Red algae on render, a rusty, orange-red discolouration that can look almost like a stain bleeding out of the wall itself, catches a lot of homeowners off guard because it looks so different from the more familiar green algae most people expect on a damp wall. It's a specific organism with its own growing conditions, and it responds well to the right treatment once you know what you're dealing with.
This guide explains what red algae actually is, why it's particularly common in coastal and North East locations, and the softwash approach that removes it properly.
What red algae actually is
The organism behind most red or orange staining on render and masonry is a genus called Trentepohlia, a type of algae that, unusually, produces carotenoid pigments (the same family of pigment that makes carrots orange) which give it its distinctive rusty red-orange colour rather than the green most algae species show. It grows on the surface of render, brick and stone wherever there's enough moisture and a suitable surface to establish on.
It's easy to mistake for a rust stain from a metal fixing, or even a general weathering discolouration, but unlike a rust stain it tends to spread gradually across a wider area over time and has a slightly fibrous, textured appearance up close rather than a flat stain.
Why it's common in coastal and North East locations
Trentepohlia is particularly well suited to environments with high atmospheric moisture and airborne salt, which makes coastal and near-coastal locations especially prone to it. Render on properties within a few miles of the coast, or exposed to prevailing damp weather systems coming in off the sea, often shows red algae staining more prominently and more quickly than equivalent inland properties with otherwise similar conditions.
It also tends to favour certain aspects and textures, rougher render finishes and walls that don't get much direct drying sun show it more readily, similar to the pattern seen with green algae, just with this particular species preferring slightly different micro-conditions.
The softwash treatment that removes it
Red algae responds well to the same soft wash principle used for green algae and general render cleaning: a low-pressure application of a biocide treatment that kills the algae at the surface, left to work, then rinsed gently rather than blasted with high pressure. Because Trentepohlia's pigment can be quite persistent even once the organism itself is dead, it's worth expecting the colour to fade progressively over the days and weeks following treatment rather than disappearing instantly during the wash itself.
As with other render staining, pressure washing alone risks damaging the render's decorative surface coat without necessarily fully addressing the algae's ability to regrow, since physical pressure can miss organism embedded slightly into the render's texture that a biocide would properly kill off.
Prevention and realistic maintenance
Given how strongly coastal moisture and salt air favour this particular algae, complete prevention isn't realistic for properties in the most exposed locations, but regular soft wash treatment, roughly every 1-2 years for the most affected elevations, keeps it from building up to the point of being a prominent, unsightly feature of the wall.
It's genuinely one of the more persistent staining types we treat, so setting expectations around a maintenance cycle rather than a one-off permanent fix gives a more honest, realistic picture for property owners in the more exposed parts of the region.
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