Green Algae Streaking Down An Outside Wall

Key takeaways

• Algae needs sustained moisture, so the green marks the path water takes, not a defect in the wall.
• Follow the stripe to its top and the source is usually within a few centimetres of where the green begins.
• North facing and shaded walls go green faster because they dry more slowly, not because they are wetter.
• Killing the algae without stopping the water means it returns in the same shape within a year or two.
• It is a warning sign rather than damage in itself, and reading it early is genuinely useful.

What the green actually is

It is a colony of algae, sometimes with lichen and moss mixed in, growing on the surface of the masonry or render. Algae are simple organisms that need light, a surface and moisture, and the limiting factor on a British wall is almost always moisture rather than the other two. That single fact makes them a useful diagnostic tool: they mark, precisely and permanently, the parts of a wall that stay wet long enough to support life. A wall that dries between showers stays clear. A wall with water running down it repeatedly develops a green track along exactly that route.

Following it to the source

Stand back and look at where the green starts, not where it is thickest. The top of the stripe is the closest point to the escape. If it starts at the gutter line, look at the joint or bracket directly above it. If it starts at a downpipe, look for a cracked socket or an open joint. If it starts below a window, the sill is throwing water back against the wall rather than clear of it, or a drip groove underneath the sill has been filled with paint or render. If it starts partway down a blank wall with nothing above it, look higher for something projecting, since water thrown off a feature can meet the wall lower down.

Why aspect matters more than people think

Two walls on the same house receiving similar amounts of water can look completely different. A north facing wall, or one shaded by a fence, a tree or a neighbouring building, receives less sun and less air movement, so it dries far more slowly and supports growth much more readily. This means the amount of green is not a reliable measure of how much water is arriving. A modest leak on a shaded north wall can produce dramatic staining, while a worse leak on a sunny south wall may show only a faint mark. Judge the fault by the shape and position of the stripe, not by how green it is.

When it is not a leak at all

Some green is entirely normal and not worth chasing. A general light film across a whole shaded elevation, particularly on render or on a north facing wall in a damp part of the country, is just the climate. So is growth at the very bottom of a wall where rain splashes back off a hard path, which is a splash zone rather than a leak. So is growth on a garden wall or a boundary wall with no roof above it. The distinction that matters is between a defined vertical stripe, which follows a concentrated flow and points at a fault, and general even coverage, which does not.

What it tells you about the wall underneath

Algae itself does very little harm to masonry: it sits on the surface and does not penetrate meaningfully. What matters is the condition it reveals. Masonry that stays damp is masonry that is not drying between wettings, and that has consequences: reduced insulation value, increased likelihood of damp reaching the inner face, and vulnerability to frost damage when saturated brick faces freeze. On rendered walls, prolonged damp behind the surface causes render to lose its bond and blow, which shows as hollow sounding patches. So the green is not doing the damage, but it is reliably marking the place where damage is most likely to develop.

Cleaning it, and the order of operations

Fix the water first. Clean second, once the wall has had a proper drying period. Cleaning first destroys the evidence, and the mark simply regrows in the same shape once the next winter arrives. When cleaning does happen, a proprietary fungicidal masonry wash applied and left to work is more effective and far gentler than blasting. Pressure washing brickwork hard is a common mistake: it drives water deep into the masonry, can strip soft old bricks and lime pointing, and on render it can force water behind the surface. On an older property in particular, patience with a soft wash beats speed with a lance.

Whether this needs doing anything about

If the stripe is faint, has been there for years without changing, and there is no damp inside, this is a low priority job that can sit until other roofline work is being done. If the stripe is growing year on year, if it is directly below a gutter joint, if damp has appeared on the inside of that wall, or if the mortar or brick faces in the affected strip look eroded or flaky, deal with it before winter. The underlying repair is nearly always small, because it is a single joint, bracket, sill or pipe connection, and the value of acting is preventing frost damage rather than removing the mark.

The one it is worth acting on quickly

There is a version of this that deserves prompt attention: a green stripe running down beside or behind a downpipe, close to the base of the wall, where water is discharging against the masonry rather than into the gully. That combination puts sustained water into the wall at ground level, which is the one part of the wall where the ground beside the foundations is also being soaked. Over years that affects both the wall and what it stands on. The fix is usually a shoe on the bottom of the pipe or a repaired connection into the gully, which is a small job with a disproportionate benefit. There is one further case that is worth separating out from all of the above, which is green growth appearing on a wall with no roof drainage above it at all, such as a gable with no gutter on that elevation. There the water is coming off the wall head or the verge rather than from a pipe, and the answer lies at the top of the wall rather than anywhere in the rainwater system. Following the stripe upward still works as a method; it simply leads to a different part of the building, and the repair belongs to the roof edge rather than to the rainwater system. On a gable that means the verge, the coping or the pointing at the wall head, all of which shed water onto the face below when they fail.

Frequently Asked Questions

What causes a green stripe down my wall?

Algae growing where the wall stays damp, which marks the path water takes regularly. The source is usually within a short distance of where the green begins, so following the stripe upward normally leads to a gutter joint, a downpipe connection, a bracket or a window sill throwing water back at the wall.

Is algae damaging the brickwork?

The algae itself does very little, since it sits on the surface. The condition it marks is what matters: masonry that never dries out is more likely to pass damp inside, loses insulation value, and is vulnerable to frost damage when saturated brick faces freeze and the surfaces spall.

Should I jet wash it off?

Not aggressively, and not first. Pressure washing drives water deep into masonry, can strip soft old brick and lime pointing, and on render can force water behind the surface. A fungicidal masonry wash applied and left to work is gentler and more effective, and it should come after the leak is fixed and the wall has dried.

Why is one wall green and the others are not?

Usually a combination of a water source above it and slower drying. North facing or shaded walls dry far more slowly, so they support growth much more readily. That also means the amount of green is a poor guide to how much water is arriving, and the shape of the mark matters more.

Is a light green film across the whole wall a problem?

Generally not. Even coverage on a shaded elevation, or growth at the very bottom of a wall where rain splashes off a path, is normal in a damp climate. What indicates a fault is a defined vertical stripe, because that follows a concentrated flow from a specific escape point.

How soon should I deal with it?

A faint unchanging mark with no damp inside can wait and be bundled with other roofline work. Deal with it before winter if it is spreading, sits directly below a gutter joint, has damp appearing inside, or if the brick faces and mortar in that strip look eroded, since frost damage to saturated masonry is the real cost.