Why roofs grow moss and algae
The roof is one of the most exposed surfaces of the whole house. It is constantly exposed to rain, dew and shade, and the slopes on the north side and under trees stay damp for a long time. These are ideal conditions for algae, moss and lichen, which over time form green and black deposits, and whole moss cushions in the hollows.
The process builds up slowly, so it is easy to miss until the roof clearly darkens. The longer you wait, the more the moss grows into the covering and the harder it is to remove.
What lying moss on the roof threatens
- Holding moisture on the covering, which encourages its faster ageing.
- Moss roots growing into gaps and joints, especially on older coverings.
- Clogging of gutters and drains with torn off moss.
- A worse look of the whole building, especially on light coverings.
Roof coverings and the right way to clean
Before you start, recognise what your roof is covered with, because not every covering tolerates the same treatment.
- Clay and concrete roof tiles are durable, but the top layer of concrete tiles dislikes aggressive scrubbing.
- Metal roof tiles have a thin coating that is easily scratched with a hard brush or a strong jet.
- Bitumen shingles and roofing felt are sensitive to high pressure, which can damage the granules and the top layer.
- Older coverings are sometimes brittle, so here care and the product matter more than force.
Tip
Always check the recommendations of the covering manufacturer. Some tiles and metal sheets have cleaning guidelines, and ignoring them can cost the warranty on the covering.
Choosing the product for roof cleaning
Green and black deposits and moss you remove with a product intended for roof cleaning that acts on microorganisms. This way the deposit comes off more easily, and you do not have to work with force where the chemistry is enough. The choice and use depend on the specific product, so follow its data sheet and instructions.
Safety first
Working on a roof is working at height, and that carries the greatest risk, greater than the cleaning itself. A wet, moss covered covering can be very slippery, and the slope of the surface makes moving around even harder.
Note
If the roof is high, steep or extensive, seriously consider leaving the cleaning to a company with proper equipment and safety measures. Never work on a roof without fall protection, alone or in bad weather. The safety data sheet of the product and the rules for working at height take priority over any advice.
Cleaning a roof step by step
- 1Secure yourself and the surroundings. Arrange fall protection and safe access to the roof. Cover what is below and protect the plants around the building.
- 2Remove the thick moss layer. Gently take off loose, thick moss from top to bottom, without scratching the covering. Do not scrub by force, because you can damage the surface.
- 3Apply the product. Spread the roof cleaning product according to the instructions and let it dwell for the stated time.
- 4Rinse with moderation. If the covering allows it, rinse it with a moderate jet from top to bottom. On shingles and metal be especially careful with the pressure.
- 5Clean the gutters. Remove torn off moss from the gutters and drains, so the water can flow freely.
What to do after cleaning the roof
After thorough cleaning and drying of the covering, part of the roofs can additionally be protected with a product that limits water absorption and the return of growth. Whether and which product is suitable depends on the type of covering, so follow the product description and the roof manufacturer guidance. How to do it we describe in the guide on protecting a roof.
The most common mistakes in roof cleaning
- Working at height without fall protection, the most serious mistake.
- Too high pressure, which damages the coating of tiles, metal or the granules of shingles.
- Scrubbing with a hard brush, which scratches the covering.
- Cleaning when wet and in rain, when the covering is most slippery.
- Skipping the gutters, where moss residue stays.
Frequently asked questions
Does moss on the roof have to be removed?
Moss holds moisture, grows into the covering and clogs the gutters, so it is worth removing. The longer it stays, the more it grows in and the harder it is to remove later.
Can I clean the roof with a pressure washer?
It depends on the covering. Clay tiles tolerate more than metal tiles or shingles, which are easily damaged by high pressure. It is safer to work with a product and rinse with moderation, according to the instructions and the covering manufacturer guidance.
Can I clean the roof myself?
On low, gentle slopes with proper safety measures it is possible. On high and steep roofs seriously consider a contractor, because the greatest risk is working at height, not the cleaning itself.
When is the best time to clean a roof?
In dry, windless and rain free weather, above freezing, when the covering is dry and less slippery and the product can work.
