Soft washing glossary
Plain-English explanations of the terms you will come across.
- Biocide
- A substance that kills or controls living organisms such as algae, mould, moss and lichen. In Great Britain, biocidal products are regulated by the Health and Safety Executive.
- Dwell time
- The time a soft wash solution is left on a surface before rinsing so that it can kill the growth. Too short and it may not work; too long and it can dry and mark.
- Efflorescence
- White, powdery or crusty salt deposits left on masonry when water moves through it and evaporates. It is mineral, not organic, so a biocide does not remove it.
- Lichen
- A crusty, slow-growing organism made of a fungus and an alga living together. Forms grey, black, white or yellow circles on roofs and walls and fades slowly after treatment.
- Monocouche render
- A through-coloured, single-coat cement-based render that is usually not painted. Cleaning must avoid damage, as patch repairs are hard to colour match.
- Pre-wetting
- Soaking plants, lawns and sometimes the surface itself with plain water before applying a soft wash solution, so that less solution is absorbed.
- Quaternary ammonium compounds
- Often called quats, a group of slower-acting biocides that keep working for days or weeks. Commonly used on roofs, stone and delicate surfaces.
- RAMS
- Risk assessment and method statement. A document setting out the hazards of a job and how the work will be done safely, commonly required for commercial sites.
- Safety data sheet
- A document supplied with a chemical product describing its hazards, safe handling, first aid and disposal. A contractor should be able to show it on request.
- Sodium hypochlorite
- The active ingredient in bleach and the basis of many fast-acting soft wash solutions. It is diluted to a strength that suits the surface and the type of growth.
- Soft washing
- A low-pressure exterior cleaning method where a biocide solution is applied, left to kill organic growth, then rinsed off. Used on surfaces that high pressure could damage.
- Surfactant
- An additive that helps a cleaning solution spread and cling to vertical surfaces and roofs rather than running straight off, giving the biocide time to work.
- Thin-coat render
- Modern renders such as silicone and acrylic applied in a thin decorative top coat. They are easily scarred by high pressure, so low-pressure cleaning is preferred.
- Tiger stripes
- Trade term for the black vertical streaks on the outside of uPVC gutters, caused by pollution and carbon carried by rainwater. Usually need a dedicated cleaner.
- Trentepohlia
- A type of red or orange algae often seen on render and painted masonry, sometimes mistaken for rust. It responds to biocide treatment.
Last updated 6 November 2026