How to Select Lumber for a Deck
If you've ever completed a woodworking project, you know how important it is to use the right lumber. We'll walk you through the best ways to select lumber for a deck, examine quality, and more.
Selecting lumber is a balancing act—among budget, beauty, and the function of the lumber (whether it will be used for framing or for decking and railings). We'll walk you through everything there is to know about different types of decking lumber, lumber grades, exotic woods, and more.
Types of Lumber
Lumber used in deck construction generally falls into one of the following categories:
Pressure-treated lumber (PT), usually pine or fir, is infused with chemicals that make it extremely rot-resistant. The chemicals also give the wood a green or brown cast, which you can hide with stain or paint or let weather to a dark gray. Pressure-treated lumber is the least expensive, but you'll have to choose carefully to get stock that is straight and free of loose knots.
Naturally resistant species, such as cedar, redwood, and cypress, are resistant to rot and insects, a quality most characteristic of the heartwood, the dense centermost core of the tree. You can seal or stain these woods to retain their natural beauty, or let them weather to various shades of gray. Exotic species, such as ipe, cambara, and meranti, display similar color characteristics and are generally more durable, more difficult to work, and more expensive.
Where to Use Different Types of Lumber
Framing: Unless your design requires the same wood throughout the entire structure (and your budget can withstand the sticker shock of natural heartwoods), pressure-treated lumber is a good choice for framing. Use lumber rated for ground contact for posts and framing members within 6 inches of the soil. Look for a grade stamp that says "ground contact" or indicates a treatment depth of 0.40 or greater. Some pressure-treated species are less porous than others, so they're incised before being treated. These incisions still show after treatment, so place this lumber where it will be less visible. Wood that has been kiln-dried after treatment (KDAT) is the highest quality.
Posts: 4x4s or 6x6s are standard. Cut cleats and stiffeners from 2x4s, joists and beams from 2x6s, 2x8s, 2x10s, or 2x12s. Pick PT lumber with care. Some boards will be smaller than standard nominal sizes. An untreated 2x10 will measure 9-1/4 inches, a PT 2x10 may only be 9-1/8 inches wide.
Decking and railings: These are the most visible parts of your deck, so you'll want to use the best lumber you can afford. Redwood, cedar, and cypress, as well as the exotic species are good choices, but since most decking boards from domestic woods are sapwood and not rot resistant, you should treat them.
For the deck surface, you can use 2x4s, 2x6s, or 5/4x6s. The 5/4 decking (pronounced "five-quarter"), available in cedar and pressure-treated fir, is 1 inch thick and 5-1/2 inches wide with rounded edges that make for a splinter-free surface.
Cedar 1x lumber usually has one rough side and one smooth side. Put the rough side down.
Grades and Moisture Content
Lumber is graded in an almost bewildering number of grades, which describe the prevalence of knots, its overall appearance, and its strength. For structural members choose a No. 2 grade or lumber graded as Standard. For decking and railings, Select grades are free of knots, but expensive. Choose the best your budget will allow.
A lumber grade stamp will indicate the quality of the stock and will also note its moisture content. For framing, air dried lumber is adequate. Use S-dry or MC-15 lumber for decking and rails.
Grade stamps differ from species to species, and board markings differ from dimension lumber stamps. A PT stamp specifies the treatment chemical, treatment depth, and other data. Pay special attention to both the grade and the moisture content of untreated lumber.
B-grade redwood has only tiny knots and is all heartwood—desirable but expensive choice for decking. Construction heart has knots but no sapwood. Construction common has large knots and is partly sapwood.
If you want to retain the brown color of these boards, plan to stain them every year or two; otherwise, allow them to weather
Chromated copper arsenate (CCA) or ammoniacal copper arsenate (ACA) have been widely used for pressure-treating lumber. Research has shown arsenic compounds to be a potential health hazard, however, so production of such treated wood for residential use has been halted, effective in 2003. You may still find these products on the market, however, because the law allows suppliers to sell their existing stock. Lumber treated with other chemicals, such as ammoniacal copper quaternary (ACQ), is not considered hazardous.
No matter what kind of PT wood you purchase, wear protective clothing, a dust mask, and safety glasses while working with it. Sweep up thoroughly and dispose of scraps. Do not burn PT waste. Call your local environmental agency and ask about proper disposal methods. Keep children out of the work area.
After it is cut, lumber is dried, planed, and smoothed, all of which reduces its thickness and width. The nominal size of a board refers to the size before drying and planing; actual size means the size you actually get, and it's less than its nominal size in thickness and width.
A very old, rough 2x4 may actually measure 2 inches by 4 inches. Today a nominal 2x4 is actually 1-1/2 inches by 3-1/2 inches.
Lumber from 2x6s on up can vary as much as 1/4 inch in width—even if taken from the same stack at the lumberyard. Posts larger than 4x4 are prone to twists and cracks; consider sandwiching 2xs instead. Measurements of common dimension lumber:
1x2 — 3/4" x 1-1/2" 1x3 — 3/4" x 2-1/2" 1x4 — 3/4" x 3-1/2" 1x6 — 3/4" x 5-1/2" 1x8 — 3/4" x 7-1/4" 1x10 — 3/4" x 9-1/4" 1x12 — 3/4" x 1-1/4" 2x2 — 1-1/2" x 1-1/2" 2x4 — 1-1/2" x 3-1/2" 2x6 — 1-1/2" x 5-1/2" 2x8 — 1-1/2" x 7-1/4" 2x10 — 1-1/2" x 9-1/4" 2x12 — 1-1/2" x 1-1/4" 4x4 — 3-1/2" x 3-1/2"
Exotic hardwoods offer a more costly, but extremely durable alternative to domestic woods. Ipe, for example, can triple your material costs—and that's if you use it only on the deck surface and railings. On the other hand, ipe is twice as strong as PT Douglas fir.
As more retailers bring these woods to market, prices might come down a bit, but they will probably never be competitive with PT lumber. Sustainability is also an issue with these woods. Some of each species is cut from sustainable forests, but these products can be difficult to obtain. Check with the Certified Forest Products Council for information on sustainable wood products.
All of the exotic hardwoods are extremely dense and may prove difficult to work with. Pre-drilling is a must, as undrilled fasteners will split the boards. Fasteners made specifically for these woods are preferred to nails or screws. Ipe is so dense that it neither needs nor will accept stains or finishes. Other species can be finished with hardwood stains or oils.
Redwood and cedar look better than PT lumber, but only the heartwood naturally resists rot. The light-colored sapwood may rot in a few years unless treated regularly with preservative.
Buying the Right Amount of Lumber
For a small deck (10x12 feet, for example), determine how much lumber you'll need by counting all the pieces of each size—12-foot 2x4s, 8-foot 2x6s, and so on. Add 10 percent to framing quantities and 15 percent to decking to allow for waste.
Lumber lengths are in even 2-foot intervals. Most longer stock is slightly longer than stated—a 12-foot board may measure 144- 1/4", for example. If you will cut several lengths from a board, remember to allow for the saw kerf when estimating.
For larger decks, you can calculate the total square footage of decking you need by multiplying the length of the deck surface times the width. Allow for overhangs. Then buy enough lineal feet of decking to make up the deck area plus the waste allowance. Make actual counts of posts, beams, joists, and other framing members.
Once you get your materials to the work site, protect them from direct sunlight and moisture, especially if you expect your deck building to spread out over several weekends. If your lumber is not kiln-dried, let it dry for several weeks. Stack the boards flat—keep them off the ground with concrete blocks or 4x4s and insert 2x2 spacers (called stickers) between them. The stickers will let air circulate evenly throughout the stack. Kiln-dried lumber is ready to use, but you'll need to protect it from the elements too.
Milled Railings, Post Caps, and Finials
The style of railing you employ in your deck design can enhance the appearance of your deck more than any other element. Square-cut posts and balusters are the most common, of course, and will complement almost any architectural style. You can dress them up with mortises, bevel cuts, and post caps. If you want something more stylish, look at the supply of milled stock carried by your home center.
You might be surprised at the array of styles in different species. Turned balusters are available with numerous configurations and go well with Victorian or classic landscape designs. Milled stock increases your design options geometrically—you can keep the style consistent throughout, or mix various baluster styles with a consistent post design for more variety.
Add style to your railings by adding milled post caps or finials. Finials come in a bountiful number of configurations and are proportioned to fit 4x4 or 6x6 posts. Some come complete with lag screws that only require predrilling the posts. Other styles need drilling for screws or dowels. To keep rainwater from rotting the top of the post, caulk the bottom edge with silicone before tightening the finial.
Post caps cover the tops of the posts completely and shed rain from the end grain. You'll find them fluted, corniced, and chamfered—in styles that will match any deck design.