadvantages.

Natural woods:
The rich, natural color of redwood and cedar looks fantastic when the deck is new. These woods are also naturally stable and tend to lie flat with minimal checking or cracking. But natural woods will turn gray within the first year or two unless you regularly maintain them with a sealer or finish. They're also softer and more likely to get scratched and gouged when you drag furniture across them or shovel them in winter. This decking is more expensive.

Treated lumber:
This material is strong, long lasting and the least expensive of your options. But often boards that aren't kiln dried after treatment will shrink appreciably after they're installed, creating wider spaces between the boards. And treated lumber has a greater tendency to crack once in place; apply a water repellent every year or two to stabilize it. Treated lumber is the least expensive.

Composite deck boards:
Composite boards are made from wood and plastic (often recycled materials) and resist rot and insect attack. Since there are no defects, there is very little waste, and many now come in a variety of colors. They have their drawbacks, however. Not all brands are code compliant in all communities. Some people object to their homogenous look and “plasticky” feel. And like any other outdoor material, they can become dirty and stained; expect to do an occasional pressure washing. And the colors do tend to fade eventually. Composites are more expensive.

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Tip 7: Consider all three dimensions when selecting your deck boards

Width: Six-inch wide boards are ideal in most cases. Four inch wide material takes longer to install, creates more gaps and requires a lot more fasteners—but you can use it. Eight inch wide boards, because of their greater width, have more of a tendency to crack and cup; avoid using them.

Thickness: Radius-edge, 5/4 material (which can measure anywhere from 1 to 1-1/4 in. thick) has become extremely common. In most cases, it requires a joist spacing of 16 in. for proper support. If your joists are spaced at 24 in. or you're running your deck boards at a diagonal, you may need to use 2-by (1-1/2 in. thick) boards. Avoid boards that are only 3/4 in. thick. They have a wimpy feel underfoot even with closer joist spacing.

Length: When possible, buy decking that can run the full length of your deck. 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. Full-length material allows you to avoid butting boards end to end, which can invite trouble; 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, making the ends even more vulnerable.

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Tip 8: A little curve, twist or crown to your joists is OK (but “scalped edges” and uneven depths are a pain)

It's actually preferable to use joists with a slight crown, an upward bow of 1/4 to 1/2 in. (Figure A). Joists will settle and sag slightly as they support weight and movement. A perfectly flat joist will wind up with a dip. Check the crown by sighting along the edge of the board. Then check that crowned edge for scalped edges, called “wane” (see photo).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.

The depths of joists can vary by as much as 3/8 in.; the 12-ft. long 2x10s at your lumberyard may measure 9-1/8 in. in depth, 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. Use joists that vary less than 1/8 in.

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Tip 9: Perfect deck lumber doesn't need to be perfect

You can be too picky. It's OK if 5 percent of your boards are “ dogs”. Lumber with moderate defects can be used.

Deck boards: If a deck board isn't perfectly straight, you can work the bow out of it 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 joists that are curved or twisted, straighten them with blocking or as you install the decking (Figure A). Extremely bowed or knot-filled boards can, again, be cut up and used as blocking or as joists for smaller landings and deck sections.

Tip: As you haul your material from the driveway to the back yard, stack it into piles of “pretty” and “ugly”, then use them accordingly.

Figure A: Uses for Less-than-Perfect Lumber

Use lower quality lumber where it's not visible or for shorter spans. Sometimes you can straighten warped boards.

Note: Figure A can be downloaded and printed from Additional Information below.

Uses for less-than-perfect lumber
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Tip 10: Buy straight, solid 2x12s for your stair jacks

Be fussy here. Look for material that's straight, with no splits or large knots. You'll cut deep triangles into each 2x12 to accommodate the treads; avoid splits along the top edge and knots along the lower edge, which can weaken the already thinned body of the jack.

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