Moss itself doesn’t cause dampness on your roof, but the water and moisture it is so adept at retaining can.

Moss is a species that thrives in moist environments, and relies on absorbing moisture to provide it with much-needed nutrients. If your roof is susceptible to acquiring moisture from the way it's structured or positioned, or tends to be covered in shade, there’s a good chance you’ll have moss on your roof. If you’re already aware of it, you might be concerned about the moss causing dampness on the roof that will filter through to your internal walls.

What will moss do to your roof?

Moss has the potential to damage your roof both directly and indirectly. Since it has an innate ability to hold and retain water, this may cause the material of your roof to rot over time. This may also cause damage in the winter, as if the water retained by the moss freezes, then once it thaws it can rip away roof tiles or cause them to crack.

On the other hand, moss on your roof isn’t necessarily a bad thing. You might find it aesthetically displeasing, but its growth on your roof tiles can be a positive thing as if it grows in between them, it actually plugs gaps and fills any unwanted spaces. So it essentially becomes a support to any loose roof tiles. If you removed any moss that provides this function, you could end up with serious damage to your roof as the tiles will become unstable.

Can moss create penetrating damp?

Sometimes, excess moss growth on your roof can lead to penetrating damp, which is also known as lateral damp. This type of damp occurs when moisture develops through roofs and walls, and up through a building’s ground floor. Damp walls and roofs provide an ideal environment for moss to grow, as the organism thrives in moist conditions. Symptoms of penetrating damp caused by moss growth include damaged brickwork and localised damp patches.

Left untreated, penetrating damp can cause structural damage to your home, leading to rotting timbers, and damage to walls, floors and ceilings. It can also significantly age a property both structurally and aesthetically, affecting its sales or rental value.


Moss has the potential to damage your roof both directly and indirectly


Long term effects of moss on your roof

Moss can obstruct the flow of water down a sloped roof. Rather than running down off the roof and into the drainpipes, like it should do, the water can be absorbed and retained by the moss. This can cause severe leaks, as well as roof decay and rotting.

As the moss retains more and more water, the pressure on the roof will increase due to the weight. This build-up of water may start to seep into the roof’s shingles or membranes and inside the property.

A large accumulation of moss on your roof will need to be dealt with, as it will eventually lead to leaking and roof decay. Getting rid of moss before it leads to these problems will prevent structural damage to your roof, and save you from having to fix expensive problems it can cause.

Is moss bad for my roof?

Overall, moss isn’t particularly good for your roof. Even though it can provide support to loose tiles and fill in gaps, in an ideal world you wouldn’t have those gaps in your roof in the first place.

Moss can damage the structure of your roof by disrupting shingles and creating moisture that encourages the growth of bacteria and mold. This may lead to holes in your roof which will result in leaks.

How to treat moss on your roof

If moss is becoming a problem on your roof, there are several ways you can treat it.

  • Use a garden hose to lightly spray the moss with the intention of loosening it. When it becomes loose, gently scrape the moss away by hand. Avoid using a pressure washer as this may damage your roof tiles. Make sure someone foots your ladder, and be extra careful when walking on the roof as the moss may cause a slip hazard, so wear boots with a solid grip.
  • Bleach can help kill moss on your roof, however it’s important to note that bleach is a highly corrosive substance. Therefore it can damage other vegetation and potentially discolour your roof tiles. If you decide to use bleach to treat moss on your roof, make sure you wear protective goggles and gloves, and dilute the substance according to the instructions on the container.
  • A less corrosive alternative to bleach, moss control powder can be an effective method of getting rid of moss from your roof. It doesn’t cause damage to the structure or colouration of your roof, although it can still harm wildlife and vegetation as it contains zinc sulfate.

Can moss benefit your roof?

Although moss may generally be seen as a hazard on roofs, there are such things as ‘green roofs’ which are popular in communities that focus on being eco-friendly. That’s because moss can cut down on heating costs due to its natural insulation ability, as well as provide a natural habitat for local wildlife, such as birds and insects.

It’s not quite as simple as just allowing moss to grow on your roof, though, and hoping it will just become a ‘green roof’. Left unchecked, this type of free moss growth will likely cause the problems stated earlier in this article. So if you are considering creating a ‘green roof’, you should ensure it's properly designed and includes a root barrier which prevents damage to the surface of your roof.