How to kill moss and algae on roof tiles

Written by pat olsen
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How to kill moss and algae on roof tiles
Moss grows easily on damp, shady roofs. (sinngern/iStock/Getty Images)

Anyone who lives in the UK's damp climate might experience moss and algae on roof tiles, especially if the roof is in shade for most of the day. Over 25,000 species of bryophytes exist in the world, constituting the largest group of land plants. Left alone, these invasive culprits will colonise the roof in just a few years. Several ways to remove moss and algae exist, but vigilance is always necessary to reduce the damage from spores that continue to reappear over time.

Skill level:

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Things you need

  • Stiff brush
  • Hose
  • Power spray
  • Power washer
  • White Vinegar
  • Citric acid (orange juice)
  • Hydrogen peroxide
  • Alkaline commercial moss killer
  • Zinc strips

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  1. 1

    Climb up on the roof at the first sign of moss formation. It usually begins on the lower edge of a tile as a single line of green and grows upwards, attaching itself to the tile and feeding off the nutrients it finds. Use a stiff brush to scrape it off where it has taken root.

  2. 2

    Hose the brushed bits of moss off the roof with a garden hose. This is effective if there is very little moss and algae accumulated. Use a power washer but with caution. Never spray upwards because the power may loosen the tiles. Use a harness if you are on top of the roof so you don't slip and fall. Work on a sunny day so that the bleaching and drying effect of the sun will help.

  3. 3

    Advanced cases of moss and algae invasion are treated with an alkaline based moss cleaner. To care for the ecology, be sure that the eradicator you use is safe for the flora on the ground and will not run off into storm drains.

  4. 4

    Make your own organic moss killer by pouring hydrogen peroxide in a power spray container and apply it to the roof. Use white vinegar or a combination of orange juice and vinegar. Each of these formulas is alkaline and effective at killing moss over time. Do not hose the mixture off because it sits on the moss and changes the hospitable pH factor, and that kills it over time. If it rains, reapply when the storm has passed. When the moss is brown and crumbly, hose it away.

  5. 5

    Zinc strips are effective at killing moss. Nail in strips starting at the peak of the roof, and apply every few feet on top of the tiles. As rain falls, the zinc leaches out of the strips, flows over the moss and kills it.

  6. 6

    Once your roof is clean, trim trees that overhang the roof, and make sure that the sun can reach the roof. Sunlight is an effective way of eliminating moss and algae, especially in a damp climate.

Tips and warnings

  • Avoid using bleach. Although it will instantly kill moss, it may also bleach the tiles and the run-off below will make the environment hostile for plants.

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