The short answer
Moss grows where a surface stays damp, and the side of a roof that gets little direct sun, usually the north-facing slope in the northern hemisphere, dries slowest. Shade from trees or taller buildings has the same effect. The sunny slope dries quickly after rain and stays clean, while the shaded one holds moisture for days and gives moss time to settle.
Moisture and sun do the sorting
Moss is a simple plant with no true roots. It takes in water through its surface, and it needs the surface it sits on to stay damp for long stretches. After a shower, a sunny slope warms up and dries within hours. The shaded slope may still be wet the next day, and in winter it can stay damp for weeks. Spores that land on both slopes survive on one and fail on the other.
In Britain the sun sits to the south for most of the day, so a roof slope facing north gets the least direct light and the least warmth. A house with a ridge running east to west, as many terraces do, therefore has one slope in sun and one in shade, and the difference is plain from the street. A ridge running north to south gives two slopes that each catch sun for part of the day, and the contrast is smaller.
Moisture in the air itself plays a part. Houses in sheltered valleys, near water or in damp hollows stay humid, so even a sunnier slope may carry some moss there, while an exposed hilltop house on the same orientation might stay almost clean.
Shade is not only about direction. A tall neighbouring building, a high boundary wall or a mature tree can put any slope into permanent shade, whichever way it faces.
Other things that make one slope worse
Direction is the first suspect, but a roof has other quirks. A slope that takes the prevailing rain-bearing wind gets wetter, though it may also dry faster in the same wind, so the effect is less clear-cut than shade. Slopes under overhanging branches collect leaf litter and drips, and that debris holds water against the covering. We look at that cause in whether overhanging trees cause roof moss.
The pitch matters. A shallow slope sheds water more slowly than a steep one, and its surface stays wet for longer. Valleys, the sunken channels where two slopes meet, carry runoff from both sides and tend to grow moss first. So do the areas below chimneys, where water is channelled round the stack, and the bottom courses near the gutter, where overflow and splash keep things damp.
Finally, the covering. A rough, porous tile or a lightly weathered stone slate holds more moisture than a smooth, new surface. An old roof is more likely to be mossy on its shaded face than a newly laid one, simply because it has had longer to gather a texture that moss can grip.
What moss does when it is left alone
On the shaded slope, moss does not stay a thin green film. It builds into a thick cushion that acts like a sponge. That mat soaks up rain and keeps it pressed against the covering, which makes the surface wetter still. In winter, frost can work into the mat and the tile or slate beneath it. The weight can lift the lower edges of tiles, and clumps that drop off end up in the gutter and choke the outlet.
It is easy to dismiss a mossy side as a cosmetic matter. What matters more is the condition of the covering under the moss. Our answer on how to remove moss from a roof explains the method, and our page on whether moss damages a roof looks at the harm it can do.
A mossy north side also warns about the rest of the roof. If moss has collected in the valleys, behind the chimney and along the eaves, the lead and flashings there deserve a look. A flashing is the dressed metal, usually lead, closing the gap between roof and wall or stack.
What can be done about it
The treatment is the same as for any moss: lift the cushion off by hand, treat the surface with a biocide, and let the roof rinse clean over the following weeks. What differs is the plan for afterwards. A slope that is damp by nature will regrow, and a clean on its own buys a number of seasons, not a permanent cure.
Reducing the shade helps more than anything. Cutting back branches that reach over the roof lets in light and air. Keeping gutters flowing means the lower courses are not sitting in standing water. Some owners fit zinc or copper strips below the ridge, so that rain washing over the metal carries a little of it down the slope and slows regrowth. We say more in our page on zinc strips and moss, including where they are worth fitting.
If you would like the shaded slope to look like the sunny one again, expect to treat it more often than the other side. A single survey will show how much work each slope needs.
Why the colour may differ after cleaning
Once the north side has been cleaned, it can look lighter and more even than the south side, which may have weathered in the sun. The two will move closer together as the roof settles. We explain the pattern in our answer on roof colour after cleaning.
To learn how much of your roof is affected, ask for the free survey, which includes pictures of every slope. Late autumn or early spring often suits the clean, when the moss is soft and the gutters can be cleared of the debris straight afterwards. Our roof cleaning service page has the full picture, and the office number is 01225 285181.
