Deck Boards Are Green and Slimy
The green film on deck boards is algae and mildew growing in the damp surface of the wood or composite, fed by shade and trapped moisture. An oxygen based cleaner lifts and kills that growth without the damage pressure and chlorine bleach can do, especially to composite boards.
The short answer
For green, slippery boards, Olympic Premium Deck Cleaner is the sensible first step, for wood and as preparation before restaining. Wet the deck, apply the cleaner, give it its working time, scrub, and rinse, and the growth film comes off with the dirt it holds. It holds 4.5 stars from 1,737 reviews on Home Depot (November 2026), with 86 percent of reviewers recommending it. Know the difference before starting: composite boards come with maker warnings against pressure washing and chlorine bleach, so the gentle cleaner matters most there.
Why does this happen?
The green is alive. Algae and mildew colonise the board surface wherever damp lingers: under leaf fall, on the shaded north side, in the grooves where water sits after rain. The growth holds moisture against the board, which feeds more growth, and the slime underfoot is the mature stage of a film that began as a faint discolouration a season earlier. Because the organisms live in the surface rather than on it, a rinse or a stiff brush with plain water just smears the problem around. The color lightens for a week, the roots of the growth remain, and the green returns on the same boards in the same pattern.
Oxygen based cleaners attack that film chemically. The chemistry releases oxygen in solution, breaking down the growth and the dirt bound up in it, so a scrub and rinse remove what pressure alone would drive at the surface. The deck material decides the method. Wood tolerates, and before staining requires, such cleaning. Composite is the case to be careful with. Its makers warn against pressure washing, which can scar the surface, and against chlorine bleach, which can damage it. An oxygen cleaner used with a brush is not just the gentle option; it is the one the makers recommend. Deep grease and rust are different chemistry and need targeted products.
The fix, in order
Work down the list: free and cheap steps first. Stop when the plant turns the corner; the product is only for what the free steps cannot do.
Free step
Sweep, clear, and identify the deck material
Remove furniture, sweep off leaf litter and standing dirt, and confirm whether the boards are wood or composite before choosing any tool. Check a suspect board end for softness: rot is not a cleaning problem. This ten minutes decides the method, because the composite warnings against pressure and chlorine bleach change everything downstream of this step.
The product step
Clean with Olympic Premium Deck Cleaner, brush in hand
Wet the boards, apply Olympic Premium Deck Cleaner as directed, and give it the working time on the label before scrubbing with a deck brush, working along the boards in sections. Rinse each section clean. The cleaner loosens the growth and the brush removes it. Skipping the waiting time or the scrub and expecting the rinse to do both jobs is the common shortcut that fails.
The product for this step
Olympic Premium Deck Cleaner
Free step
Let it dry fully and judge the boards honestly
A wet deck hides its real color, so wait for a full dry before deciding the result. Remaining shadows are usually a second application on the worst sections, while grease and rust marks are a different problem needing targeted products, as the cleaner own limits state. Drying also reveals the true condition of wood that is a candidate for staining.
Habit step
Keep the film from coming back
Sweep leaf fall off the boards through autumn, keep planters on feet so moisture is not trapped under them, and trim back the planting that holds one corner in permanent shade and damp. Algae returns where damp returns; the annual clean gets shorter every year the moisture habits improve, and the slippery corner is the place to start.
The evidence, in tiers
Tier 1 · The method
The method distinction is documented in composite decking maker guidance: pressure washing and chlorine bleach are warned against for composite boards, while oxygen based cleaning with agitation is the aligned approach, and oxygen cleaners are the standard preparation for wood decks before staining. The product chemistry matches that guidance.
Tier 2 · This product
Olympic Premium Deck Cleaner holds 4.5 stars from 1,737 reviews on Home Depot (November 2026), with 86 percent of reviewers recommending it.
We have not cleaned a test deck for this guide. The protocol follows the product directions and composite maker guidance on pressure and bleach.
Tier 3 · What owners report
Owners report the color change the chemistry promises, with wood decks in particular coming back close to their original tone, and they note the work is in the scrubbing, not the applying. Disappointed reports focus on deep grease and rust, which the product does not claim, and on composite owners who expected cleaning to undo surface wear, which no cleaner can.
The alternatives, conceded where they win
| Option | What it is | The honest verdict |
|---|---|---|
| Olympic Premium Deck Cleaner | Oxygenated cleaner, brush, and rinse | The pick: it removes the growth film chemically, suits wood, respects the composite warnings, and doubles as the correct preparation if the deck will be stained. |
| Pressure washing | High pressure water jet | Wins on raw speed over a large, sturdy wood deck in sound condition, where it clears a season of grime quickly. It loses, and can do real damage, on composite boards whose makers warn against it, and it can scar wood grain when handled carelessly. |
| Chlorine bleach solution | Household bleach and water scrub | Wins on being already in the house and harshly effective on visible mildew color. It loses on composite, where maker guidance warns against it, on the planting around the deck, and on doing nothing for the dirt film bound up with the growth. |
The honest price comparison
The honest comparison is a container of cleaner and an afternoon with a brush against a contractor resurfacing or cleaning quote, or against replacing boards whose only fault is a film of growth on sound wood. Cleaning is the small intervention; it is worth doing properly before anyone prices the large one.
Where it falls short
These weaknesses come from the product's own reviews, not from a rival's marketing. Read them before you buy.
- It cleans, and that is the boundary: deep grease and rust stains survive it and need targeted products, which owners discover mid job if nobody warned them.
- The result depends on the scrub: applied and merely rinsed, it underdelivers, and the reviews that disappoint tend to describe exactly that shortcut.
- It cannot restore wear. A composite deck whose surface is worn or a wood board going soft has a material problem that presents as dirt and survives every cleaning.
The product, in short
Olympic Premium Deck Cleaner
Questions people ask next
Is it safe on a composite deck?
The oxygen based approach is the one composite maker guidance aligns with, unlike pressure washing and chlorine bleach, which makers warn against for composite boards. Use a brush rather than any pressure, follow the cleaner directions, and check the deck maker guidance for your specific boards before starting.
Do I need to scrub, or will it rinse clean?
Scrub. The chemistry loosens the algae and dirt film, and the brush is what lifts it out of the surface and grooves. Owners who apply and rinse get a lighter deck for a week; owners who scrub get the color change the reviews describe. Dwell time matters too.
The green came back in the same corner. Why?
Because the conditions did: that corner stays damp, shaded, or under leaf fall, so the film re-establishes where it always lived. Clean it again and change the condition, with planters raised on feet, planting trimmed, and leaf litter swept, or the same corner will be green every spring on schedule.
Can I stain straight after cleaning?
On wood, this cleaning is the standard preparation before staining, which is one of its uses. Let the deck dry first, since stain over damp wood fails early, and deal with grease or rust spots with products beforehand, because the cleaner leaves those and stain will seal them in.
Home Depot product record for Olympic Premium Deck Cleaner (November 2026) · Composite decking maker care guidance on pressure and bleach
The Fix JournalCase 16Plants & Garden