Roof Cleaning Methods Compared: Biocide Soft Wash vs Pressure Washing

How Biocide Soft Wash Works

A biocide solution (typically a proprietary diluted algaecide/biocide) is applied at low pressure — similar to a domestic garden hose — across the entire roof surface. The biocide penetrates the moss, lichen, and algae colonies at root level and kills the organisms. The dead growth then weathers off naturally over the following three to six months through rain action, wind, and freeze-thaw. A second application of protective biocide after the initial wash inhibits re-colonisation for three to five years in typical Aylesbury conditions.

The advantage of this method is that it achieves the biological kill without any mechanical force on the tile surface or mortar. The roof is left looking unchanged immediately after treatment — the improvement becomes visible gradually as the dead growth weathers off. This sometimes leads homeowners to assume the treatment hasn't worked. It has: the growth is dead and is being removed by natural weathering.

Why Pressure Washing Is the Wrong Method

High-pressure washing removes moss mechanically — the stream of water physically dislodges the growth from the tile surface. It works, in the sense that the roof looks clean immediately after. The problems are:

• The high-pressure stream erodes the surface aggregate from concrete tiles. The smooth surface that results weathers faster and accelerates re-colonisation. On older tiles where the surface is already slightly worn, pressure washing can remove enough material to shorten tile life significantly.
• Water at high pressure is forced under tile laps and into the batten zone. On roofs with aged felt or where the felt laps have opened, this wets the structure below the covering and can cause internal damp problems that weren't present before treatment.
• Mortar at ridge and verge joints is eroded by the water stream. Repointing that would otherwise last another five years can be stripped back to a condition requiring immediate repair.
• There is no residual biocide effect — with no biological kill, re-colonisation begins immediately and the roof returns to a similar state within two to three years.

Checking What You're Being Quoted

When getting quotes for roof cleaning, ask specifically: what method is used? What is the active biocide ingredient and what dilution? Is a protective treatment applied after the initial clean? What's the expected re-treatment interval? A contractor using soft wash will answer these questions in detail. A contractor who can't explain their method or who describes a pressure wash as "soft wash" (common greenwashing of the term) is worth treating with caution.

Price is rarely a differentiator between the two methods — both are typically quoted in the same range for a standard Aylesbury semi. The choice is purely about what you want to happen to your tiles and mortar over the following five to ten years.