Prep first
Before you start
- Confirm the roof covering can take bleach before you spray.
- Brush off loose moss and debris before any liquid goes on.
- Wear old clothes and rubber gloves, and keep the solution off plants.
- Rinse thoroughly with a garden hose once the dwell time is up.
Those black streaks on a roof are usually algae rather than mold, and the green or dark growth spreading across shaded planes is moss. Both settle wherever a surface stays damp and travel on wind-borne spores, so they keep coming back unless the surface dries out. Heavy moss may need repeat treatments, so plan on a second pass.
For moss on roof shingles, skip the spray and sweep it off by hand with a stiff broom on a dry day. Moss on concrete or a patio near the roof edge can take the same treatment, so do both in one pass. If the roof is steep or the moss is thick, a pro is the safer call than climbing up with a bucket.
5 items
What you'll need
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- Laundry-strength liquid chlorine bleachIt's the active ingredient in the 50:50 roof spray. (opens Amazon)
- Garden sprayerIt puts the solution on evenly across the roof.
- Stiff broomIt clears debris and loose moss before the spray. (opens Amazon)
- Garden hoseIt wets the roof first and rinses it at low pressure afterward.
- Rubber gloves, old clothes and shoesThey protect you while you mix and spray. (opens Amazon)
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5 steps
Step by step
Step 1 of 5
Confirm the roof can take bleach
The bleach method is for sealed, non-porous exterior surfaces such as stucco, painted wood, siding, tile, brick and patio stone, and asphalt shingles aren't on that list. If your roof is shingled or you can't confirm it can take bleach, skip the spray and only clear loose growth by hand and with a hose.
Step 2 of 5
Clear debris and loose moss
Sweep leaves, sticks and other debris off the roof with a stiff broom. Then brush the loose moss off the shingles so the solution can reach what's left.
Step 3 of 5
Wet the roof at low pressure
Run a garden hose over the roof at low pressure so the surface is damp before the spray goes on. A damp roof lets the solution spread more evenly across the growth.
Step 4 of 5
Mix the bleach solution
Mix one part laundry-strength liquid chlorine bleach with one part water, which makes a 50:50 solution. Pour it into a garden sprayer so you can apply it evenly.
Tip. Mix only what you'll use in one treatment.
Step 5 of 515 to 20 minutes
Spray, let it dwell, then rinse
Spray the solution across the roof, working from the top down so it runs over the moss, and keep the surface wet for 15 to 20 minutes. Then rinse it off with a garden hose at low pressure until no solution is left.
If you'd rather buy than mix
Best products for algae
For stone, choose a cleaner made for biological growth such as algae, moss and lichen, and follow its label for dilution and dwell time.
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 these surfaces, 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.
See what to do for what you're seeing →Heavy buildup usually takes more than one round.
Heavy or long-standing buildup →When to call a professional →Pick what it is on
Algae on every material
Clothes & laundry1
Kitchen & bath4
Carpet & rugs1
Metal & glass1
Furniture & upholstery1
Walls1
Leather & suede1
No match. Try the main search for anything else.
How to tell it's working
Algae streaks wash away with later rains, so they may fade without a second treatment. Moss loosens over time after a bleach treatment, so expect it to lift in the days and weeks after the rinse rather than right away.
What changes for yours
Adjust for where the stain is
Asphalt shingles
Shingles need the gentlest approach, so the low-pressure hose rinse in the steps above is all the water they should get. Moss can lift shingle edges and cause leaks, so check the edges once the roof has been treated.
After treatment, a leaf blower can clear moss that has loosened, and heavy moss may need repeat treatments. Algae streaks alone wash away with later rains.
Concrete, patios and brickwork
Bleach is for sealed, non-porous surfaces such as patio stone and brick, so check the surface before you spray it. If you can't confirm that bare concrete or cement is sealed, skip the spray and use the brush-and-rinse steps alone.
For driveways and full concrete methods, the Concrete, Brick & Driveways guide covers the rest.
Pool liners, walls and floors
Pool algae is handled differently from roof moss. Start by adjusting the pH to 7.2 to 7.4 and waiting at least one hour, then shock the pool with a pool shock product, following its label for the dose for your pool size.
Brush the walls thoroughly, then run the filter for at least 24 hours until the water clears. Vacuum the pool, backwash or clean the filter, and add an algaecide. To prevent regrowth, keep the pH between 7.2 and 7.8 and shock and add algaecide weekly.
Siding, paint and other outdoor surfaces
Sealed siding, painted wood, tile and stucco are the surfaces the bleach method is written for. Use the same 50:50 spray and the brush-and-rinse steps on those surfaces, and keep the dwell time within 15 to 20 minutes.
If the growth is on vinyl siding and the basic spray doesn't clear it, the stronger siding mix in the stubborn-stain section is the next step.
Yards, grass and trees
Moss in a lawn or under trees grows where it stays damp and shaded, so the lasting fix is more light and drier ground. Trim branches to let in more sun, and clear debris so the ground can dry out.
Because the roof spray is meant for hard surfaces, lift the moss by hand instead of spraying it across the yard.
Fountains, tanks and water features
Water features are a different case, since the bleach and pool steps are written for roofs, outdoor surfaces and pools. Scrub the algae off by hand with a soft brush, then rinse the feature fully before refilling it.
For any water feature that has fish or plants living in it, skip bleach and pool chemicals entirely and rinse with plain water.
Moss and algae smells
These steps are written to remove the growth itself, so start there rather than adding a second cleaner for the smell. Once the surface is rinsed and dry, check whether the smell is still there before you add anything else.
Heavy or long-standing buildup
Old, heavy growth on a roof usually needs a second pass, not a stronger mix. Keep the 50:50 spray and hold the dwell time within 15 to 20 minutes, and don't let the solution dry in between.
Dried moss takes longer to let go, so it may not lift right after the rinse. A leaf blower can clear it once it loosens, usually over the days after a bleach treatment.
If the moss is still there after one treatment, repeat the spray, since heavy moss can need more than one pass. Decks have their own stronger mix, so the Decks & Wood Fences guide is the place to go for them.
Vinegar, baking soda, peroxide and more
Do home remedies work on algae?
Most home remedies don't do much on roof moss or algae. The bleach spray is the only remedy in these steps, and the natural options here either don't clear established growth or can damage the surfaces they're used on.
Works1Skip5Avoid1
What makes it worse
Mistakes to avoid
Don't pressure-wash asphalt shingles
It strips the granules and shortens roof life. A garden hose at low pressure is the only water the shingles should get.
Don't mix bleach with other cleaners
Never mix chlorine bleach with ammonia, vinegar, other acids or household cleaners, because toxic gas can form. Use the bleach mix on its own.
Don't let the bleach solution dry on the roof
Dried solution can keep the roof from rinsing completely. Keep the area wet through the dwell time and rinse fully afterward.
Don't nail zinc or copper strips onto an existing roof
Exposed nails can leak, and breaking the sealant bond risks wind damage. If you're replacing the roof, ask about algae-resistant shingles instead.
Don't mix pool chemicals together
Pool chemicals can injure people when mixed together. Use appropriate protective equipment and good ventilation whenever you handle them.
Don't store pool chemicals carelessly
Keep them in their original labeled containers, kept dry and below 95°F. Secure them from children and animals.
Troubleshooting
If the stain is still there
Pick what you're seeing:
The solution dries before the dwell time is up
Cut the area down to a patch you can keep wet for the full 15-20 minutes, and rinse that patch before moving on. Do not let the solution dry completely on the roof.
Moss is still sitting there after one treatment
Severe moss may need more than one treatment. Give the treatment time to loosen it, then blow it clear with a leaf blower rather than pulling at the shingles.
The black streaks come back
That is regrowth from wind-borne spores. Trim branches for more sun, clear debris with a leaf blower, and keep gutters draining, and use algae-resistant shingles when the roof is next replaced.
When to call a professional
Call a roofer if the shingle edges are lifting, if moss is heavy across the whole roof, or if you can't confirm the covering can take bleach. A roofer can check for leaks and tell you whether the shingles need replacing.
Questions
Is roof moss dangerous to my shingles?
It can be. Moss can lift shingle edges, which can let water in and cause leaks, so check the edges once you've treated the roof. Clear loose moss off rather than letting it sit.
Will the moss come back after I remove it?
It can, because moss and algae both spread by wind-borne spores. Trimming branches for more sun and clearing debris so the roof dries out faster slows regrowth.
Is the black stuff on my roof mold?
Usually not. Those black streaks are algae, most commonly blue-green algae, and they're often mistaken for mold or mildew. The 50:50 roof spray in the steps is the method for those streaks.
Can I swim after shocking my pool?
Not until the product label says it's safe. The wait depends on the product, so follow the label for both the shock and the wait.
Is algae removal safe around pets, fish and plants?
Keep pets away from the solution while you mix and apply it. Around any fish or plants, skip the bleach and pool steps and rinse with plain water instead.
Do I need a store-bought moss cleaner for the roof?
No, a homemade bleach-and-water mix clears roof moss without a store-bought product. If you do buy a cleaner, choose one labeled for algae and moss and follow its label.
Can I use vinegar instead of bleach?
Not on these surfaces. Vinegar is unsafe on marble, limestone and cement grout, so skip it for roof growth. Brushing and hosing off loose growth helps, but it won't clear the growth on its own.
Related
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.