How do you stop moss coming back on a roof?

Why moss grows on one roof and not another

Moss needs moisture and it needs the surface to stay damp long enough to use it. A pitch that dries out within an hour of rain rarely holds much. A pitch that is still wet at teatime holds plenty.

That is why the same house can have a heavily mossed north-facing slope and a nearly clean south-facing one. Aspect, shade, tree cover, the porosity of the tile and how exposed the roof is to sun and wind all feed into it. On the Fylde Coast, damp air is a constant, which is why so many local roofs green up despite being nowhere near woodland.

Removal alone is not the whole job

Scraping moss off a roof removes the visible growth, but spores and root material remain in the texture of the tile and in the laps. If nothing else is done, that residue is the head start the next generation grows from.

This is why the roof is treated with sodium hypochlorite and biocide as part of the clean. The treatment keeps working after the visit, breaking down what is left rather than simply relocating it into the gutters.

What genuinely slows regrowth

Some of the conditions behind moss can be changed, and those are worth dealing with at the same time as the clean rather than a year later.

• A biocide treatment applied as part of the clean, continuing to work over the following weeks
• Gutters and downpipes cleared, so wet debris is not banked against the lowest courses
• Overhanging branches cut back where they keep a pitch permanently shaded
• Leaking or overflowing rainwater goods repaired rather than worked around
• Clearing the moss before it becomes thick enough to hold water like a sponge

What does not work

Pressure washing a roof to keep it clean is a false economy. On older or softer tiles it removes surface material and can drive water under the laps, and an eroded tile is more absorbent afterwards — so the roof holds damp better and regrows moss faster than it did before.

Repeated harsh cleaning on a fragile covering also shortens its life. Where a roof is too far gone to clean safely, the honest answer is that it needs a roofer, not a cleaner.

So how often will it need doing?

There is no honest fixed figure. A treated roof in an open, sunny position can go a long time looking fine. A shaded pitch under trees will not. The sensible approach is to keep the gutters clear, keep an eye on the pitches you can see from the ground, and have the roof looked at again when growth is visibly re-establishing rather than on a schedule.