After a summer of outdoor barbecues, pouring rain, relentless sun, and blowing debris, a wood deck usually looks nothing like it did in spring. Left alone, that film of dirt, mildew, and algae holds moisture against the boards through autumn and winter, which is exactly when rot gets a foothold. A good cleaning before colder months arrive is cheap insurance, and it does not have to mean expensive commercial products.
Four homemade recipes cover most decks: a mildew and algae cleaner built on trisodium phosphate, a soapy scrub for oily stains, an oil soap blend for stubborn growth, and a powdered oxygen bleach wash for light maintenance. Before mixing anything, it helps to know what kind of structure you are maintaining. The continuous multiple span deck common on attached homes behaves differently from a freestanding one, and the way water drains across the framing affects where mildew collects in the first place.
When and Why Decks Need a Seasonal Cleaning
Wood decks accumulate three enemies during a season of use: mildew, algae, and embedded dirt. Mildew appears as dark speckled patches in shaded, damp corners. Algae shows up as green streaks on the surface. Dirt and grime build up in the grain, especially in high-traffic paths and under furniture. All three trap moisture against the wood, and moisture trapped against wood is the precondition for decay.
The worst damage usually hides in the spots you cannot see from the kitchen window: under planters, behind grills, and along the low side of the deck where water lingers. Cleaning before winter matters for a specific reason: the freeze-thaw cycle. Moisture trapped under a layer of grime freezes and expands, which works the boards apart at the grain and opens the surface to deeper staining. A fall cleaning removes the grime layer so the wood can dry out before the first hard freeze.
Maintenance intervals matter because how long a treated wood deck lasts depends less on the preservative in the lumber than on how consistently the surface is cleaned and resealed. A deck that gets one thorough cleaning a year and a fresh sealer every two to three years will outlast a deck that is never touched.
Signs a Deck Is Due for a Wash
- The surface feels slick or slimy after rain.
- Dark patches or green streaks are visible in shaded areas.
- Water no longer beads up on the surface.
- A musty smell lingers around the deck after wet weather.
- Dirt is ground into the grain along traffic paths.
Before You Mix Anything: Prep, Safety, and Choosing Ingredients
The right cleaner depends partly on the decking material. Pressure-treated pine, cedar, and composite boards all respond differently to bleach-based washes, and the pressure treated vs cedar wood comparison is a useful starting point: cedar is naturally rot resistant but softer and more easily etched by harsh cleaners, while pressure-treated pine takes more aggressive washing without damage.
The Safety Routine
Whichever recipe you choose, follow the same safety routine:
- Test the cleaner on a small, inconspicuous corner first and check the result after rinsing.
- Protect nearby plants. Bleach kills vegetation, so wet down garden beds or cover them with plastic before washing.
- Wear gloves, eye protection, and clothes you do not mind bleaching.
- Never mix bleach with ammonia or acidic cleaners. The combination releases toxic chlorine gas.
- Work outdoors with the wind at your back, and rinse surfaces before the solution dries.
Choosing Between Bleach and Oxygen Bleach
Household bleach (sodium hypochlorite) kills mildew and algae fast and is the cheapest option, but it can lighten wood and kill plants on contact. Powdered oxygen bleach (sodium percarbonate) releases hydrogen peroxide when mixed with water: gentler on wood and vegetation, slower acting, and a better fit for routine cleanings between deep washes.
Four Homemade Deck Cleanser Recipes
The four recipes below use common household ingredients and cover the full range of deck conditions, from heavy mildew to light surface dust. If the deck sits over a crawl space or storage area, check the under deck drainage systems before you start, because the runoff from a bleach wash has to go somewhere and standing water under the deck will undo the cleaning above. The table summarizes each recipe, and the sections that follow give the full mixing instructions.
| Recipe | Key Ingredients | Best For | Notes |
|---|---|---|---|
| Mildew and Algae Cleaner | TSP, household bleach, warm water | Heavy mildew, mold, dirt, algae | Soak 10 to 15 minutes before rinsing; also works on composite |
| Soapy Deck Scrub | Dish soap, household bleach, warm water | Oily stains and grime | Protect plants; rinse thoroughly |
| Oil Soap Mildew Cleaner | Oil soap, bleach, rubbing alcohol, warm water | Stubborn mildew and algae | Alcohol and soap lift growth after bleach kills it |
| Powdered Bleach Scrub | Oxygen laundry cleaner, dish soap, hot water | Light cleaning, wasp prevention | Preferred for pressure washer use |
Recipe 1: Mildew and Algae Cleaner
Mix 1 cup trisodium phosphate (TSP), 1 cup household bleach, and 2 gallons of warm water. Hose the deck down to soak the wood, apply the solution, and scrub with a long-handled brush or stiff broom. Let it sit for 10 to 15 minutes, then rinse with clean water. This one solution kills mold, dirt, algae, and mildew in a single pass, and it doubles as a homemade composite deck cleaner.
Recipe 2: Soapy Deck Scrub
Combine 1/4 cup of ammonia-free liquid dish soap, 2 quarts of household bleach, and 2 gallons of warm water. This is the best homemade wood deck cleaner when the deck suffers from oily stains, dirt, and grime. Protect nearby plants before applying, scrub the surface, and rinse everything away when done.
Recipe 3: Oil Soap Mildew Cleaner
Mix 1 gallon of warm water, 1 quart of household bleach, 2 cups of rubbing alcohol, and 2 tablespoons of a mild oil soap. The bleach kills algae and mildew, while the alcohol and soap lift the dead growth off the wood. Scrub the mixture in, let it soak, and rinse it away.
Recipe 4: Powdered Bleach Scrub
Stir 2 cups of powdered oxygen laundry cleaner into 2 gallons of hot water until completely dissolved, then add 1/4 cup of liquid dish soap and stir gently. Besides killing algae and mildew, this solution discourages wasp nests and keeps yellow jackets away. It suits decks in good condition without major stains and works well for light cleaning between yearly scrubbings, and it is the preferred homemade deck cleaner for a pressure washer.
Step-by-Step: Cleaning a Deck From Start to Finish
A full cleaning takes an afternoon when the weather cooperates. Work on a dry day with temperatures above 50 degrees Fahrenheit so the wood can dry before the next step. Avoid cleaning in direct afternoon sun, where the solution can dry before it finishes working. Start at the far end and work back toward the house so you do not walk through the wet solution. The sequence:
- Clear the deck of furniture, planters, grills, and rugs.
- Sweep the surface and remove leaves and debris from between the boards.
- Pre-soak the deck with a hose so the cleaner works into the grain instead of evaporating.
- Apply the chosen cleaner with a pump sprayer or brush, working from one end to the other.
- Scrub with a long-handled brush, paying extra attention to shaded corners and traffic paths.
- Let the solution soak for the time in the recipe, then rinse thoroughly with clean water.
- Allow the deck to dry for two to three days before sealing or staining.
If you use a pressure washer, keep the nozzle 12 to 18 inches from the surface and move it with the grain. Pointing the spray directly into the gaps between boards drives water into the joists and can lift the finish off the wood.
Why a Code-Compliant Deck Cleans Up Faster
A deck that is built correctly is much easier to clean. Deck construction according to code includes proper gaps between boards and correct flashing, which lets water and debris drain instead of pooling, so mildew has less chance to establish itself in the first place.
After the Clean: Sealing, Staining, and Protecting the Surface
Cleaning exposes bare wood, and bare wood needs protection. Once the deck has dried for two to three days, inspect it for raised grain, popped nails, and damaged boards, then apply a sealer, stain, or water repellent. Most finishes need reapplication every one to three years depending on sun exposure, foot traffic, and climate. Penetrating oils soak into the grain and fade gradually, while film-forming stains sit on the surface and peel when they fail, so the two types demand different inspection habits.
Before sealing, check how the deck sheds water at its edges. A freestanding deck design supports the deck without attaching it to the house, so there is no ledger joint trapping moisture against the siding, and water runs off all four sides. Attached decks need the ledger flashing inspected at the same time, because that joint is the most common entry point for water into the house wall.
The Water Bead Test
A simple test tells you when the finish has worn out. Sprinkle water across the deck. If it beads up and runs off, the sealer is still working. If the water soaks in and darkens the wood, the deck needs a new coat. Do the test in a few spots, especially where the deck gets the most sun.
Routine Maintenance That Keeps a Deck Clean Longer
The fastest way to cut cleaning time is to keep debris from sitting on the boards. Leaves, pollen, and dirt hold moisture against the wood and feed mildew, so a weekly sweep during the growing season does more than most treatments.
A Short Maintenance Routine
- Sweep weekly, and clear leaves from between the boards in fall.
- Move planters, grills, and furniture occasionally so the boards underneath get light and air.
- Trim back shrubs and vines so the deck edge dries out between rains.
- Check the underside and the support posts twice a year for standing water or soft wood.
- Plan a deep clean twice a year in humid climates, once a year everywhere else.
The structure under the deck matters as much as the surface. Freestanding deck foundations engineered to keep posts out of standing water resist rot at the base, and dry, well-ventilated framing gives mildew nowhere to hide. Between a solid structure, a seasonal cleaning, and a finish refreshed on schedule, a wood deck can stay presentable for decades with nothing more than household ingredients and an afternoon of work.