The short answer
The north-facing slope gets the least direct sun, so rain and dew stay on the tiles for longer. Moss needs a damp surface to root, and a slope that stays wet most of the day gives it one. Shade from trees or a taller neighbouring house makes it worse. Cleaning clears it, and opening the slope to light and air keeps it from returning quickly.
How sun, shade and drying work together
A roof is a wet surface for a good part of the year. What separates a clean slope from a green one is how quickly each dries after rain, and drying depends on sun and moving air. Across Britain, a pitch facing north receives hardly any direct sunshine, particularly in winter when the sun is low. The tiles take longer to warm, the film of water lingers, and the cool damp surface suits moss.
That is why you can stand in the garden and see a roof that is clean on the south side and thickly green on the north. The coverings are the same and the age is the same. Only the drying differs.
On a typical Swindon pair of semi-detached houses the rear slope often faces a different way from the front, so one side of the house can show the problem long before the other. Our page on why moss grows sets out the full list of conditions, and orientation is only one of them.
What else makes a shaded slope worse
North-facing slopes are rarely shaded by their orientation alone. Several things pile on.
- Trees. Mature trees to the north block what little light there is and drip onto the tiles long after a shower has stopped.
- A taller building. A neighbouring gable or an extension next door can keep a slope in shadow most of the day.
- Low pitch. Rain leaves a shallow slope reluctantly, which makes a low pitch facing north the worst combination.
- Porous tiles. Concrete tiles soak up and hold moisture more readily than clay or slate.
- Poor air movement. Walled gardens and tight terraces slow the wind that dries a roof.
Where several of these stack up, regrowth tends to be quicker. Where only the orientation is against you, a clean can hold for longer. The survey shows which case your roof falls into, with photographs of each slope.
Is the moss doing any harm?
Wet moss sits against the tiles like a sponge, and in a freezing spell that water can expand and flake the surface of older tiles. A thick cushion of it can also lift and block the gaps where tiles overlap, so water backs up rather than draining. It is heaviest when soaked, and it sheds into gutters and valleys, where it causes the blockages described on our page about moss in gutters.
That said, a light green film on one slope is cosmetic for a while, and nobody needs to panic. A sensible habit is to glance up from the garden in spring and autumn, note whether it is spreading, and plan a clean when the cover thickens or the gutters start to fill. We would rather tell you a slope can wait than sell you a visit it does not need.
Selling is one reason to act sooner. A dark north slope is the first thing a buyer sees from the road, and our page on cleaning a roof before selling weighs that against what a survey will pick up.
Never try to clean it by scraping from a ladder. Dry scraping tears the weathered face off concrete tiles, so you wear out the covering you meant to protect.
What helps a north slope stay cleaner
The biggest gains come from more light and more air. Cutting back or lifting the crown of overhanging trees opens the slope to the sun and cuts the drip. Our page on trees that overhang a roof explains how to go about it. Check the position of the trees before pruning: some are protected, so ask your council's tree officer or look at its website before you cut anything significant.
Keep gutters flowing so that water leaves the slope quickly, and keep valleys and the lower courses free of leaves. After a clean, a biocide treatment gives the surface a residual film that slows regrowth. It does not replace the conditions that drive moss, which is why a shaded slope is the first to need attention again.
Zinc or copper strips below the ridge can help on a long north pitch, though they protect only the area beneath them. Do not expect a product to defeat shade by itself.
How the north side affects a clean
A shaded slope often has heavier growth, so it may take longer to treat and rinse than the sunny side. Growth that has sat for years roots into the tile face, so the washing has to be gentle enough to leave that face intact. We use low pressure and a biocide wash rather than blasting; the how often to clean a roof page covers the timing, and the roof cleaning and moss removal hub shows how a visit runs.
If you are weighing the price, MyBuilder's roof cleaning guide, updated in August 2026, says costs start from around £300 and puts a typical pitched roof at £450 to £850. Those are national figures. Your own figure turns on how much roof there is, how it is reached, and how thick the growth lies on each slope. A house with a heavily shaded north side and a clear south side can cost the same as one that is lightly green all round, because the work follows the worst slope. The roof cleaning cost page goes through the factors, and the survey gives you one written total for your own roof.
Ring 01793 626096 or send a picture of the north slope by WhatsApp or through the form, and our reply will describe the growth, and whether a clean now or a wait suits the slope better.