Removing moss from a roof: a gentle and effective method
Moss on the tiles is not just an aesthetic problem: it holds moisture, slips under the tiles, lifts the slates in frost and can eventually encourage infiltrations. Yet cleaning a roof is never harmless: you work at height, on fragile and slippery materials. Before getting out the pressure washer, you must therefore weigh up the method carefully, because a roof damaged by a bad clean is far more costly than a mossy roof.
When to clean your roof?
The best time to treat a roof is a dry period, with moderate temperature, no frost and no strong wind. Spring or autumn are often ideal, outside the heavy rain periods. Absolutely avoid walking on a wet or frozen roof: terracotta and slate become extremely slippery. If you have the slightest doubt, delegate: working at height on a roof is one of the most dangerous operations there is.
Assessing your safety before climbing up
Before any work at height, honestly assess your level of comfort and the condition of the roof: if the slope is significant, if the roof is high or in poor condition, or if you must walk on the covering rather than work from the ground, the use of a harness attached to an anchor point or calling in a professional is a must. Safety equipment (harness, lifeline, scaffolding) is available for hire in DIY shops and specialist hire companies, but setting it up correctly requires experience. Never take the risk of a fall to save a few euros.
Gentle cleaning: the method to favour
- Assess the condition of the roof from the ground with binoculars: locate the mossy areas and check that no tile is broken or displaced.
- For light moss, a treatment with spray or gel moss remover, applied from the ground with a sprayer on a telescopic pole, is often enough.
- Leave the product to act for the stated time (generally several hours to a few days), then rinse with clean water if the product recommends it.
- For more soiled areas, use a soft broom or a bristle brush to lift the moss, without ever scrubbing too hard.
- If possible collect the debris that falls into the gutters, then clean them once the work is finished.
- Then apply a preventive moss product to delay regrowth: a treated roof stays clean for several years.
Why avoid the pressure washer on the roof
The pressure washer is tempting, but it is often counterproductive: at too high a pressure, it can shift the tiles, break the hooks, lift the slates of the roof and make water penetrate under the tiles. Moreover, it projects water everywhere, including into the gutters and onto the walls. If you insist, use a very reduced pressure, a grazing angle and a large distance, and only on a roof in good condition. In doubt, a professional with suitable equipment is preferable.
The mistakes to avoid
- Walking on a wet, frozen or steep roof: it is the most frequent cause of serious DIY accidents.
- Using aggressive products or acid that attacks the terracotta and discolours the tiles.
- Scrubbing with a metal tool or a hard brush that scratches the protective glaze of the tiles and accelerates the return of the moss.
- Projecting water under the tiles with a too-powerful pressure washer.
- Neglecting the gutters after cleaning: the moss and soil residue can block them and make the water overflow.
Calling in a professional: how much does it cost?
If the roof is pitched, high, in poor condition, or if you are not comfortable with the ladder, call in a roofer or a moss-removal company. The price of a professional roof clean generally ranges between 10 and 30 € per m² depending on the accessibility and the method, often with a preventive treatment included. It is an investment that protects your roofing for years and, above all, that avoids a significant risk of falling.
Preventing moss regrowth
Moss develops especially on the north-facing slopes, in the shade and in damp areas. To limit its return, install protections at the top of the roof if necessary, make sure the gutters and valleys are clean, and cut the tree branches that overhang the roof and keep it in the shade. A preventive treatment applied every three to five years completes the protection.
Removing moss from a roof is useful maintenance, but it must be done with care: gentle methods, suitable products, working from the ground as much as possible and a professional as soon as the height or slope becomes risky. Treated and maintained, your roof will keep its watertightness, appearance and longevity, for a modest investment compared with the damage that neglected moss can cause.