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How much bleach do I mix with water to kill roof moss?

Updated February 21, 2017

Moss can shorten your roof's lifespan by keeping the wood shingles damp, potentially causing them to rot. A thick green carpet also makes your shingles look unsightly. If your roof gets a lot of shade, you're probably familiar with moss. You can choose between two types of bleach that both work as effective roof cleaners.

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As a general guideline, the ratio of bleach to water for a roof cleaning solution is 1-to-4. Combine 1 litre (1 quart) bleach with 4.5 litres (1 gallon) of water, and add 59 ml (1/4 cup) of trisodium phosphate (TSP) or a similar tough-acting cleaner. Mix all the ingredients together, pour them into a garden sprayer and spray the roof. Leave the solution on for 15 minutes to clean the moss from the roof, then rinse off with water.


Chlorine bleach is very effective at removing roof moss and algae, but it's not that environmentally friendly. Bleach solution that runs off the roof can damage or kill plants and flowers planted below. Bleach also discolours wooden roofing shingles and speeds corrosion of metal gutters. If your roof gets very mossy and you need to clean it frequently, chlorine bleach really takes a toll on your roof. A safer alternative is oxygen bleach.


If you want to use oxygen bleach to prolong the lifespan of your roof, it will kill moss. It's slower acting than chlorine bleach. Follow the directions on your container of oxygen bleach and mix the right amount of this powdered bleach with water. Leave the bleach solution on for 20 minutes, then rinse off. If you still see traces of moss, repeat the alternative bleach treatment.


Moss roots are thin and loosen from your roof through contact with the bleach. To really get all moss and stains off your roof, use a stiff bristled scrub brush. Brush down, from the top of the roof toward the ground, to loosen and remove the moss. Do this at the end of the cleaning solution's time frame, just before you rinse the roof with water.

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