Green algae on walls: causes, fixes and what surveyors look for

A well-maintained garden is a genuine pleasure. Fresh air, green space and a sense of nature on your doorstep are all well-documented benefits. That's why the idea of escaping to the countryside holds such appeal for so many people.

But there is greenery that nobody wants. We have covered problem plants before, with Japanese knotweed being one of the more alarming examples. But what about that other unwelcome growth: the green algae that creeps across exterior walls, timber frames, plastic fascias, fence panels and roof surfaces?

It is more than an eyesore. Left unmanaged, it is a flag that something is wrong with the property, and a chartered surveyor will treat it as exactly that.

Like black mould inside a property, algae thrives where conditions are persistently damp. That typically means areas that receive little or no direct sunlight and have poor airflow, so surfaces never fully dry out. North-facing elevations, walls in the shade of overhanging trees and narrow side passages are typical examples. In the UK's climate, with significant rainfall throughout much of the year, these spots are far from uncommon.

Blocked or leaking gutters are a particularly common cause. Where rainwater is not being properly directed away from the building, water runs freely down the walls and saturates the wall surface. Over time this can also drive water damage inside the property, creating the conditions for black mould growth internally.

In winter, poorly insulated walls create cold spots where condensation forms, and that's another environment in which algae can take hold.

The problem is compounded by the fact that most construction materials, including brick, stone, render, concrete and timber, are porous to some degree. They hold moisture and, in doing so, provide a ready surface for biological growth.

Often, yes. Algae on a north-facing or shaded wall can simply reflect the aspect of the building. Heavy or patchy growth in one area usually points to a specific source of moisture, such as an overflowing gutter, a leaking downpipe or a cracked sill. If the wall behind the growth feels cold or the room inside shows signs of damp, the two are likely to be connected.

Algae itself is largely cosmetic. The real issue is what it tells you and what it holds against the wall. A layer of algae, moss and lichens keeps the surface damp for longer after rain. On porous brick, stone and render, that retained water can freeze in winter, causing the face of the masonry to flake or spall and mortar joints to crumble. Render that stays wet is also more likely to crack and come away from the wall behind it.

On timber window frames and fascias, persistent damp speeds up decay and peeling paint. On paths, steps and the patio, algae makes hard surfaces slippery. A surveyor will look past the algae itself to the source of moisture feeding it and to any damage it has already caused.

For existing growth, the approach is fairly straightforward. Use a stiff brush to remove algae and loose growth, and pressure washing will shift the worst of it. High pressure can damage soft brick, old mortar and render, though, so use a low setting or rinse with a garden hose. Many contractors now use a soft wash for exterior cleaning instead, applying a biocide at low pressure and letting it work before rinsing.

A 50/50 solution of white wine vinegar and water is an effective traditional remedy. A proprietary algae remover or biocidal cleaner is also available, though these should be used with care around other plants and wildlife in the vicinity. After cleaning, allow the wall to dry before applying any preventative treatment.

There are also preventative spray products designed for walls, roofs and other exposed surfaces that inhibit algae from taking hold in the first place. None of these require specialist application. If you are repainting, choose a breathable masonry paint that contains a fungicide or anti-algae additive, and treat the wall with a biocide first. Fixing the source of the moisture will make any treatment last longer. Otherwise algae removal is only ever a temporary fix.

For buyers, first impressions form the moment you pull up outside a property. Evidence of poor maintenance registers before you have even stepped through the door, whether that's rotting window frames, a neglected garden or dilapidated outbuildings. Algae growth on exterior walls falls into the same category. If the current owners have not addressed something as straightforward as green algae on the front elevation, it is reasonable to wonder what else has been left unattended.

For sellers, cleaning the front elevation is a simple job that costs relatively little in time or money. It can still meaningfully affect how a buyer perceives the property from the outset, so sorting it before going to market is a sensible step.

For buyers who want certainty about everything behind the surface and not just what they can see, a RICS Level 2 or Level 3 survey will report on all identified issues, their severity and the recommended remedial action.

Surveyor Local connects you with a nationwide panel of RICS-accredited chartered surveyors who will assess the full condition of a property before you commit to buying.

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