Prep first
Before you start
- Find the cause first: location, smell, feel and how the mark appeared decide the treatment.
- Brush or vacuum dry soot and mildew before wetting them.
- Test any cleaner on a hidden spot first.
Black marks on a deck usually come from mildew, soot, manganese in well water or greasy residue, and each one needs a slightly different fix. Fresh dirt and mildew tend to wash off, while old marks that have set into the wood may only fade. Boards that have weathered past their natural color can also stay silver even after a thorough clean.
Black stains on a wood deck usually come up with a scrub and a mild cleaner before anything stronger. Cedar decking takes gentler handling, so test a small patch before you go over a whole board. If a dark stain on cedar still holds, wet the board again and repeat the wash with moistened detergent, then rinse it well with clean water.
Before you start
What decks & wood fences can take
Safe to use2
- Water
- Oxygen bleach
Use with care5
- Soaking
- Chlorine bleach
- Solvents (alcohol, spot removers)
- Acids (vinegar, lemon)
- Scrubbing and abrasives
6 items
What you'll need
Tap an item to get it on Amazon, or tick what you already have.
- Garden hoseGives a low-pressure spray for the final rinse.
- Mild detergentLifts the dirt and mildew in the wash and on stubborn spots. (opens Amazon)
- Stiff synthetic-bristle brushWorks along the grain on tough marks without leaving iron behind. (opens Amazon)
- SpongeApplies the detergent and scrubs the stubborn spots. (opens Amazon)
- Vacuum or stiff brushRemoves dry soot and mildew before anything gets wet. (opens Amazon)
- Rubber gloves and eye protectionKeeps the cleaning solution off your skin and out of your eyes. (opens Amazon)
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6 steps
Step by step
Step 1 of 6
Identify the black mark first
Look at where the mark sits, how it smells and how it feels, because the cause decides the treatment. Mildew, soot, manganese from well water and greasy residue all look black, so narrow it down before you pick a cleaner.
If your water comes from a well, treat the mark as possible manganese and keep bleach away from it.
Step 2 of 6
Brush off dry soot and mildew
Brush or vacuum dry soot and mildew before you wet anything, since water turns them into a smear. Brush mildew off outdoors and sweep up whatever comes loose.
Step 3 of 6
Test a hidden spot and wash
Test your cleaner on a hidden spot first, then wet the whole deck with water. Wash from the bottom up with water and a mild detergent to reduce streaking, working one section at a time.
Careful. Don't let cleaners or soap dry on the deck.
Step 4 of 65 minutes
Scrub the stubborn marks
Apply moistened detergent to the stains and leave it for about 5 minutes. Then scrub with a sponge and rinse the area off.
Tip. For tougher marks, switch to a stiff synthetic-bristle brush and work along the grain.
Step 5 of 6
Rinse and let the deck dry
Rinse the area well with clean water, using a garden hose on a low-pressure spray. Let the deck dry before you judge the result.
Careful. Narrow-stream and rotating tornado pressure-washer tips can damage the wood.
Step 6 of 6
Repeat if the mildew returns
If mildew returns in several days or weeks, repeat the process from the wash. Check the deck once it's dry to see whether any dark patches have come back.
If you'd rather buy than mix
Best products for black stains on decks & wood fences
A commercial mildew remover labeled safe for painted or stained wood can also be used on this deck. Read the label and test a hidden spot first.
Dawn Ultra Dish Soap Refill Jug
These are the everyday supplies the steps above use, in the versions most people buy on Amazon.
Finished the steps? How does it look?
Nice work. Let it dry completely and check it again in good light. Here is what done looks like:
How to tell it's really gone →A faint mark usually needs one more round, not a stronger product.
Heavy buildup usually takes more than one round.
Heavy or long-standing buildup →When to call a professional →How to tell it's working
Once the deck is dry, the wood should look even, with the dark marks lighter or gone. If a patch comes back within a few days or weeks, it's mildew, so repeat the wash.
What changes for yours
Adjust for your decks & wood fences
Cedar decking
On cedar, keep the cleaning gentle, because the wood is soft and the marks may lift with a wash alone. Use the hose for the rinse and a soft brush along the grain for stubborn spots, and stop as soon as the mark lifts.
If a dark patch returns, check whether it's mildew or manganese before you choose a stronger treatment. The two need different approaches, so the set-in options only apply once you know which one you have.
Mildew on decking
Mildew is one of the common causes of black marks on a deck, and it's the one most likely to come back. Give stubborn patches the full 5 minutes of detergent before you scrub, since that's where the growth sits.
Keep the scrubbing along the grain so the wood isn't roughened, and rinse the patch before it dries.
Soot
Soot is dry and dusty, so it has to come off before any water touches it. Brush or vacuum the deck thoroughly and move the loose soot away from the wood first.
Only then wash with water and a mild detergent and rinse well. If a soot mark remains after that, treat it as a stain that may not come out fully.
Manganese from well water
Manganese staining comes from the water rather than the wood, so the wash alone often won't clear it. Identify it before you clean, since the mildew steps don't suit it.
Treat it as a stain that keeps returning, and see the set-in section for what's realistic.
Heavy or long-standing buildup
Marks that have soaked into the wood need more than a wash. A dark moisture ring can be lightened by sponging chlorine bleach around and then onto the stain, repeating until it fades, but the wood lightens too.
Weathered, grayed decks can be brightened with a commercial deck cleaner, brightener or restorer, which lifts mildew and dirt so the wood's own color shows. Old stain or sealer comes off with a stain and sealer remover, then a brightener, and a light sanding once it's dry. If all the natural color has leached out, the wood stays silver when dry, and it's best left as it is.
Manganese staining keeps returning over time, so a clean deck may not stay clean. Some black marks are damage that can't be fully removed, and the realistic goal then is a lighter, more even deck.
Vinegar, baking soda, peroxide and more
Do home remedies work on black stains on decks & wood fences?
Most kitchen and laundry remedies don't belong on a wood deck. A few are worth a try on a wood deck, and water with a mild detergent is the place to start.
Skip6
What makes it worse
Mistakes to avoid
Don't use bleach before you know the cause
Chlorine bleach can intensify manganese and iron stains, so rule those causes out before you reach for it. If you do use it on an old ring, do it only after the rest of the cleaning is rinsed away.
Don't use chlorine bleach on pressure-treated decks
Bleach breaks down the lignin in the wood, corrodes metal fasteners, harms plants and doesn't kill mildew spores. If you can't tell whether the deck is pressure-treated, leave the bleach out.
Don't paint over mildew
Mildew keeps growing under a coat of paint, so the mark will come back through it. Remove the mildew and let the deck dry before you recoat.
Don't use steel wool or wire brushes
Leftover iron in the metal reacts with the wood's extractives and stains the boards. A synthetic-bristle brush does the same scrubbing job without the stain.
Don't power wash or scrub cedar hard
Caustic cleaners and power washing strip soft woods like western redcedar, removing wood from the surface. Stick to the gentle steps here, even when a mark looks stubborn.
Don't mix bleach with ammonia
Bleach mixed with ammonia, or with a detergent that contains ammonia, forms a toxic gas. Read the label on any detergent before you use it.
When to call a professional
There's no clear rule for when a deck needs a professional. The firm case is old CCA lumber, which should be replaced rather than cut or burned, since sawdust and smoke contain arsenic. Keep kids and pets off wet CCA wood while you work.
Questions
How do I keep a dark-stained deck clean?
Sweep or vacuum off debris, and wash the deck from the bottom up with water and a mild detergent once marks start to show. Rinse well with clean water and let it dry. If mildew comes back in several days or weeks, repeat the wash.
What causes black marks on a deck?
Mildew, soot, manganese from well water and greasy residue are the usual causes, and some black marks are damage to the wood itself. Look at where the mark sits, how it smells and how it feels to narrow it down.
Will black marks come out completely?
Not always. Old moisture or burns in wood finishes may not be fully removable, and manganese staining can keep coming back.
Should I sand a deck to remove black stains?
Only lightly, and only once the deck is dry after cleaning. Light sanding is used when old stain or sealer is being removed, and everyday scrubbing uses a synthetic brush instead.
How often should I clean a deck?
Clean it when black marks start to show, and again whenever mildew returns. Mildew can come back in several days or weeks, so there's no set schedule.
Can I use a commercial mildew remover on my deck?
Yes, if the label says it's safe for painted or stained wood. Test a hidden spot first and follow the label's directions.
Black Stains on other materials
Other stains on decks & wood fences
Start from the bigger picture
How we know this
Every method, amount and warning here comes from university extension services, industry bodies and product makers. We read them, check every step for unsafe combinations and changed amounts, and write it up so you can follow it.