The short answer
Because the north slope gets the least direct sun in Britain, it stays wet longest after rain and dew. Moss, lichen and algae need moisture to grow, so a slope that dries slowly gives them the best chance. Trees, tall buildings and a shallow pitch make it worse, and the same thing happens on any shaded face, whichever way it points.
Moss does not care about the compass. It cares about damp, and the north side of a roof stays damp the longest.
Sun, shade and slow drying
In the northern hemisphere the sun sits in the southern half of the sky. A roof slope facing north receives little direct sunshine, particularly in winter, when the sun is low. After rain, or a heavy dew, the water on that slope dries slowly, if at all, between showers.
Why do moss and algae favour damp tiles?
They are simple plants that take in moisture straight through their surface. A tile that stays wet for most of the day gives them what they need, while a sunny slope dries out and holds them back. Spores land on every roof, so the difference is in which slopes let them settle and spread.
That is why a house can look clean on its sunny side and green on the other. The tiles are identical, and the covering is the same. The only change is how long each face stays wet. Our page on roof cleaning and moss removal explains how growth is dealt with on whichever slope it appears, and what the roof needs afterwards.
What makes a shaded slope worse
Facing away from the sun is one factor, and others stack on top of it. Where several apply together, growth can be dense within a few seasons.
Do trees and neighbouring buildings matter?
Very much. Branches overhead cast shade and drip water long after rain has stopped. They also drop leaves, twigs and seeds that settle on the tiles and in the gutters, where they rot and feed growth. A tall neighbouring wall or a high hedge can shade a slope that would otherwise get some sun, so a south-facing roof can still grow moss if something blocks the light.
Pitch plays a part too. A shallow slope sheds water slowly and holds a film on its surface. A steeper one drains faster. Rough, worn tile surfaces grip moisture, which is why older roofs often grow moss earlier than new ones. Low, damp ground and nearby water raise the humidity around the house as well, and the roof shares it. Our answer on whether moss damages a roof explains why this is worth acting on.
Does the north side always grow moss first?
Often, but not always, and it is worth checking your own roof rather than assuming. The pattern comes from shade and damp, and the compass is only one clue to where those are.
Why is the moss on my south slope instead?
Something is shading it, or the water is not leaving. Look for a tree, a taller building or a roof section that shelters it. Check the gutter and the valley, where the two slopes join, for debris holding water against the tiles. A south face under a canopy of leaves can be greener than a bare north one. Lichen, which grows as crusty grey or orange patches, can appear on exposed slopes too, as our page on lichen on roof tiles explains.
Photos help. A few shots of each slope, taken from the garden on a dry day, show where the growth sits and how it is spreading. Compare the shaded ground below the roof with the open parts of the garden: the places where the grass stays wet are usually the places where the tiles do. That tells you where to look first, and tells a roofer where to begin.
What to do about a mossy north slope
A thin film of moss on a sound roof is not an emergency. Thick growth that holds water, spills into the gutters or sits over slipped tiles deserves a proper look, because it hides faults and keeps the slope wet.
Can the shade be reduced?
Sometimes. Cutting back branches that overhang the roof lets in light and air, and clearing gutters and valleys of leaves stops water pooling. Neither needs a ladder, and each slows regrowth after a clean. A tree surgeon is the right person for large limbs, while loppers cover a low hedge. Our answer on moss growing back after cleaning goes into what lengthens the gap.
The north slope is surveyed along with the rest of the roof, with pictures of the tiles, the ridge and the gutter line, and a single written total follows. Seeing the shaded side next to the sunny one makes the contrast easy to judge. Where the tiles underneath are cracked or the covering is tired, that is raised first. Send us a photo of the shaded side, or ask for a visit, and you will get a view on whether it needs a clean, a repair or simply a regular check.
More on shaded roof slopes
Is moss on one side of the roof a sign of a problem?
Not by itself. It shows that side stays damp longer, which is normal for a shaded slope. Thick growth is a concern because it holds water against the tiles and can cover cracks or slipped tiles. A look at the tiles and the gutter below tells you whether anything needs doing.
Will cutting back trees stop moss on the north side?
It helps, because more light and air dry the slope faster and fewer leaves land on it. It will not remove moss that is already there, and a north face still gets little sun. Think of tree work as a way to slow regrowth between cleans, not a cure.
The full picture: Roof cleaning and moss removal in Taunton ยท More short answers