Cleaning a Wood Deck Without Splintering the Surface
Dirt does not damage a deck. The method used to remove it does. Pressure damages the surface fibers, and with a strong cleaner it can take wood with it. A wire brush leaves iron behind, which stains tannin-rich wood. A coating applied before the wood has dried raises the risk of blistering and peeling. Clean with the mildest product that lifts the soil, a non-metallic bristle brush and enough water to rinse it out — then expect the grain to rise, and deal with it before you coat.
The method is set by the species and by the finish already on the boards. The deck and fence guide owns the prior question — whether the structure is sound enough to be worth cleaning — and this page owns the surface.
Identify what is on the boards
Four conditions look similar from standing height and each needs a different product. The Forest Service’s finishing chapter and the cedar association publish the same test: put a drop or two of household bleach, 5% sodium hypochlorite, on the discoloration and watch. Mildew bleaches out within seconds to half a minute; dirt, extractive bleed and iron stain do not change color.
| What you see | What it is | What removes it |
|---|---|---|
| Green, gray-green or black patches, powdery or slimy | Mildew and algae | A mildew cleaner or diluted oxygen bleach, scrubbed and rinsed; oxygen bleaches are less likely to damage wood than chlorine-based ones |
| Brown streaks with the grain, where water reaches the wood | Extractive, or tannin, bleed driven by moisture | A mild oxalic acid solution, which removes water, tannin and iron stains |
| Blue-black patches around screw heads, or where a wire brush or steel wool was used | Iron stain, from iron reacting with the wood’s tannins | Oxalic acid, then thorough rinsing; the stain returns if iron remains |
| Gray, dry, fibrous surface, no growth on it | Weathered wood fiber, its color removed by sunlight | Nothing restores the color; the gray layer goes by sanding, or by a cleaner that takes wood with it |
The last row catches people. Weathering removes colored components from the surface, the Forest Service states, so color comes back by staining, not cleaning. The cedar association warns that some cleaners remove wood from the weathered surface — a product that appears to restore gray boards may be sanding them chemically.
Why the surface goes rough
Wet wood does not stay the shape it was planed to. Planing presses the dense latewood bands down into the softer earlywood and crushes it, the Forest Service explains; when the board gets wet, the crushed cells spring back and push the hard bands out, and the face becomes bumpy. If a film has formed before the grain releases, it is prone to cracking.
That is why a deck can feel rougher the day after it is cleaned than it did before. The Forest Service’s diagnosis needs no instrument: wet the wood, let it dry, and feel for bumps. One wetting is usually enough to release the grain; once the wood is dry again it can be coated normally, and sanding is what smooths it. So the sanding has to happen after the wood dries and before the coating goes on.
The pressure washer question
The authorities part company here, and the split follows the material.
The Western Red Cedar Lumber Association does not recommend power washing on a cedar deck “or any other cedar product”, stating that power washers “can seriously damage cedar deck surfaces”; its longer finishing guide puts the rule absolutely, that power washing “should never be used to clean cedar decks”.
For pressure-treated lumber, one preservative maker publishes a number instead of a prohibition. Its guidance limits a pressure washer to experienced users or professionals and, if one is used, keeping it “under 500 psi” with a fan tip about 18 inches from the deck. The Forest Service sits nearer the preservative maker than the cedar association: if a power washer is used on wood, “keep the pressure low to avoid damaging the wood” and keep the stream pointed down.
All three protect the same thing. The Forest Service describes what a strong cleaner and a washer do together: some cleaners pulp the wood surface, “and subsequent power washing removes the pulped surface”. The color is restored because wood has been removed — the result sanding would give, reached by accident instead of control. The method comparison covers the machine itself.
Brushes, and the one metal to keep away
Both the Forest Service and the cedar association say never to use steel wool or wire brushes on tannin-rich wood: iron left in the surface reacts with the wood’s tannins into a dark blue-black stain. The reaction needs wood moisture above about 16%, which is why iron stain is an outdoor problem.
Use a stiff bristle brush with non-metallic filaments, and scrub along the grain rather than across it. The cedar association’s instruction for weathered cedar is exactly that: sand, brush “with a non-ferrous bristle brush”, and wash. For boards that are merely dirty, the Forest Service’s preparation advice for a lightly pigmented finish is a stiff-bristle brush.
Choosing a cleaner
Deck cleaners are not one strength. The Forest Service sorts the common classes by what they do to wood:
- Oxygen-based cleaners, usually sodium percarbonate, are the active ingredient in many commercial deck products. The Forest Service states that oxygen bleaches “remove mildew and have been reported to be less likely to damage wood surfaces than chlorine-based bleaches, particularly with low-density woods like western redcedar, Alaska yellow-cedar, and redwood”.
- Chlorine bleach is the most familiar mildew treatment and the most contested. The Forest Service publishes a dilution — one part household bleach to five parts water, and “in no case” stronger than one to three — and warns never to mix it with ammonia or an ammonia-containing cleaner, because the combination is toxic. One preservative maker says never to use household bleach at all on treated lumber, because it “can strip the preservatives and damage the wood fibers”.
- Sodium hydroxide cleaners are the strong end. The Forest Service calls sodium hydroxide “a strongly alkaline chemical that pulps wood”, says such cleaners may be needed where mildew is embedded in a finish, and that they “should be used only as a last resort”.
- Oxalic acid is a stain treatment, not a cleaner. The Forest Service states it removes extractive bleed and iron stains but “is not effective for removing mildew”, gives a saturated solution for iron stain, and requires thorough rinsing, because traces of iron left behind let the stain return. The cedar association adds that it can be harmful and its instructions are to be followed.
Two rules cover the gaps. Never mix cleaning products, and read the container: the label sets the dilution, the dwell time and the protection for that product.
Rinse, dry, and the number that decides
Rinsing is part of the method, not the cleanup. The Forest Service’s instruction is to rinse thoroughly with a garden hose, keeping the stream pointed down so water is not driven up behind the boards. Where that water goes is a separate question with rules of its own, covered in the wash-water page.
Then the wood has to dry, to a figure the coating maker sets rather than a feeling. One deck and siding stain’s technical data sheet states the wood’s moisture content “should not exceed 15% to allow for proper penetration”, and asks for air and surface temperatures between 40°F and 90°F. The cedar association sets its own limit for refinishing at below 20%. The Forest Service explains why the figure matters more than the schedule: most wood products are sold at 20% moisture content or less, and “painting wet wood increases the risk of blistering and peeling”.
A moisture meter reads the number directly. Without one, the coating maker publishes a substitute: a few drops of water on the prepared surface, which should penetrate quickly if the wood is ready.
The documents also disagree about why a smooth new deck needs sanding. The coating maker instructs sanding new decking, or treating it with a brightener, “to break the mill glaze” and allow penetration. The Forest Service reports that its scientists could not reproduce mill glaze, and that the failures blamed on it came from raised grain, weathering and moisture. The sanding is right either way; the Forest Service’s reason is the one you can see on your own deck.
A sequence that keeps the surface
- Sweep, and clear the gaps. Debris between boards holds water against the wood; the preservative maker asks for the gaps to stay clear so rain can carry it away.
- Test a small area with a drop of bleach to establish whether the dark patches are mildew, and choose the cleaner from that answer.
- Apply the cleaner at the label’s dilution, scrub along the grain with a non-metallic bristle brush, and rinse one section at a time.
- Rinse thoroughly, stream pointed down, and settle where the water is going before it reaches a drain.
- Let the wood dry to the coating maker’s limit, and check it rather than assume it.
- Sand the raised grain if the deck is to be coated, once the wood is dry.
- Coat inside the maker’s temperature and moisture window. One preservative maker advises applying a coating within one week of cleaning.
When cleaning is the wrong job
Cleaning is maintenance on sound wood, not a treatment for a deck that has stopped being sound. Soft timber at a post base, a board that gives underfoot, a ledger pulling away, fasteners standing proud or rusted through, a guard that moves, or a film finish peeling in sheets come before the surface, and none is improved by scrubbing. The deck and fence guide sets out what they mean and when the work belongs to a qualified professional.
If the deck carries old paint, sanding or scraping can release lead dust; the coating maker’s label carries that warning, and lead-safe preparation is a different subject with different rules.
The next useful action
Clean once a year, in spring, as the cedar association advises, and read the structure in the wet season, when the evidence is visible. If the deck is sound, the work is a brush, a cleaner matched to what you found, a thorough rinse, and enough patience to let the wood dry before anything goes over it.