The short answer
A roof should be cleaned when moss, lichen or algae start to build up, not on a fixed date. In shaded or tree-covered settings that can mean a return visit every few years, while an open, sunny roof may go much longer. Treated with a biocide, a roof usually stays cleaner for longer, so the interval depends on the setting and the covering.
Why a fixed interval does not work
Ask five roofers how often to clean a roof and you may hear five answers. The reply that holds up is that the roof sets the schedule. Moss, lichen and algae need moisture, shade and a surface they can grip. A roof that gets sun all day and dries quickly gives them little. A roof under trees, or one whose north slope stays damp all winter, gives them everything.
The covering matters as well. Rough, porous surfaces such as old concrete tile hold growth more readily than smooth dense slate. The pitch plays a part: a shallow slope drains slowly and stays wet, while a steep one sheds water fast. Even the first clean matters, because a roof that was only scraped, with no treatment afterwards, will regrow sooner than one that was treated.
So instead of a calendar date, we suggest an eye on the roof. A look each spring and autumn from the ground, and after any big storm, will tell you when growth is thickening. roof cleaning and moss removal explains what is involved once it is time.
What shade, trees and aspect do
The setting does most of the work in deciding the interval. Trees overhead drop leaves, seeds and twigs that rot into a thin soil on the roof, and they shade the slope so it dries slowly. A roof under a mature tree will usually need attention more often than the same roof in the open, and its gutters and valleys will need clearing more often too.
North-facing slopes stay cooler and wetter than south-facing ones, which is why moss so often appears on the north side first. Roofs close to open fields or damp low ground can stay humid. A house in a sheltered Bath valley, with trees at the back and a stone wall on one side, will hold moisture in a different way from one on an open hilltop. We look at what is around the building, not only at the roof.
If the main cause is overhanging trees, cutting them back is worth discussing with the owner of the tree. Our page on trees and moss on roofs goes through that.
Interval by covering
Different coverings tolerate growth differently, so the point at which a clean is worth doing varies.
- Natural slate. Slate is dense and absorbs little, so it often copes with moss for a long time, and a light clean at the right moment is enough. Pressure washing is not suitable for it, as the edges are easily damaged. See cleaning slate roofs.
- Clay tiles. These can hold growth, particularly older handmade ones. Clean them gently and treat afterwards. Our guide to cleaning roof tiles has the detail.
- Concrete tiles. Their surface wears, and moss accelerates it. Keeping them clean is a way of slowing that wear, but a very worn concrete tile cannot be restored.
- Stone tiles. Porous and heavy, they need a gentle hand and a lighter touch than any other covering.
- Flat roofs. Membranes collect silt and algae more than moss, and the gutters and outlets need clearing on a regular basis.
None of these is a promise. They are starting points that we adjust after seeing your roof.
How treatment changes the gap between cleans
Removing moss without treating the surface leaves the spores behind. They regrow from the first damp spell. A biocide is a chemical wash that kills what remains and leaves a surface that is harder for growth to take hold on. It is applied once the moss is off, and it works over the following weeks and months rather than at once, so a roof can look patchy for a few weeks after treatment.
Treatment stretches the time between cleans, but it does not stop growth for good. Spores arrive on the wind all the time, and a shaded damp roof will invite them back. Our page on how long before moss comes back covers that in more detail, and the guide to roof biocide explains what the treatment does and how it should be used.
A zinc or copper strip fixed below the ridge is sometimes used to slow regrowth, as rain washes metal salts down the slope. It is a supplement to cleaning, and does not replace it.
Signs that a clean is due
Rather than counting years, look for the signs.
- Moss is thick enough to stand proud of the tile or slate edge, or has started to fill the gaps between courses.
- Green or black staining has spread over a whole slope.
- Moss debris is turning up in the gutters or around the downpipe outlets.
- Water overflows the gutter during rain, although the gutter looks clear.
- Damp patches or stains show on a bedroom or landing ceiling under the roof.
The last two are not signs of a dirty roof so much as signs of a fault, and a clean alone will not cure them. If you are unsure which it is, our free survey comes with photographs of the roof itself, so you can see the same thing we do, and the quote after it is a fixed price in writing. Moss is also a matter of degree, and a photograph shows degree better than words.
For typical costs, see roof cleaning cost, and for the practical steps involved in clearing growth, read about how to remove moss from a roof.
