How do I stop moss growing on a roof or path?

Moss is a symptom of conditions rather than an infestation, which is why it returns within a year of every removal that does not change something.

Safety

Step by step

1. Work out why it is growing there.Shade, damp, poor drainage, overhanging trees, a north-facing surface. Moss needs moisture and low light, and until one of those changes it will keep coming back.
2. Cut back what is shading it.Overhanging branches and shrubs. More light and better air movement does more than any treatment, and it is the change that actually lasts.
3. Fix the drainage.Water pooling on a path, a blocked gutter overflowing down a wall, a soakaway not working. Moss follows standing water, and this is the second-largest factor.
4. Never pressure wash a roof.It strips the granular surface from concrete tiles, drives water under them and can dislodge them entirely. It shortens the roof's life significantly and is one of the most damaging things done to roofs.
5. Do not go on the roof yourself.Tiles break, and roofs are the most common cause of serious DIY falls. A scaffold tower for a low roof, or a contractor. This is not a job for a ladder.
6. Brush roof moss off from a safe position.A stiff brush working downwards, never upwards under the tiles. Then clear the gutters, which will be full of what came off.
7. Consider a zinc or copper strip near the ridge.Rain washes traces of the metal down the roof, which inhibits regrowth. A genuinely effective long-term measure and one that needs doing only once.
8. Use a proprietary moss killer on hard surfaces.A benzalkonium chloride-based treatment applied to a dry surface and left. It kills the moss over a few weeks so it brushes off easily, rather than needing scraping.
9. Be careful about pressure washing paving.Concrete and stone tolerate it; block paving loses its joint sand and old or soft paving pits. Use a wide fan and keep it moving, and re-sand block paving afterwards.
10. Scrape narrow joints by hand.A block paving knife or an old screwdriver along the joints removes moss where a jet only pushes it around, and it does not wash out the sand.
11. Be cautious with home remedies.Bleach damages plants and kills soil life and can discolour paving. Vinegar etches stone. Salt kills anything it drains into. A proper moss killer is more predictable and less damaging overall.
12. Treat at the right time.Autumn or spring, on a dry day with no rain forecast for a day or two. Treatment washed off before it works is the usual reason it seems ineffective.
13. Seal paving after cleaning if appropriate.A breathable sealer on block paving reduces regrowth and makes the next clean easier. Not suitable for every surface, so check first.
14. Keep gutters clear.Overflowing gutters keep walls and paths permanently wet, which is a common and easily fixed cause of persistent moss on a wall or path below.
15. Treat slippery paths as a safety issue.Mossy paving is genuinely dangerous in wet and frosty weather, particularly by a door or on steps. That is reason enough to deal with it promptly.

Tips

• Cutting back overhanging branches does more than any treatment. Moss needs shade and damp, and light is the thing you can change.
• Never pressure wash a roof. It removes the tile surface and forces water underneath, and it is more damaging than the moss.
• Treat on a dry day with no rain forecast. Most moss killer that seems not to work was washed off before it acted.

Common mistakes

• Pressure washing a tiled roof — It strips the granular surface from the tiles, drives water under them and can dislodge them. It shortens the roof's life considerably.
• Removing moss without changing the conditions — It grows back within a year. Reducing shade and improving drainage is what makes the removal last.

If it doesn't work

Questions people ask