The short answer
Cleaning a commercial roof is worth doing when it keeps water moving (outlets, gutters and valleys), when growth is holding moisture against a coating or membrane, or when a roof is being prepared for a coating or survey. On fibre cement roofs that may contain asbestos, HSE guidance limits the methods: wet biocide treatment and gentle scraping, or specialist machines that collect the slurry, and high-pressure jetting only in exceptional cases. Cleaning restores drainage and appearance; it does not fix failing coatings, laps or membranes.
On this page
Roof cleaning gets sold on looks: a green, streaked warehouse roof turned grey and bright again. For a building owner the better reasons are practical. Dirt and growth block outlets, hold water against coatings and seams, and hide defects from anyone surveying the roof. Some cleaning is genuinely worth paying for; some is cosmetic; and on older fibre cement roofs the wrong method creates an asbestos problem that did not exist before.
Where cleaning pays for itself
Outlets, gutters and valleys
This is the cleaning that matters most. Leaves, silt, moss and litter build up in box gutters, valley gutters and flat roof outlets until water backs up and overflows into the building. Many commercial roof leaks reported as “the roof failing” turn out to be a blocked outlet. Clearing them twice a year, usually in spring and after leaf fall in autumn, is the core of most planned maintenance contracts.
Before a coating or overlay
Any coating, liquid membrane or cut edge treatment needs a clean, sound surface to bond to. Cleaning is the first stage of the job, and a specification should say exactly how it will be done and to what standard.
Before a survey
Moss and dirt can hide splits, blisters and corrosion. A surveyor who cannot see the surface cannot grade it. Light cleaning of key areas may be part of a condition survey.
Where growth holds moisture
Thick moss and lichen on metal or fibre cement sheets keep the surface damp, which is hard on coatings and on the cut edges of steel sheets. On flat roofs, vegetation rooting in the gravel or at the edges can damage the membrane.
Where cleaning is mostly cosmetic
On a sound roof with clear drainage, a general clean improves appearance and sometimes reflectivity, but does little for the roof’s life. It is a reasonable spend before a sale or letting; it is not maintenance in the sense of preventing leaks.
Cleaning by roof type
| Roof | Usual approach | Watch for |
|---|---|---|
| Profiled metal and composite panels | Low-pressure wash with a suitable cleaner, biocide where there is growth | Damaging soft coatings; water forced into laps and fixings |
| Single ply membranes | Low-pressure wash with the maker’s approved cleaner | Some cleaners and solvents damage membranes; check the guarantee terms |
| Felt and asphalt | Brush and clear debris, treat growth | Pressure washing strips mineral surfacing |
| Fibre cement sheets | HSE methods only (see below) | Asbestos fibres; fragility |
| Rooflights | Gentle cleaning from safe access | Rooflights are fragile; never lean or step on them |
Fibre cement roofs and asbestos
Fibre cement sheets put up before 2000 may contain asbestos. As the surface weathers, fibres sit near the surface, and aggressive cleaning releases them into the air and into the slurry that runs off the roof.
HSE’s Asbestos Essentials sheet a12, on cleaning weathered asbestos cement roofing and cladding, sets out the accepted methods for non-licensed work:
- From safe access, wet methods only: an approved biocide applied with a low-pressure sprayer, the surface kept wet, and growth removed by gentle scraping.
- Specialist roof cleaning: a contractor using a cleaning machine that collects the slurry and passes it through a filtration system.
- High-pressure jetting: only in exceptional circumstances, and by a specialist.
- No dry sweeping of dust or debris; a Class H vacuum or damp rags instead.
- Gaps into the building sealed so slurry cannot get in, the area restricted, and waste double-bagged, labelled and taken to a licensed site.
The same sheet repeats a point worth underlining: asbestos cement roofs are fragile and cannot bear a person’s weight. Cleaning is done from platforms or other safe access, never by walking on the sheets. Our note on fragile roofs covers the rules.
If a roof is in such poor condition that cleaning it is a serious undertaking, it is worth asking whether overcladding would be the better use of the money.
What cleaning will not fix
Cleaning does not repair anything. It will not:
- stop cut edge corrosion once the coating has lifted;
- reseal leaking laps or perished fixing washers;
- rejuvenate a membrane at the end of its life;
- dry out wet insulation under a flat roof.
A contractor who offers to “clean and seal” a roof should be clear about which of these they are addressing and with what system. A coating applied over a poorly prepared, dirty or corroded roof can fail within a few years and make the eventual repair harder.
Questions to ask a roof cleaning contractor
- How will you access the roof without anyone standing on fragile sheets or rooflights?
- What method and pressure will you use, and is it suitable for this roof’s coating or membrane?
- If the roof is fibre cement, has it been checked for asbestos, and how will you deal with the slurry and waste?
- How will you protect the building, drains and anything below from run-off?
- Will you clear and check every outlet, gutter and valley, and report any defects you see?
Done well, a clean and gutter clear is cheap insurance and a good moment to have the roof looked at properly. Done badly, on the wrong roof, it can cost more than it saves.
To have outlets cleared, a roof cleaned or a maintenance visit priced, use the form below. The Commercial Roofing UK home page covers the bigger decisions if cleaning turns up more.