Roof cleaning guide
How roof cleaning works on residential properties
Most London homeowners notice moss or discolouration on their roof tiles but treat it as a cosmetic issue. In most cases it is not. Moss retains moisture, accelerates tile and mortar deterioration and blocks gutters. This guide explains exactly how professional roof cleaning is carried out, which methods suit which roof types and what long-term protection looks like.
A residential roof in London is exposed to persistent moisture, variable temperatures, shade from surrounding trees and buildings, and significant levels of airborne pollution. These conditions create an ideal environment for the establishment of moss, algae, lichen and other biological growth on roof tiles. Most new roofs will show the first signs of biological colonisation within 2 to 7 years of installation, depending on the roof's aspect, pitch, surrounding vegetation and tile material. The north and east-facing slopes of a roof receive less sunlight and dry more slowly after rain, making them significantly more prone to moss and algae growth than south and west-facing slopes. Left untreated, this growth creates a cycle of increasing moisture retention, tile deterioration and structural risk that becomes progressively more expensive to address.
2 to 7 years
typical time before biological growth first appears on a new residential roof in London, depending on aspect and surrounding environment
Softwash first
the National Federation of Roofing Contractors advises against aggressive high-pressure moss removal which poses structural risks to roof integrity
3 years clean
most professionally softwashed and biocide-treated roofs remain visually clean for approximately 3 years before significant biological regrowth requires retreatment
What moss and algae actually do to a residential roof
Moss is not merely unsightly. It is a plant that actively retains moisture against the roof surface through its sponge-like structure. A thick layer of moss on a concrete or clay tile roof holds water in contact with the tile surface for days or weeks after rain, rather than allowing it to run off as the roof was designed to do. This sustained moisture contact accelerates freeze-thaw cycling within the tile material in winter, which progressively expands microscopic cracks and causes spalling, delamination and surface loss on concrete tiles in particular. Moss also grows into the mortar of ridge tiles and hip tiles, where its expanding root system physically breaks down the cement, eventually causing mortar failure and displaced ridge tiles.
Algae produce the dark green, black and red staining commonly seen on London roofs. While algae are less structurally damaging than moss, they are a precursor to lichen establishment. Lichen are composite organisms formed from algae and fungi and they are the most difficult to remove from roofing materials. Lichen attach to the tile surface using a network of penetrating root-like structures called rhizines that physically bond to the tile surface. Once established, lichen is very difficult to remove without damaging the underlying tile. The circular white, grey and brown patches commonly visible on older concrete and slate tiles are typically lichen. As the Benz Softwash guide notes, lichen can only be removed by killing it and waiting for the growth to detach itself from the surface, a process that can take several weeks to months after treatment.
Beyond tile damage, moss and algae falling from the roof surface are the primary cause of gutter blockages in London residential properties. Clumps of moss dislodged during heavy rain or wind collect in gutters and downpipes, creating dense blockages that cause overflow, fascia board saturation and eventually internal damp if left unaddressed.
The cleaning methodsHow professional roof cleaning is carried out
Softwashing (recommended for most residential roofs)
Softwashing uses a biodegradable biocidal cleaning solution applied at low pressure, typically equivalent to garden hose pressure. The chemical solution kills algae, moss, lichen and mould spores at a cellular level, breaking down the organisms at their root. The dead growth then detaches from the surface naturally over the weeks following treatment, aided by rain and wind. A post-treatment biocide application after the initial clean kills remaining spores and provides residual protection that significantly delays regrowth, typically for 2 to 3 years. Softwashing is the preferred method of the National Federation of Roofing Contractors and is safe for all common UK residential tile types including clay, concrete and slate.
DOFF steam cleaning (for heritage, delicate and stone tiles)
DOFF steam cleaning uses superheated steam at low pressure to remove biological growth and surface contamination without chemical solutions. It is particularly suited to older properties where tiles are fragile, natural slate roofs, and heritage properties where chemical applications may be restricted. The high temperature of the steam kills all biological organisms on contact. DOFF is gentler than pressure washing and leaves no chemical residue.
Pressure washing (for sound modern concrete tiles only)
High-pressure washing can be used on sound, modern concrete tiles with good pointing and no missing or cracked tiles. It produces an immediate visual clean but does not kill the biological spores, meaning regrowth typically occurs faster than after softwashing. The National Federation of Roofing Contractors advises against aggressive pressure washing because it can dislodge loose tiles, damage ridge mortar and force water under tiles. It is never appropriate for old, fragile or handmade clay tiles or natural slate.
Manual moss removal (for thick, established growth)
For roofs with very heavy moss accumulation, manual removal using plastic scrapers, stiff brushes and hoe-style tools is often carried out before softwashing. This removes the bulk of the growth physically, reducing the load on the biocidal treatment and improving its penetration to the tile surface. Any mechanical removal is followed by biocidal treatment to kill remaining spores.
What a professional roof cleaning visit involves from start to finish
1. Roof inspection
The contractor inspects the roof for missing or cracked tiles, loose ridge mortar, visible entry points for water and any areas of structural concern. Cleaning is not carried out on a structurally compromised roof without the issues being repaired first. The homeowner is informed of any defects identified.
2. Protection of surrounding areas
Plants, garden furniture and any features below the cleaning area are protected or moved. Downpipes are often temporarily capped to prevent debris from entering the drainage system during cleaning.
3. Biocidal solution application
The biocidal cleaning solution is applied to the entire roof surface using a pump sprayer, low-pressure application equipment or a water-fed pole system at low pressure. The solution covers all tile surfaces including ridge tiles and valleys.
4. Dwell time
The biocide is left to dwell on the roof surface for the time required to kill the biological organisms. Depending on the product, the level of contamination and the weather conditions this is typically 30 minutes to several hours.
5. Low-pressure rinse
The roof is rinsed at low pressure to remove dead growth and product residue. The roof will continue to improve in appearance over the following weeks as the dead biological material weathers away naturally.
6. Post-treatment biocide and gutter inspection
A second biocide application after rinsing kills any remaining spores and provides the residual protection that extends the clean period. The gutters and downpipes are checked and cleared of any debris dislodged during cleaning.
Roof cleaning London
Professional residential roof cleaning across London by Cloud Nine
Cloud Nine cleans residential roofs across London using professional softwashing with biocidal treatment. We inspect the roof before every clean, use the correct method for the tile type and apply post-treatment biocide to protect results for up to 3 years. Contact us for a free quote.