Render cleaning glossary
Plain-English explanations of the terms you will come across.
- Acrylic render
- A thin-coat render bound with acrylic resin. It is flexible and comes in many colours but is generally less water-repellent and less breathable than silicone render.
- Biocide
- A chemical product designed to kill living organisms such as algae, mould, moss and lichen. Biocides used on render are regulated products and must be used according to their label.
- Drip
- A groove or overhang on the underside of a sill, coping or bead that makes rainwater drop clear of the wall instead of running down the render and causing streaks.
- Dwell time
- How long a cleaning product or biocide is left on the surface before rinsing. Longer dwell times help reach growth in textured finishes and kill it fully rather than just bleaching the surface.
- Efflorescence
- White, powdery salt deposits left on render or masonry when moisture carries dissolved salts to the surface and evaporates. It is usually brushed off dry rather than washed.
- Hollow render
- Render that has lost its bond with the wall behind it, also called blown render. It sounds dull when tapped and can crack or fall away, so it should be repaired.
- Lichen
- A slow-growing combination of fungus and algae that forms grey, yellow or black crusty spots on render. Dead lichen can take weeks or months to weather away after treatment.
- Monocouche
- A French term meaning single coat. A through-coloured, cement-based render applied in one thick layer, usually with a scraped finish, which does not need painting.
- Pebbledash
- A finish made by throwing small stones or pebbles onto a wet render coat so they bed into the surface. Spar dash uses crushed, often sparkly, stone in the same way.
- Silicone render
- A thin-coat render containing silicone resin, which makes rainwater bead and run off. It is applied over a base coat and mesh and is common on modern homes and extensions.
- Soft washing
- A low-pressure cleaning method where a biocide or cleaning solution is applied to a surface and left to kill growth, then rinsed gently, instead of relying on high-pressure water to strip it off.
- Through-coloured render
- Render with pigment mixed right through the material rather than applied as paint on the surface, so the colour is the same all the way through the coat.
- Trentepohlia
- A type of algae that appears orange, red or rust-coloured rather than green. It is often mistaken for rust staining and is treated with a biocide like other algae.
- Tyrolean
- A textured render finish flicked onto the wall with a hand-cranked machine, creating a rough honeycomb pattern. It is often seen on bungalows, gable ends and older extensions.
- Water repellent
- A clear treatment applied to masonry or render that reduces how much rainwater soaks in. Breathable types still let water vapour escape from inside the wall.
Last updated 6 November 2026