The short answer
Overhanging trees do not plant moss, but they create the conditions it thrives in: shade that stops the roof drying, drips that keep it wet, and a steady fall of leaves, twigs and seeds that settles on the slope and in the gutters. A roof under a tree usually grows moss faster than one in the open, and trimming back branches is often the most effective single step.
Three ways a tree feeds the problem
The first is shade. Moss is slow to establish on a surface that dries quickly in sun and wind. A tree canopy blocks light and holds still air against the roof, so the slope below dries slowly after rain. The shaded patch becomes the part of the roof that is damp for longest, which is exactly where moss prefers to start.
The second is water. Even after rain stops, a tree goes on dripping from its leaves and branches. A roof beneath a mature tree can be wet for hours longer than one a few metres away, and the drips fall in the same places, which is why moss often forms in distinct patches.
The third is debris. Leaves, twigs, seed cases, blossom and bird droppings land on the roof and settle in the laps and corners. They rot slowly into a thin soil, and moss spores take hold in it. Debris also collects in the valleys and gutters, and a blocked channel overflows onto the slope below. The result is a damp, nourished surface that is hard for any roof to resist.
Which trees cause the most trouble
Any tree close to the house can contribute, but some are worse than others. Evergreens such as conifers and holly shade the roof all year and drop needles or leaves that are slow to clear. Deciduous trees like beech, lime and sycamore let light through in winter, though their leaf fall in autumn can fill gutters in a week. Trees that drop sticky sap or honeydew, such as limes, leave a film that grows black mould and holds dirt.
It is not only trees on your own land. A neighbour's tree, a hedge on the boundary or a wooded lane behind the house can have the same effect, and you may not have any say over it. Courtesy and a conversation often work better than a dispute.
Roots matter in a different way. A large tree close to a wall can affect the ground and the drains, which is a separate concern from moss, but it is a reason to think about what is planted near the house when you next plan the garden.
Distance matters. A branch that touches the roof causes physical damage as well, scraping the covering in wind and dropping its load of leaves in one spot. Branches that reach within reach of the eaves also give squirrels and birds an easy way onto the roof, where they may disturb ridge tiles and nest in the eaves.
What to do about the branches
Cutting back is usually the most effective step. Bringing a canopy back from the roof lets in light and air, and the slope will dry out more quickly. As a rule, aim for a clear gap between the branch tips and the roof, so that the wind cannot whip them against the covering and leaves fall elsewhere.
Before cutting, check the position. Some trees are protected, and about half of Bath lies in its city-wide conservation area, so ask the council before pruning anything big. Where birds are nesting, the work should wait, because nesting birds are protected by law.
Tree surgery should be done by a qualified arborist, not by the roofer and not from a ladder. If the tree is a neighbour's, ask them first. Many neighbours are glad to have an overhanging limb dealt with once it is explained.
Looking after the roof under a tree
Where a tree has to stay, you can still reduce its effect. Keep the gutters clear, ideally in autumn once the leaves are down and again in spring. A gutter guard may help in some cases, though it needs checking too. Clear leaf litter from valleys and from behind chimneys, using a long-handled brush from a safe position, or have it done as part of a visit. Our answer on scaffolding and roof access explains how a roofer reaches those spots safely.
Have the moss treated before it builds into a thick mat, because a thin growth is quicker and gentler to remove. Some owners also fit zinc strips near the ridge, which can slow regrowth on a slope that has to stay shaded. We cover how they work, and where they are worth the trouble, on our page about zinc strips and moss.
If cleaning is needed more than once, we will tell you whether the cause is the tree or the roof, and whether a different approach would help. Some roofs under heavy cover are better served by a regular light treatment than by occasional heavy cleaning.
Choosing the cleaning method for a shaded roof
Moss under trees can be thick, and the covering beneath it is often wet and soft at the surface. A gentle soft wash, as explained on our page about soft washing roofs, is better suited than a hard jet. The moss is lifted by hand first, then the treatment goes on, and the roof is left to clear over the following weeks.
When we survey, we photograph the roof and the trees above it so you can see what is going on, and we tell you what we would cut back and what we would leave. Afterwards you get a single fixed price on paper, and before any branch is cut, check with the council whether the tree is protected or stands in a conservation area. Our roof cleaning service page has the full picture, and you can reach us on 01225 285181.
