The short answer
Soft washing is a way of cleaning a roof using low-pressure water together with a treatment solution that kills moss, algae and lichen at the root. It relies on the chemistry to do the cleaning rather than the force of a jet, so the tile surface is left intact. It suits concrete tiles, clay tiles and slate that would be damaged by a pressure washer. It is usually combined with hand removal of any thick moss before the solution is applied.
How soft washing differs from pressure washing
A pressure washer works by force. A narrow jet of water hits the surface at high pressure and blasts off the growth, along with anything loose underneath. On a roof that is a problem. Concrete tiles have a thin surface layer that can be stripped away, the jet can drive water up under the laps into the roof space, and it can dislodge the nibs and mortar that hold the tiles and ridge in place.
Soft washing flips that approach. Water comes out at about the pressure of a garden hose, so it spreads and rinses rather than cuts. A mild cleaning solution, usually based on a biocide, does the real work by killing the organism so that it loosens and washes away. The roof surface is not attacked, and the mortar and tile edges are left alone.
It is the same idea we describe in the page on whether pressure washing damages roof tiles, where we look at the risks in more detail.
Pressure also varies with the nozzle and the distance. A low-powered wash held well back is not the same as a hard jet held close, and some operators use the same machine for both. The word soft is meant to describe the method as a whole, which is low pressure plus a treatment, rather than a gentle setting on a powerful machine.
What goes into the solution
Most solutions contain a biocide approved for use on roofs, in a diluted form that is sprayed onto the tiles. Some products also include a mild surfactant that helps the liquid spread and cling. The mix is chosen for the kind of growth and the type of tile, and it is applied from the ground or from the roof depending on access.
Household bleach is not the same thing, and it is a poor choice for roofs because it can discolour tiles and harm planting below. Our answer on whether you can use bleach on roof tiles explains why. A proper roof treatment is made for the job, and it is applied with care so the garden, the pond and the pets are protected.
The solution needs time on the surface. It is left to soak in rather than being rinsed off at once, so it can reach the root structure of the growth. Rinsing too soon wastes the product and leaves the organism alive. On some roofs a second light application is made after a few weeks to catch any patches that were missed.
Where soft washing works best, and where it does not
It works well on algae, light moss and lichen, and on any roof whose surface should not be disturbed. Natural slate, old clay and weathered concrete all do better under a soft wash than a jet. It is also good for the run-off areas such as porches, conservatory roofs and garage roofs, where the covering is delicate.
It does not remove thick moss by itself. A deep mat has to be brushed or lifted off by hand first, because a spray cannot get through it to the roots. Soft washing is also slower to show a result than blasting, since the treatment works over days and weeks and the dead growth comes away with the rain. Some people expect an instant white roof and are surprised when it improves gradually. That gradual change is normal.
Soft washing is gentle, but it is still a chemical treatment, so weather matters. Rain right after application dilutes it, and strong sun can dry it too quickly. A calm, mild, dry spell gives the best result, which is why we agree a window for the work rather than a fixed hour.
What to ask before you book
Ask what the solution is, how it is applied and how the garden will be protected. Ask whether thick moss is removed by hand beforehand and whether the gutters are cleared as part of the visit. A good answer is specific. A vague promise to wash the roof does not tell you what will happen to your tiles.
Our approach starts with a survey and photographs of the slopes, so the method fits what is growing there. The fixed written quote states what is included. See the full page on roof cleaning in Basingstoke to see how soft washing fits with the other methods we use.
Soft washing is not a way to repair a roof. If the survey finds cracked tiles, failed pointing or worn flashings, those are quoted as repairs, and the clean is sensible only if the roof underneath is sound.
Every roof is different. A free survey with photos settles it for yours.
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