Soft Washing vs Pressure Washing for Roofs: What the Evidence Shows

Those dark streaks that appear on roofs over time — often mistaken for soot, mould, or simple dirt — are usually caused by a specific organism: Gloeocapsa magma, a cyanobacteria (blue-green algae) that feeds on the limestone filler used in asphalt shingles. Understanding what it actually is explains why the recommended treatment method is soft washing, not pressure washing.

What Gloeocapsa Magma Is, and Why It Spreads

Unlike surface dirt, Gloeocapsa magma is a living organism capable of photosynthesis. Its spores are airborne, land on a roof, and colonise the shingle surface, feeding on the limestone filler within the shingle itself. As the colony grows, it spreads across the roof, creating the characteristic dark streaking, and — left untreated — traps moisture against the shingle, accelerating deterioration.

Why Pressure Washing Doesn't Solve the Problem

High-pressure cleaning can strip the visible growth off a roof, but it doesn't kill the organism. Because Gloeocapsa magma has a root-like structure embedded in the shingle, pressure washing removes the surface colony while leaving the roots and spores intact — meaning the staining can return, sometimes on a damaged shingle surface that offers an even better foothold for regrowth. The Asphalt Roofing Manufacturers Association (ARMA), the US/Canada trade body representing asphalt shingle manufacturers, addresses this directly in its own technical bulletin on the subject: ARMA advises against using high-pressure washing systems on asphalt roofing for algae removal or any other purpose, warning that the practice is likely to damage the roofing.

How Soft Washing Works Instead

• Low pressure: soft washing uses a fraction of the pressure of a standard pressure washer, low enough to avoid disturbing the granule surface of the shingle.
• Targeted solution: a cleansing solution is applied to the roof and left to dwell, breaking down the algae's cell structure rather than just displacing it physically.
• Gentle rinse: once the solution has had time to work, it's rinsed away at low pressure, along with the now-dead algae.

ARMA's own guidance is candid that any cleaning technique — pressure or soft wash — produces a temporary result, and that discolouration is likely to recur over time as new algae spores land and establish themselves. The advantage of soft washing over pressure washing is not permanence; it's that it treats the biological growth without the physical damage to the shingle that high pressure causes.

Not Just a Roof Treatment

The same low-pressure, solution-based principle applies more broadly to soft washing on rendered walls, painted surfaces, and other areas where high-pressure water risks damaging the substrate — surfaces where the goal is removing organic growth (algae, moss, lichen) without stripping paint, mortar, or protective coatings.

Sources

• Asphalt Roofing Manufacturers Association (ARMA) — Algae Discoloration of Roofs, technical bulletin