Part 5 - Cleaning Wood Decks

A wood deck requires regular maintenance just like any other major part of a home. Regular cleaning and application of a high-quality stain and sealer will help protect your deck from the weather and ultraviolet ray damage caused by the sun and will extend the usable life of the deck.

Wood deck cleaners come in various ingredients and concentrations for varied applications—spray, scrub and power wash. The purpose of wood deck cleaners is to remove dirt, mold, algae and oxidation (graying).

Because most deck cleaners raise the pH in wood, deck brighteners are important to neutralize cleaners to leave the wood in a neutral state.

Cleaners containing chlorine bleach are commonly used to clean decks but they are not recommended for cleaning treated wood. Excessive use of chlorine bleach containing cleaners (sodium hypochlorite or calcium hypochlorite) can damage treated wood leaving it with an unnatural whitewashed appearance. Chlorine bleach also can raise the wood fibers and cause a fuzzy-looking surface.

Chlorine bleach breaks down the lignin in wood causing excessive damage to wood cells.¹ The natural pH of wood is slightly acidic, and bleach is a basic solution. As a result, use of bleach on wood shifts the pH from near neutral pH to a basic pH that will damage the cellular structure.²

Bleach corrodes metal fasteners, screws and nails.
Bleach can cause damage to surrounding plants.
Bleach, after several months, lightens wood's natural color.
Bleach does not eliminate the spores from which mold and mildew grow.

Bleach Alternatives

do not use chlorine bleach on wood decks

Hydrogen peroxide is environmentally friendly and a safer substitute than bleach. Its chemical formulation is H₂O₂. As the oxidizing process releases the excess molecule of oxygen, H₂O (water) remains as its residue. For tough jobs, hydrogen peroxide kills the mildew on contact within several minutes.
Oxygen bleach is sodium percarbonate, an environmentally friendly and excellent detergent and bleaching agent with a hydrogen peroxide base. Sodium percarbonate is a cleaning and bleaching agent with a strong fungicide effect. It has been tested and found to be more effective attacking the deeper-rooted organic growth on the porous wood surfaces.
Sodium percarbonate is a white particle powder, nontoxic with no contamination, nonflammable, non-explosive, and soluble in water. It is also biodegradable and leaves no harmful by-products or residues that can harm the environment. Sodium percarbonate is effective in cleaning most average wood preparation jobs.
Sodium percarbonate is a chemical formulation is an adduct of sodium carbonate ("soda ash" or "washing soda") and hydrogen peroxide (that is, a perhydrate) whose formula is written as 2 Na₂CO₃ · 3 H₂O₂.
Apply oxalic acid (deck brightener) after using sodium percarbonate or hydrogen peroxide in the cleaning process. The deck brightener will restore the wood to its natural pH and neutralize the sodium percarbonate cleaner. Rinse well. This will provide a beautiful finish when applying the final stain.
The formula of oxalic acid is C₂H₂O₄. Its usual form is that of the crystalline hydrate, (COOH)₂·2H₂O.³
Deck brighteners are designed to neutralize and brighten wood without damaging wood fibers.

Download top rated deck cleaners according to consumer ratings in 2020. Download the updated 2023 list.

Basic Deck Cleaning

There are a number of commercial products in the marketplace that are recommended for the cleaning of preservative

treated wood decks. For best results always follow the manufacturer’s instructions. The following suggestions are provided as supplementary guidance.

Clear the deck of all furniture, grills, etc., and the surrounding area of all debris and obstacles to create a safe work zone.
Remove all debris trapped between deck boards and the edge of the house and sweep or blow the deck to remove all debris.
Prepare the surrounding area and protect your shrubs and plants with a plastic drop cloth. Spray water on plants in the surrounding area to dilute any over-spray of the deck cleaner that lands on desirable plants.
For mild stains and dirt use a mild dish detergent diluted in a bucket of water. Mop a small area of the deck surface with the detergent and then use a stiff bristle brush to work the dirt free from the surface. Rinse the solution with a garden hose and re-clean areas as needed.
For more severe stains and dirt use a deck cleaner. Before use carefully mix/stir the product in accordance with the manufacturer's directions. Use eye protection and rubber gloves as directed.
Apply the deck cleaner according to manufacturer's instructions.
Unless directed otherwise by the manufacturer, apply the cleaner only to the amount of deck surface you can work at one time. Work in sections and let the deck cleaner do its work. Many cleaning solutions should not be allowed to dry on the wood so periodic spraying/misting may be required.
Let the cleaner set on the deck boards for the time period recommended by the manufacturer.
Once the cleaner has worked for the specified time, use a hard-bristled broom with synthetic bristles to scrub the deck clean. Scrub parallel with the wood grain at all times.
Rinse well with water and repeat the process on the next section of the deck.
Once cleaning is completed inspect your work. The surface should be consistently clean and unmarred or damaged. Re-clean any stubborn areas that still look dirty.

Power Washing Your Deck

Use the lowest possible pressure that effectively cleans the surface.
Always start by pointing the spray away from people and glass windows and at least 24" away from the wood deck. In general, avoid spraying closer than 16-20 inches unless the pressure is very low.
Use a fan-tip only, set for an angle of spread between 40 and 60 degrees.
Never use a narrow stream or a rotating “tornado” type of tip.
Once the fan spread is properly set, slowly begin feathering the spray approximately 18 inches from the deck.
Test your spray in an inconspicuous area and not the primary deck surface.
Sweep the sprayer along the deck boards, maintaining a consistent distance between the tip and the deck surface.
Start cleaning deck boards closest to the house and work from the house outward to the far edge of the deck.
Work with the grain by feathering the spraying lengthwise with the deck boards and overlapping each area slightly. The objective is even cleaning with no visible differences on any board.

Cleaning Mold on Your Deck

Maintain a deck that is dry and clean.
Ensure gutters/downspouts and dryer vents do not discharge directly on decks.
Ensure adequate ventilation under and between decking boards.
Minimize water puddles under decks and the use of wet mulch up against the deck structure.
Cleaning a deck just after the last of the major pollen events will minimize the seasonal outbreak of mold and mildew.
Periodically rinse off your deck using a garden hose with a spray nozzle, especially after the major pollen events. Skilled professionals may use pressure washers with wide fan tips but in the wrong hands, your deck can be damaged. Exercise extreme caution when using pressure washers.
Ensure the gaps between the decking boards remain free of debris so that rain showers can remove pollen and organic debris between cleanings.
Avoid fertilizer over-spray.

References and Resources

* Disclaimer: The information contained in this bulletin is provided in good faith and should be used for general information only. Viance, LLC makes no representations or warranties of any kind, express or implied about the completeness, accuracy, suitability and fitness for purpose of the deck cleaning products mentioned in this report. Viance, LLC expressly disclaims, and does not undertake or assume any duty, obligation, or responsibility for any decisions, reactions, responses, actions, failure to act, by you or any other person or party as a result of or in reliance on, in whole or in part, the information contained in this bulletin, or for any consequences or outcomes arising from or caused by any such decisions, reactions, responses, actions, or failures to act. This bulletin does not cover other products or surveys that may have been conducted this year and does not imply other products are not as effective.