Expert advice on how to clean, restore, refinish and maintain a wood deck, including protecting against wood rot, how to acid wash a deck, and apply a finish to decking.
A new wood deck provides a warm, handsome connection with the outdoors, adding to a home’s beauty and setting the stage for a world of outdoor activities. But decks take a lot of abuse, both from use and through continued exposure to weather. After a few years, without proper care, they become weathered and uninviting.
Fortunately, most deck surface problems are cosmetic, not structural, because decking is built of durable woods, usually redwood, cedar, or pressure-treated pine. Redwood and cedar heartwoods have a natural resistance to termites and decay; pine is pressure-treated with a pesticide to give it insect- and decay-blocking power.
Even so, ultraviolet radiation (UV) from the sun breaks down surface fibers and lignin, causing graying and surface erosion. Moisture encourages surface mildew and causes stains, particularly in damp or humid climates where surfaces never completely dry. And natural extractives in redwood and cedar can discolor the surface.
The road to restoring a deck’s great looks is usually simple, regardless of the type of wood. First, you clean it, and then you diagnose and treat any discoloration. Last, you protect the result with a durable finish.
If you’re working on a deck that is above a usable area, such as a second-story deck above a patio, before beginning be sure to protect the area below it with plastic sheeting or dropcloths.
Give your tired-looking deck a facelift sooner rather than later. Wood that is left unprotected will soak up moisture, leading to serious damage. Because decks are exposed to regular abuse—harsh sun, foot traffic, rain, snow, and ice—you may need to treat your deck periodically, perhaps even once a year.
Inspect a Wood Deck for Rot
If your deck was built with pressure-treated lumber rated for ground contact, the wood will be very resistant to rot. If it was built of untreated redwood, cedar, or other wood, there is a greater risk of rot, particularly if the wood is light-colored sapwood rather than dark-brown heartwood.
Just about any wood, however, can rot if it stays wet long enough. (Look at tags at the ends of boards, or printed stamps on boards, to find out about your wood. A deck may be made from several types of lumber.)
Take the time to examine all the wood carefully to see if any boards are rotten. Unfortunately, most wood rot occurs in places that are hard to see—under the decking boards, at the ledger (the board that is attached to the house), on the underside of stair treads, and so on. If possible, crawl underneath the deck to make your inspection.
If the rot is less than 1/2 inch deep, the board can probably be left in place. More extensive rot calls for a replacement board. Use a flat pry bar to carefully remove rotten boards. Replace them with rot-resistant wood. If nails or boards are popping up or coming loose, do not pound the old nails down again; remove them and replace them with longer nails, special decking nails, or decking screws.
If any part of your deck stays wet for a day or more after a rainfall, take steps to see that it can dry out. You’ll probably need to use a leaf blower or a broom to sweep away leaves and dirt from between boards or where the deck meets the house. Perhaps a bush or tree limb needs to be trimmed back, or a gutter downspout moved to direct water away from the deck.
Debris between deck boards looks bad and soaks up water, promoting rot. Use a putty knife or, even better, a laminate scoring tool like the one shown here. Sweep away the dirt and debris, using a sturdy broom. to clear out the debris.
Next, clean the wood. Thoroughly scrub the surface with a sudsy mixture of water and laundry detergent, using a stiff fiber brush on a long handle. Work in small areas and rinse periodically. This may be all it takes to return much of the wood’s natural tone.
For a large deck, you can rent a power washer that delivers up to 1,200 psi of pressure to blast and rinse the decking. It should have a nozzle that fans an arc of about 25 to 40 degrees. Wearing safety goggles, hold the nozzle about 6 inches above the deck’s surface and spray slowly in line with the wood grain, overlapping your path.
After washing the deck’s surface, allow the deck to dry for several clear days before applying a finish.
First use a leaf blower or broom to blow all of the debris off of the deck’s surface and then wash the deck.
To clean a deck, purchase a commercial deck cleaner or TSP heavy-duty cleanser and follow directions—usually this means scrubbing and rinsing. Be sure to wear rubber gloves and safety glasses. You may need to repeat the process.
Using a stiff fiber bristle brush on a broom-type handle, thoroughly scrub the surface with a sudsy solution of water and detergent. Rinse with clear water and allow to dry.
If you decide to use a pressure washer, get one that delivers 1,200 psi (pounds per square inch) of pressure or less and has a spray nozzle that fans an arc of abut 25 to 40 degrees. Be careful—the powerful spray can erode soft wood grain. Allow the deck to dry for several days before applying any type of finish.
Over time, the beauty of a natural wood deck can lose its luster as the ultraviolet rays from the sun break down the wood’s structure. If the deck has taken on a dull gray color, the problem may easier to solve than it looks.
Dark stains may be due to mildew; cleaning with soap and water or bleach will clear up the problem. Several products are available for dealing with discoloration and stains. Commercially available powder or liquid concentrates have a base of non-chlorine bleach or oxalic acid; a detergent may be part of the formula. Bleach-based products eliminate mildew. Acid-based products handle graying and stains. Some products may darken woods such as redwood and cedar, so be sure to test any product in an inconspicuous place.
Test for mildew by applying a drop of undiluted liquid household bleach to a small, black spot. If the spot disappears after a minute or two, clean the deck with a mild cleanser (no ammonia) and rinse with a solution of 1 part household liquid bleach to 4 parts water, and then rinse. For stubborn mildew, mix 1 cup trisodium phosphate (TSP) and 1 cup household liquid bleach in a gallon of water and scrub with a stiff brush. Rinse after about 15 minutes.
Non-mildew stains are often caused by natural wood extractives or corroding hardware and nails. Familiar, general graying of the decking is usually the result of surface wood cells that have been broken down by UV radiation and wear. Bleach used to kill mildew can also leave a surface drab and washed out.
Always wear rubber gloves, goggles, and old clothes when working with these chemicals, and follow the directions explicitly. Never mix detergent containing ammonia with household bleach; the resulting fumes can be highly toxic.
For these problems, an acid-based deck restoration product is best. You can buy a pre-mixed oxalic acid deck cleaner or purchase oxalic acid crystals from a hardware store or home improvement center and mix a solution of 4 ounces crystals to 1 quart water in a non-metallic container. Wearing rubber gloves, eye protection, and old clothes, apply with a rag one board at a time and scrub with a soft brush. Allow to dry, and then rinse with water.
You can strip and clean a deck that has an existing stain finish using a commercial “deck scrub” that you brush in and rinse off. Follow the label directions.
Caution: Never mix detergent containing ammonia with household bleach; the resulting fumes can be highly toxic.
After cleaning a deck, allow it to dry thoroughly before sealing it, but don’t wait too long or it will get dirty again. Determine if it needs to be sealed by sprinkling a few drops of water on the wood. If the water doesn’t bead up and instead soaks right in, the deck needs to be sealed.
Consult with a paint supplier or other expert to choose the finish best suited to the lumber used to build your deck. It should contain either oil or paraffin to keep moisture from soaking in. In addition, it should have UV blockers or pigment (color) to keep rays of the sun from turning the deck gray, unless that’s the look you want. If your home is subject to wood-eating bugs, see that the finish contains insecticide as well.
Apply a finish using a paint roller, brush, or pump-type sprayer. Stand back every few minutes and examine the finish to make sure the color is being applied evenly.
Though some people paint decks with deck paint, most choose to take advantage of the natural beauty of expensive decking woods by applying a clear or lightly stained finish.
If you do choose to paint, use a stain-blocking oil or alkyd primer first.
In general, the best finishes are those that soak into the wood, not those that provide a surface film. A heavily pigmented, solid stain isn’t really recommended for decking because it shows wear patterns and may tend to peel. You want something that really soaks in.
There are three important characteristics to look for in a finish.
- It should be water repellent or waterproof, not just water resistant.
- It should offer UV protection.
- If mildew is a potential problem, it should contain a mildewcide, which a wood preservative does. Regular preservatives should be reapplied once a year; newer and better “toner” products offer more UV protection and may last up to four years.
Whatever product you choose, read the label to be sure it’s right for your deck. Buy quality materials and follow the manufacturer’s directions for application. Your result will be a deck that provides you with years of lasting beauty and enjoyment.