Why does moss grow on one side of the roof?

Moss has no interest in compass points. It needs moisture, a little warmth and a surface to cling to. A slope that dries quickly after rain gives it very little to work with, while one that stays damp for hours or days gives it everything. The side that sees the least direct sunshine is the side that dries slowest, which is why the green so often lines up with one end of the building.

You can see the same effect on a garden wall, a fence or a patio, where the cooler, shadier face goes green first. A roof behaves in the same way, only the slope is steep and the damp lingers in the overlaps, along the lower courses and in the valleys, the angled channels between slopes. Once a patch takes hold it holds water itself, like a sponge, and keeps the surface wet for even longer.

Sun is only part of the story. A tree that overhangs the roof drops leaves, twigs and sap into the lower courses and shades the surface below it. A tall building next door, a high boundary wall or a large chimney stack can throw a shadow across one slope for much of the day. Even a hedge by the gutter line can slow the airflow that dries the eaves.

That is why a house in a sheltered garden can look very different from one on an open plot, and why a roof can grow moss on a side that gets plenty of daylight. Our visit takes in what stands around the roof as well as the roof itself, because moss will return to the same spot if the shade and the leaf fall stay as they are. A bungalow with a mature oak at the back and a clean front slope is a typical picture.

Moss alone will not finish a roof, but it does no favours. A thick mat holds water against the tiles or slates, lifts at the edges and can work into the joints. When pieces break off they fall into the gutter and can block the outlet, which sends water down the wall or over the fascia (the timber board behind the gutter). Our answer on whether moss is bad for a roof goes through the damage in more detail.

What matters more is the condition under the moss. Growth can hide cracked tiles, slipped pieces and failing pointing along the ridge and verge, so the first step is a close look, not a spray. If the covering is sound, the green slope is mainly a cleaning job. If it is not, the repair comes first and the clean follows, so that you are not paying to treat a roof that is about to leak.

A treatment kills the growth that is there. It does not change how long the slope stays wet, so it is worth thinking about the cause at the same visit. Cutting back an overhanging branch lets light and air reach the roof. Clearing the gutter along the green side stops water backing up under the lowest course. Where a neighbour's tree is the problem, it is a conversation to have over the fence, since we cannot cut what is not on your land.

On the roof itself, the loose growth is lifted gently, the surface is treated and the debris is cleared away. After the survey a written quote follows, giving one figure for the whole job. If the shady slope is the only one affected, we will tell you whether treating that side alone is sensible, or whether both sides are better done while the access is already standing.

Stand across the road or at the back of the garden and study each slope in turn. Note which side is greenest and what stands next to it. Look along the gutter for green or black debris, and check the downpipe for a stain running down the wall. A phone photograph, zoomed in, will show a lot without anyone climbing onto the roof.

Send us what you have and we will say whether a visit is needed. Do not go up there yourself to look: a mossy slope is slippery, and the best view of a problem is usually from a safe distance. Our page on roof cleaning and moss removal shows how a survey runs, and the answer on whether moss will come back explains how the shade question affects regrowth.