Lumber for Decks
Pound for pound, wood is almost as strong as steel. Its warm, natural beauty and remarkable workability make it ideal for decks and other outdoor structures.
The uses of lumber are endless. It can be used to add privacy screens and planters, create ornate railings, build ornamental trellises for climbing plants, or ease the transition between one area of the yard to another with wooden steps.
If you are extending a deck, the lumber you use will probably be similar to the existing structure. But if you’re planning to build a shade structure or a deck addition— or to add decorative elements not tied to the deck structure—your lumber choices become more flexible.
Not all woods are alike, of course, so guide your selection by ranking appearance, cost, and durability at the top of your list of lumber-selection “rules.” Whatever the scope of your plans, here are some tips to keep in mind when making your selection.
Choosing the Right wood
Wood used on exterior surfaces must be rot-resistant; several species are suitable for outdoor use. For example, pine, hemlock, or fir will start to fall apart after a few years if not treated or painted. These woods are fine if a painted surface complements your overall design and if you like how paint looks. But if you want the beauty of the wood grain revealed, you need a naturally resistant species or pressure-treated wood.
Wood has two main enemies—rot and bugs. Rot (even “dry rot”) occurs for the most part when wood remains damp for a long time. Termites and other wood-eating insects are often attracted to moist wood. They usually live in the ground and tunnel in through your structure to get food.
Protect against rot and bugs by using pressure-treated lumber or resistant species wherever moisture may be a problem. To protect structural wood from damage, take steps to ensure that it can dry out; sometimes just sweeping away dirt can do the trick. You can also brush the wood with sealer that contains preservative and insecticide.
You can count on the appearance and durability of naturally rot-resistant redwood, cedar, and cypress. These woods are more expensive than others (redwood is the most costly), and their cost (as well as that of any species) increases the farther you live from their natural growth areas.
REDWOOD: Beautiful and expensive, redwood is used when cost is not an issue. One of the species used as untreated lumber, redwood has a close grain and is naturally resistant to weathering, warping, cupping, and shrinkage. Its more than 30 grades are based on appearance, strength, and color.
Be careful! Not all redwood is long lasting. The dark heartwood resists rot and insects, but the cream-colored sapwood can be seriously damaged in just a couple of years. Often the two are mixed—in the same board and in the same bin.
“Common” redwood, often sold as “construction common,” is partially composed of sapwood. Grades that use the term “heart,” such as “B heart” or “construction heart,” are heartwood grades with some knots and are as rot-resistant as heartwood grades that are “clear.” Common redwood that has been treated is nearly as rot-resistant as pressure-treated lumber.
If you use heartwood, you can let redwood “go gray,” meaning that you apply no stain and let it weather to a silvery color.
CEDAR: Less expensive than redwood, cedar has a lighter color and is generally regarded as less attractive. If you let it “go gray,” it will not have the stately sheen of redwood. It works easily, but it’s more likely to split than other woods. Because cedar does not have as many grades as redwood, its quality varies within a grade. Don’t rely on grading alone as an indication of quality. Inspect every board for knots and imperfections.
Like redwood, only the darker-colored heartwood of cedar is rot-resistant. Unfortunately, most of the cedar sold today is sapwood, and many homeowners are dismayed to find their cedar decks rotting within a few years. If you use cedar for decking and rails, make sure it can dry out between rainfalls, and give it a thorough coating of sealer-preservative.
CYPRESS: Similar to cedar in several respects, cypress grows in swampy areas of the South, a reason for its natural resistance to rot and decay. Lightweight, strong, and easy to handle, it also weathers to a silvery gray.
The characteristics of the wood you buy depend on the type of tree it came from, how it was cut, and how it was handled.
Vertical and flat grain: The best lumber comes from old trees, with fewer knots than younger trees, and trees that have tight rings for a close grain in the lumber. Certain species produce wood that is resistant to warping and rot, whereas others do not. Much lumber sold today comes from quick-growth trees, so the grain is not tight and the boards develop twists if not stored and installed carefully.
There are two basic methods of cutting wood:
- Plain sawing, in which the boards are cut roughly parallel to the growth rings, produces flat-grained wood, with a pattern of widely spaced wavy lines, V shapes, and ovals. This lumber can be strong, but it is prone to cupping and warping.
- Quarter sawing—cutting perpendicular to the rings— produces vertical grain in closely spaced parallel lines. As long as it has few knots, this type is stronger and more stable than plain-sawn (flat-grained) lumber, making it more desirable for finish work.
But if there is a spike-shaped knot extending across much of the width of a board, the board will be seriously weakened. Quartersawn lumber tends to be expensive because it often comes from old-growth trees, and much of the log becomes sawdust and waste wood.
wood movement and moisture:
Wood changes with the climate. Freshly cut wood shrinks until it is dry, and dry lumber expands and contracts with changes in humidity. However, given its longitudinal grain, wood can change significantly in width but hardly at all in length. Most lumber is air-dried after it is cut. This removes much of the moisture, but leaves enough that wood may shrink, bow, and twist after you buy it. For framing and rough work, air-dried lumber is adequate as long as you stack the wood flat and fasten it in place before it has a chance to move.
Kiln-drying removes more moisture, so the boards are more stable. Dry lumber (S-DRY or MC 15) is less likely to warp, works more easily, holds fasteners more tightly, and finishes better. This is your best choice for trim work on your deck.
Pressure-treated lumber: A less expensive substitute for redwood, cedar, and cypress, wood treated with various chemicals under pressure is extremely rot-resistant.
Both chromated copper arsenate (CCA)— identifiable by its green tinge—and ammoniacal copper arsenate (ACA) have been used widely, but wood treated with these and other arsenic compounds is no longer available for residential use. Ammoniacal copper quaternary (ACQ) is a newer preservative with no hazardous ingredients. Grade stamps on the lumber tell you which chemical has been used.
Grade stamps also tell you how much preservative the lumber holds. Posts and boards (skids, joists, and skirts) that are in contact with the ground should have a retention level of 0.60 or higher. Other components can be built with lumber treated to 0.40. Look for a “Ground Contact” or “LP-22” stamp, or both.
SYNTHETIC DECKING: Made of wood byproducts, wood and plastic, or 100 percent plastic, composition materials offer long life and extreme rot and insect resistance. They cost more initially, but require less maintenance over time. Some varieties snap together, making installation easy.
Working with Pressure-Treated Lumber
Pressure treatment is not a substitute for finishing the wood. A couple of weeks after construction, you will need to apply a water-repellent sealer—several coats on surfaces you have cut.
After it dries for two to three months, apply paint or stain. Refinish pressure-treated stock from time to time. An unfinished structure built with treated wood will gradually turn to a pleasant, weathered gray but may not last nearly as long.
Wear gloves when handling treated wood—and a respirator when sawing it. Avoid burning or burying the scrap. Check with your local environmental agency for proper disposal methods.
Lumber is divided into categories according to its thickness. Boards are less than 2 inches thick, and dimension lumber is 2 to 4 inches thick. Lumber is graded for its strength and appearance following standards established by independent agencies.
BOARDS: Fir and pine boards are graded in two categories—select and common.
- Select: This is the best—with few or no knots.A: contains no knots B: has only small blemishes C: has some minor defects D: has larger blemishes that can be concealed with paint
- Common: Utility grades are ranked from 1 to 5 in descending quality. A middle grade, such as no. 3, is a good choice for many decking projects.
Grade Stamps: what They mean
Manufacturers stamp their wood products to provide customers with information about the species, prevalence of defects, grade, and moisture content. A grade stamp may also carry a number or the name of the mill that produced the lumber and a certification symbol that shows the lumber association whose grading standards are used.
Pressure-treated lumber carries a grade stamp that shows the year it was treated, the chemical used as a preservative, exposure condition (whether it can be used above ground or with ground contact), and the amount of chemical treatment it received. Plywood grade stamps also show whether the wood is suitable for ground contact or only for aboveground use, and whether it can be used as sheathing. The stamp also designates the thickness of the sheet and the distance it can span. If the plywood is made to withstand exposure to the elements, look for a stamp that says “EXTERIOR.”
DIMENSION LUMBER: Fir and pine dimension lumber grades are:
- Construction grade: top of the line
- Standard grade: almost as good but cheaper than construction grade
- Utility grade: low-quality stock, unsuitable for framing
Pressure-treated lumber also comes in a variety of grades, from those treated for ground contact for posts and retaining walls, to lighter treatments for fences or decking boards. Redwood lumber is classified in “garden” or “architectural” grades. As a rule, buy standard-grade dimension lumber and common-grade no. 3 boards or better if you can afford it. You may choose to save money by buying a cheaper grade and painting it after filling holes and sanding If you want a rustic look, you may choose a lower grade.
Most wood is either air-dried or kiln-dried before sale. One of three marks tells you its surface moisture content: S-GRN (green lumber), more than 19 percent; S-DRY, up to 19 percent; MC 15, up to 15 percent. For framing and rough work, air-dried lumber is adequate. Dry lumber (S-DRY or MC 15) is less likely to warp, works more easily, holds fasteners more tightly, and finishes better.
Manufacturers produce plywood (a sheet material made from thin layers of wood that are glued together for strength) in a variety of sizes, thicknesses, textures, species, and grades. Any plywood used for outdoor construction must be an exterior-grade material—made with glues that will not deteriorate when exposed to moisture—so the veneers stay tightly bonded and the sheets remain flat and strong.
Purchase AA exterior grades for sheathing that you will stain or paint or that will be visible from both sides. Lesser grades have blemishes that will show through the finish but will work just fine for sheathing covered with another material, such as shingles.
The plywood grade stamp gives you most of the information you need to make purchases. It shows whether the wood is suitable for ground contact or aboveground use, whether it can be used as sheathing, how thick it is, and the distance it can span.
Plywood is stiff and rigid, so it doesn’t absorb or filter winds. That’s why it should be affixed to a framework that’s strong enough to support not only its weight but the lateral forces of wind shear.