Prep first
Before you start
- Start with plain water on a well-wrung sponge.
- Wear old clothes, shoes and rubber gloves, and protect nearby plants.
- Sponge the bleach mix on, since the wall is vertical, leave it 10 minutes, then rinse with a hose.
Dark streaks on a painted wall are usually algae, and moss can join them in any spot that stays shaded and damp. Matte paint is easy to dull if you soak it or press hard, so a light touch matters as much as the cleaner you pick. Whether you can use bleach at all depends on whether the paint is sealed and non-porous, and that's the first thing to settle.
Moss on a wall you plan to paint still needs a light hand, because pressing hard dulls matte paint. Algae on painted walls is usually a lighter job, so start with the gentlest wipe that will work. If the streaks remain after a full round and the paint is sealed, a mold and mildew remover is the product to reach for.
Before you start
What walls can take
Use with care4
- Water
- Chlorine bleach
- Solvents (alcohol, spot removers)
- Scrubbing and abrasives
Avoid1
- Soaking
6 items
What you'll need
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- Soft-bristle brushLifts loose, dry growth without scratching the paint. (opens Amazon)
- Non-abrasive sponge or clothHolds the water and bleach mix and scrubs without marking the surface. (opens Amazon)
- Plain liquid bleachLiquid bleach mixed with water is the only cleaner this job calls for. (opens Amazon)
- BucketFor mixing the bleach and water. (opens Amazon)
- Rubber gloves and old clothesProtect your skin and clothing from the bleach. (opens Amazon)
- Garden hoseRinses the bleach off the whole area.
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6 steps
Step by step
Step 1 of 6
Check the surface before you start
Look over the wall to confirm it's sealed paint, stucco, siding, tile or brick, since those are the surfaces that can take bleach. Put on old clothes, shoes and rubber gloves before you go near the wall.
Careful. If the surface is porous or unsealed, skip the bleach.
Step 2 of 6
Brush off the loose growth
Use a soft brush to knock off loose, dry growth before any liquid touches the wall. Brush lightly and sweep the debris away from the base of the wall.
Step 3 of 6
Wipe with plain water
Wet a sponge with plain water and wring it out well, so it's damp rather than dripping. Wipe the growth gently and rinse the sponge in clean water often.
Step 4 of 610 minutes
Scrub with the bleach mix
Mix one part liquid bleach with three parts water in a bucket, and use only those two. Sponge the mix onto the growth and scrub gently with a non-abrasive sponge, then leave it on for 10 minutes before you rinse.
Careful. Never mix chlorine bleach with ammonia, vinegar or other cleaners; toxic gas can form.
Step 5 of 6
Rinse the area thoroughly
Rinse the whole treated area with a garden hose until the water runs clear. Rinse the ground and any plants nearby too, since the bleach can reach them.
Step 6 of 6
Let the wall dry and check
Let the wall air-dry completely, then look it over for growth left in cracks, corners or shaded spots. If something is still there, the set-in section below explains the next step.
Tip. Shaded, damp corners are where moss grows, so they're worth watching.
If you'd rather buy than mix
Best products for algae on walls
Use a plain liquid bleach with no added scents or cleaners, since the mix should be bleach and water only.
Amazon Basics Microfiber Cleaning Cloths
How we picked these: each product is made for this type of stain, its maker doesn't warn against using it on walls, and its directions fit the steps above.
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
Each light pass should lift a little more of the growth onto your cloth, and the dried wall should look even, with no water marks where the water ran. If the paint starts to look chalky or rubbed, back off and use less water. If two gentle passes leave the same dark tint, the growth has likely soaked into the surface and needs the set-in approach.
What changes for yours
Adjust for your walls
Algae on painted walls
Algae on a painted wall usually shows as thin dark streaks, and plain water often lifts the lighter growth. Start with the water step, and move on to the bleach mix only if the streaks stay.
Once the streaks are gone, keep the spot dry, since algae comes back wherever the wall stays damp. The bleach mix and rinse stay the same, so nothing else needs to change.
Vinyl siding
Vinyl siding takes a weaker mix than the one in the steps: 1/3 cup of bleach per gallon of water. Scrub with it, wait 6 minutes, then rinse with a hose.
Use this mix only in place of the bleach step; the water wipe and the rinse stay the same.
Painted wood
Painted wood siding can take the same bleach mix as long as the paint is sealed, so the steps stay as they are. Bare or unsealed wood needs a different approach, so the Decks & Wood Fences guide is the one to follow for it.
Look for bare or peeling patches before you start, and keep the bleach off them.
Stucco
Stucco is on the list of sealed surfaces that can take the bleach mix, so the steps apply as written. Because stucco is rough, let the rinse run over the whole area so none of the mix is left in the texture.
Sponge the mix into the textured surface rather than pressing hard, since pressure can damage the finish.
Heavy or long-standing buildup
Old growth that has built up for a long time may not lift in one pass. Wet it again with plain water before you scrub, so the growth softens.
If the growth dried out before you got to it, it can look harder than it is. Repeat the bleach mix once, leaving it for the same time, and rinse well afterward.
If it's still there after that, stop, since stronger mixes aren't a good idea on a painted wall. Heavy growth often needs more than one pass, so give it a second round before you decide it's stuck.
Vinegar, baking soda, peroxide and more
Do home remedies work on algae on walls?
Before painting a wall with moss on it, clean the growth off with the plain water and bleach steps, rinse well, and let the wall dry fully before you prime or paint. Most home remedies people suggest have no place on a wall like this, and some can damage it.
Skip5Avoid1
What makes it worse
Mistakes to avoid
Don't let the bleach dry on the wall
Bleach left to dry on the wall can stop the rinse from clearing it completely. Work in small sections and rinse each one before the mix has a chance to dry.
Don't scrub with a wire brush
Stiff or wire brushes scratch painted surfaces and can leave marks that show. Use a non-abrasive sponge or cloth for the scrubbing, and keep the soft brush for loose growth only.
Don't make the bleach mix stronger to save time
A heavier mix doesn't clean faster, and it puts the paint at more risk. Stick to one part liquid bleach to three parts water, leave it for 10 minutes and rinse well.
Don't let the mix run onto plants below the wall
Bleach solution harms plants, so cover or move anything growing under the wall before you start. Wear old clothes and rubber gloves too, since splashes add up over a full wall.
When to call a professional
Once the wall has been wiped and rinsed, a paint store can help you match the color for a patch or a full repaint. Grasscloth wallpaper is the exception, since a cleaning professional is the safer call there than a home wipe-down.
Questions
Why does the algae keep coming back?
Algae and moss spread on wind-borne spores, so they return whenever a spot stays damp. Trimming branches for more sun and clearing debris from around the wall helps slow regrowth.
Is the black streaking on my wall mold or algae?
Those dark streaks are often algae, not mold or mildew, even though they're often mistaken for it. The steps here are meant for those streaks, so start there.
Can I use the bleach mix on tile or brick?
Yes, as long as the tile or brick is sealed, since both are on the list of surfaces that can take it. For brick paths and driveways, the Concrete, Brick & Driveways guide covers those surfaces.
What should I do if I don't know whether the paint is sealed?
Don't use bleach on it until you know. Start with plain water on a small, hidden spot and see how the paint reacts before you go further.
How long after painting should I wait before cleaning algae off?
Give the paint about two weeks to cure before any wet cleaning, since wetting it earlier can change or damage the finish. Until then, a dry microfiber cloth or a vacuum brush can lift loose growth without adding any moisture.
Will a wet cleaning make matte paint look worse?
It can, if the sponge drips or you press hard, since matte paint is the most easily damaged by both. Wring the sponge nearly dry, use light strokes and test a hidden corner first. Once it dries with no water rings and an even sheen, you're in the clear.
Can I clean algae off wallpaper?
Only if the paper is labeled washable, and then with a sponge that's barely damp and no abrasives. A few textured papers can't take water at all, so test a hidden strip before you go near the growth. Grasscloth is better handed to a cleaning service than done at home.
Should I clean from the top of the wall or the bottom?
Begin at the bottom and move up, rinsing each section as you finish it. That keeps the runoff off paint you've already cleaned, and it stops dirty water from running back into the wet areas.
Algae on other materials
Other stains on walls
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.