The short answer
Moss grows on one side of a roof, often the north-facing one, because that slope gets less direct sun and stays damp for longer. Moss needs prolonged moisture rather than warmth, so the pitch that dries slowest grows it first. Trees, tall walls and neighbouring buildings can create the same effect on any aspect, and a lower pitch or poor drainage makes it worse.
Sun, shade and how long a slope stays wet
After rain, every slope starts to dry. The south side catches sun for much of the day, and the surface warms and loses its water within hours. The north side sits in the roof's own shadow for most of the year. It dries slowly and sometimes not at all between showers in the colder months.
Moss and algae need moisture more than heat. Spores land everywhere, but only the damp patches let them take hold. That is why the green stripe often follows the line of shade rather than the line of the ridge, and why the lower part of a slope, where water lingers, can carry more than the top.
So the pattern on your roof is a map of drying time. A slope that is all green and a slope that is all clean are two ends of the same scale, and most roofs sit somewhere between.
Trees, walls and neighbours that shade a roof
Aspect is only part of it. A tall hedge, a mature tree or the gable of the house next door can put a south-facing slope in permanent shade. In a valley town with terraces stepped up a hillside, a neighbour's roof or a retaining wall can block light from one direction all year.
Trees add a second problem. Branches drip long after the rain has stopped, and leaves, seeds and twigs drop onto the slope and settle in the valleys and behind chimneys. That litter holds water and feeds growth. If one side of your roof is green and the same side faces a tree, the tree is a likely contributor.
Walk around the house and look at what stands between each slope and the sky. Often the answer is visible within a minute. Note the tallest tree, the nearest high wall and any chimney that throws a long shadow across the pitch, then compare those against the green areas. Where the two line up, you have found the main cause.
Pitch, drainage and other reasons the sides differ
Even with equal light, slopes can differ. A shallower pitch sheds water more slowly. A dormer, a chimney or a valley can create a patch that drains poorly. A blocked or sagging gutter can keep the bottom courses wet from below.
The covering itself matters. Rougher, porous surfaces such as concrete tiles hold moisture longer than smooth glazed ones, and an older tile has a more open surface than a new one. If an extension or a replacement section was added to one side, it may be a different material or age from the original, and so grow differently.
We look at all of this on a survey: the aspect, the pitch, the nearby trees, the gutters and the age of each section. Pictures from the survey show the pattern, so you can see why one side behaves as it does, and which of the causes you can change and which you cannot.
Does one-sided moss mean one-sided damage?
It can. A slope that stays wet is the slope that suffers from frost, slipped fixings and soft battens first. Cotswold District Council's 2000 stone slate guidance suggests that scraping every year might double a roof's years, which shows how much the hours a surface spends wet count.
That does not mean the clean side is flawless. Check both slopes from the ground for tilted tiles, gaps in the courses and debris in the gutter. If you can get into the loft, look for damp on the underside of the damp-facing slope in particular.
For the general picture of why moss takes hold on roofs, our guide to why moss grows on Stroud roofs under trees and shade covers the causes. This answer is about the single-sided pattern.
What to do about a single mossy slope
You do not have to treat the whole house. Cleaning one slope is a smaller job, and In MyBuilder's 2026 figures, scraping by hand is about £5 to £10 per m² and soft washing about £15 to £20 per m². Whole-roof cleaning there starts from about £300; a single elevation can cost less, though access still has to be provided.
Cutting back the cause helps more than repeated cleaning. Trim branches, clear the gutters and, where light can be let in, open it up. Without that, the growth returns on the slow side first, usually within the same few seasons, and each repeat clean costs the same as the last. Treating it with a soft wash cuts the green film, and if the wall or tree cannot be changed, you accept more frequent maintenance on that slope.
The green film that often goes along with moss is looked at in green algae on a roof.
Getting a quote for the slope that needs it
Send a few photographs of each side and tell us which way the house faces. We will say what the pattern suggests and whether one slope will do. When you are ready, you can ask for a roof cleaning quote, written once we have surveyed the roof.
Our wider guidance is at roof cleaning and moss removal in Stroud. Work we do on the roof is covered by a 10-year workmanship guarantee in writing, and you can reach us on 01453 933613.