Algae on House Walls: Removal & Prevention

What causes green algae on walls

Algae needs moisture, a food source, and a surface to grip. Rendered and brick walls provide all three: a slightly porous texture for spores to land on, ongoing damp from rain, condensation or poor drainage, and organic dust that gives the algae something to feed on. Shaded elevations, walls near overhanging trees, and areas that never quite dry out are hit hardest.

Modern render types, particularly smoother pebbledash and coloured renders common on North East housing stock, can actually be more prone to staining than older lime-based finishes because the surface holds a thin film of moisture longer after rain.

Guttering that overflows or downpipes that leak onto a wall face create a permanently damp strip that algae will colonise faster than anywhere else on the property, so it's always worth checking whether a drainage fault is feeding the problem before treating the render alone.

Why pressure washing isn't the fix

Blasting algae off render with a pressure washer might shift the visible green in the short term, but it drives water and spores deeper into the surface, can crack or chip render, and often leaves the wall looking patchy rather than clean. On brick, high pressure can strip old, softer mortar joints and damage the face of the brick itself, which is expensive to put right.

It is one of the most common mistakes we see from DIY jobs and, unfortunately, from some less careful trades too: it looks like a clean straight after, then the algae is back within a few months because the root of it was never actually removed.

The soft wash process

Soft washing uses a biocide treatment applied at low pressure, left to dwell so it can work through the algae, moss and lichen at a cellular level, then rinsed off gently rather than blasted. This kills the growth rather than just knocking the top layer off, which is why the wall stays visibly cleaner for far longer.

A typical soft wash for algae on a semi-detached house render runs from around £250 to £500 depending on size, access and how established the growth is, with larger detached properties or heavily rendered elevations costing more.

Biocide residual protection

Because biocide-trained applications leave a residual layer on the surface, they slow new spore growth from establishing, which is the real value over a one-off clean. Most properties stay noticeably cleaner for twelve to twenty-four months rather than showing regrowth within weeks, though shaded, damp elevations will always need attention sooner than sunnier walls.

Some homeowners assume a coloured masonry paint over the top will hide the problem for good, but painting over active algae growth without treating it first simply seals moisture in and often leads to the new paint failing or blistering within a season or two.