Roof Cleaning Methods

Roof cleaning removes biological growth (algae, moss, lichen, mould) and dirt from roof surfaces. It is done mainly for appearance and to stop moss and debris from holding water and lifting roof units. The method matters: some approaches clean effectively at low risk, while others can shorten the roof's life or affect manufacturer warranties.

Soft washing

Soft washing applies a cleaning solution at low pressure, typically with a pump sprayer, and relies on chemistry rather than force. The most common active ingredient is sodium hypochlorite (chlorine bleach), sometimes with surfactants; oxygen-based cleaners such as sodium percarbonate are alternatives. The solution kills algae and other organisms; residue and dead growth wash off with rain or a gentle rinse. Asphalt shingle industry guidance has recommended a diluted bleach solution, with one widely quoted recommendation being equal parts household-strength bleach and water, applied without pressure washing.

Precautions include wetting and protecting plants below, managing runoff away from drains and waterways, protecting metal gutters and fixtures from corrosion, and following product labels and local regulations.

Manual removal

Thick moss is often brushed or scraped off by hand, working downslope to avoid lifting shingles or tiles, sometimes after a pre-treatment that loosens it. This is slow and requires safe roof access.

Pressure washing

High-pressure washing removes growth quickly but carries well-documented risks:

• stripping protective granules from asphalt shingles;
• eroding the surface of concrete tiles and wood shakes;
• breaking brittle tiles and slates;
• forcing water up under laps and into the roof;
• creating slippery, dangerous working conditions.

The Asphalt Roofing Manufacturers Association discourages pressure washers and mechanical cleaning tools on asphalt shingles, and many tile and shingle manufacturers caution against them. Some tile roofs are pressure-cleaned and then recoated as part of a restoration, which is a different process with its own specifications.

After cleaning

Zinc or copper strips, algae-resistant shingles and trimming trees slow regrowth. Cleaning is not a substitute for repairs: inspect the roof once it is clean, since growth can hide cracked or slipped units.