The short answer
There is no set interval that suits every house. How quickly growth returns depends on shade, nearby trees, damp, the slope and the covering, so a roof under an oak may need attention long before one in an open spot. Looking at the roof regularly tells you more than a schedule does, and a clean is worth arranging when growth is thick or starting to hold water.
A calendar will not tell you when a roof needs cleaning. The roof, the trees around it and the weather will.
What decides how fast growth returns
Moss, lichen and algae need moisture, and they settle where a roof stays damp. That depends on the house and its surroundings far more than on the number of years since the last visit. Two neighbouring roofs can need different attention for exactly that reason.
Why does my neighbour's roof stay clear?
Look at the differences. Slopes facing away from the sun stay damp longer, which is why the north side of a roof grows moss first; our answer on moss on the north side of a roof explains it. Trees add shade and drop leaves, twigs and seeds that collect in corners and hold water. A shallow pitch drains slowly, and a rough tile surface gives growth something to grip.
The covering matters too. Concrete tiles, clay tiles and natural slate each weather differently, and a roof that has lost some of its surface layer tends to hold moisture and growth more readily. Where houses stand close together, in a sheltered lane or a damp valley, the air itself stays moist, and so does the roof. Those local conditions are why a fixed interval is a poor guide.
Inspect first, clean when it is needed
A better habit than a routine wash is a regular look. Once or twice a year, check the roof from the ground, ideally with binoculars, plus a peek into the loft where the hatch is easy to reach. You are looking for change, not perfection.
What can I spot without climbing anything?
Check the shaded slopes for growth that has spread or thickened. Look at the gutters for moss, grit and leaves, which suggest it is coming off. Scan for tiles that have slipped, cracked or gone missing, and for staining on ceilings after rain. A line of green along a ridge or a valley (where two slopes meet) is worth noting, as water gathers there.
Arrange a clean when growth has built into a mat that keeps the tile wet, or when pieces are dropping into the gutters. The roof has then moved beyond looks into maintenance. A light dusting of algae is mostly a cosmetic matter. A roof with no problems underneath can wait. Our page on whether roof cleaning is worth it helps you weigh up an appearance-only clean against an inspection.
Cleaning too often has its own cost
Every method puts some wear on the roof, even a gentle one, and each visit means access and handling. Cleaning more often than the roof needs adds that wear without adding benefit.
Can washing a roof too often harm it?
It can, if the method is rough. Marley does not recommend jet washing, and repeated blasting is the clearest example of why. Repeated blasting of a concrete tile can thin the coloured surface layer, and a thinner surface holds growth sooner. Gentle treatment is a better fit for a roof you expect to look after for years.
The gap between cleans is also affected by how thoroughly the first one was done. A treatment that reaches the roots lasts longer than a surface scrape, which is covered in our answer on moss growing back after cleaning. Keeping gutters clear and trimming branches that overhang the roof also lengthens the time between jobs, for little outlay, and both can be done from the ground. The wider options are laid out on our roof cleaning and moss removal page, for the point when a clean becomes worthwhile.
Setting up a sensible routine for your house
For most homes, a simple pattern works. Look at the roof after autumn leaf fall and again in spring. Keep a few dated photos taken from the same spots in the garden, and set them side by side each spring. Changes in colour, spread or thickness show up far better than they do from memory. A phone is all you need, and the pictures are useful to a roofer too.
When is it time to call a roofer?
Call when you see thick moss on a shaded slope, growth in a valley or at the gutter edge, tiles lifting, or water marks inside. The survey comes first, with pictures of your own slopes and one written price. Where cleaning is not the answer, you are told plainly and the repair that is needed gets priced instead. That keeps you from paying for a wash on a roof that needs mending first, and from cleaning more often than the roof requires.
Roofs that have been treated and then inspected annually seldom surprise their owners. If you are unsure whether yours is due, send us a photo from the garden, or ask for a survey, and the next step is a look at the tiles and a written recommendation.
More on cleaning intervals
How often will my roof need cleaning if trees overhang it?
Overhanging trees shade the slope and drop leaves and seeds that hold water, so growth tends to return sooner than on an open roof. Trimming branches back is the simplest help, and it lengthens the time between cleans. A survey shows how much the shade is contributing and what the roof needs.
Does a roof that looks fine from the road still need a check?
A look is worthwhile, a clean is not always. If the tiles are sound and growth is thin, regular checking is enough. Check the shaded slope, gutters and valleys, which hide problems the road view misses. Arrange a clean when moss is building up or dropping into the gutters, not on a schedule.
The full picture: Roof cleaning and moss removal in Taunton ยท More short answers