Why Does Moss Grow on One Side of the Roof?

What the sun does for a roof

Direct sun does two things for a roof. It warms the surface, and it dries it. A south-facing slope catches sunshine for much of the day, so after a shower the tiles are dry again within hours. Moss spores that land there never get long enough in the damp to take root. A north-facing slope tilts away from the sun, particularly from autumn to spring when the sun is low, and the surface may stay wet for days.

Moss does not need much light. It needs water, and a surface that holds on to it. That is why the cold, shaded side of a roof is often green from eaves to ridge, while the sunny side looks almost untouched. The difference you see from the garden is the difference in drying time, not in the number of spores landing.

The ground matters too. A roof with a damp, shaded garden on its north side sits in a more humid pocket of air, and that raises the odds that the slope stays damp long after the rain has stopped. Clay soil and low-lying plots, which are common around Basingstoke, make the effect stronger.

Other things that cause lopsided growth

North is not the only cause. A tall tree on the west or east side can cast a shadow across a slope for half of every day. A taller house next door, a high boundary wall or a hedge does the same. Slopes that sit under a canopy also collect leaf litter, pine needles and seed that break down into a damp mulch between the tiles.

The covering and pitch play their part. A shallow pitch drains slowly, and a rougher tile holds moisture. On a roof with an extension or dormer, valleys and abutments can concentrate water on one side. We go through the other factors on the page about how long before moss grows back, since the same things decide how quickly the growth returns after a clean.

The compass is a rough guide only. On a hipped roof, the north-east and north-west faces behave differently from the straight north, and a single-storey extension can shade the slope above it. Walk round the house on a dry day and see which parts of the roof are in shadow at different times. The answer often explains the pattern better than the bearing does.

Does one-sided moss mean something is wrong?

Usually not. It is a pattern more than a fault. But it is worth looking at the shaded slope more closely, because moss that has been in place for years keeps the tiles wet and hides the state of what lies beneath. Cracked or slipped tiles are harder to spot under a thick mat, and a gutter on that side that has filled with moss and debris can overflow onto the wall.

If the growth is thick on one side only, ask for it to be checked. The survey includes photographs of the slope, so any damage is documented before and after the clean. If a repair is needed, it is quoted separately and plainly. On the moss on roof tiles page we set out when growth moves from cosmetic to a genuine problem.

Photographs taken from the same spot each year help show how quickly the growth is advancing. If one slope is turning green quickly, it is a signal to pay attention to the gutter on that side as well.

What helps the shaded side

Cutting back overhanging branches is the most useful thing you can do, because it brings light and air onto the slope. Keeping gutters clear helps the roof edge drain. A biocide treatment after a clean makes the surface less welcoming for a while. Some homeowners fit zinc or copper strips along the upper slope, which release a trace of metal as the rain runs down, and our answer on whether zinc strips stop moss covers how well that works.

There is no way to turn a north slope into a south one. The aim is to keep the moss thin, so it does little harm and is easy to remove. We look at your roof first and recommend what is worth doing. The service is set out on the page for roof cleaning in Basingstoke.

A shaded slope will always go green sooner than a sunny one, so it helps to set expectations at the start. Treating the north side again after a few seasons is normal, and it is not a sign that the earlier work failed.