How Long Does a Roof Biocide Treatment Last?

A biocide kills the growth that is on the roof today and leaves a surface that is hostile to spores for a time. It does not make the roof immune. Spores arrive on the wind, from trees and from neighbouring roofs every season, and whether they take hold depends on how damp and shaded the tile is.

That is why the same treatment, put on two houses in one week, can last quite different lengths of time. One has open sky and a dry gable end, the other sits under a mature oak with a north-facing slope that never gets sun. Product makers sometimes publish long periods, but they come from test sites, and a roof in a garden full of trees is not a test site.

We give no promise of how long a treatment lasts, because it would not be true for every roof. The survey lets us read the aspect, the trees and the tile, and say which slopes are likely to go green first. Our page on how a biocide roof treatment works explains what the product does.

Aspect is the direction a slope faces. In the northern hemisphere a north-facing slope gets the least sun, stays wet longest and grows moss and algae first. East and west slopes sit in between, and a south slope dries quickest. A house in a valley or with tall hedges nearby behaves as if every slope were partly north-facing.

Shade matters as much as direction. Branches overhead cast shade, drop leaves into the valleys and drip water long after rain has stopped. Cutting the trees back gives a treatment a far better chance than a second treatment would.

The tile counts too. Concrete tiles are coarser and more absorbent, so spores find purchase on them quickly. Smooth glazed and clay tiles often stay cleaner for longer, though not always, and old weathered tiles of any kind tend to hold growth. The sunlight and damp behind all this are explained in our answer on why moss grows on the north side of a roof.

You will see it before it becomes a problem. Look from the garden, preferably after rain, at the north slope and the lowest courses of tiles first. The first signs are a faint green tinge on the tile faces, small dark specks along the edges, and tufts of moss starting in the overlaps and beside the ridge. Binoculars from an upstairs window help.

Make a note of what you see and when, because a series of dated photos tells you more about the roof than memory does. Do not wait until the cushions of moss are thick. Light growth treated early needs only a gentle rinse, while heavy moss means slow work with a hand brush, which costs more and is harder on the tile. Check the gutters too, since moss washed off the roof often ends up there and blocks the outlet.

A coating can slow growth on some roofs, though it is a different choice with costs of its own. Our answer on whether to coat a roof after cleaning sets out when it is worth it.

The sensible rule is to treat when growth is back, rather than at a set interval. Some roofs will need a repeat sooner and some later, and the survey photos from the first visit make a useful comparison. Take your own photo of the worst slope each spring, and you will see the change. Treat different slopes separately if you need to, since the north side may need attention long before the south does, and a partial visit can cost less than a full one.

When you do book again, a repeat treatment is usually quicker and lighter than the first, because there is less growth to clear. For orientation, Checkatrade's April 2026 table puts typical UK roof cleaning at £8.50 to £14 per m², and the quote we write is a single sum after the survey. Make sure VAT and any scaffold are covered in whatever you are offered.

Fix the cause where you can. Trim trees, clear gutters, and have loose or cracked tiles replaced, because a broken tile holds water and grows moss around it first. Our page on how often you should clean your roof covers the pattern by tile and aspect, and the roof cleaning and moss removal page covers the service. Attach a photo of your most worrying slope to your enquiry and we will compare it with what a typical repeat visit involves.