Roof & Render Soft Washing

Every ruined roof I have been asked to look at was ruined by a pressure washer. That is the whole argument for soft washing and it is worth understanding before you let anybody up there.

What pressure does to a roof

Concrete tiles are made with a coloured granular surface bonded to the body of the tile. That surface is what sheds water and what gives the roof its colour. A jet washer strips it. The tile underneath is a paler, rougher, more porous thing that holds water and grows moss faster than it did before, which is why a pressure-washed roof often looks worse three years later than one that was never touched.

Then there is water direction. Tiles are lapped to shed water downwards. A jet aimed up the slope drives water under the lap, past the batten, and onto the felt. On an older roof with brittle felt that is how you find out the felt was brittle.

Clay tiles and slates have their own version of the problem. Slate delaminates, old clay spalls, and the nails and the mortar bedding on the ridge are all vulnerable to a direct jet.

Soft washing, and why it is slower

Soft washing means low pressure, chemistry, and time.

I scrape the bulk of the moss off by hand or with a soft brush. Then a biocide goes on across the whole roof at very low pressure, essentially applied rather than blasted. The biocide kills the moss, algae and lichen, including the root structures in the surface of the tile that a wash leaves behind.

Then you wait. The dead growth breaks down and the weather carries it off over the next three to six months. That is not me being slow, it is how the method works, and anyone selling you an instantly clean roof is selling you a pressure wash by another name.

What you should see on the day: bulk moss gone, gutters cleared of what came off, treatment applied evenly. What you should see the following summer: a clean roof that stays clean for years rather than months.

Render

Modern silicone and acrylic renders clean up very well with a soft wash. The green and black staining on a north wall is surface growth and it lifts readily with the right chemistry and a gentle rinse.

Older sand and cement render needs more care. It is more porous, it is often already crazed, and water pushed into a crack ends up inside the wall. Pebbledash with loose stones is the same problem with an extra way to lose material.

If the render is actually failing, and you can usually tell by tapping it and listening for a hollow sound, cleaning is not the job that needs doing first. I will say so and you can get a renderer to look at it. There is no point making a failing wall look nice for a season.

Chimneys, conservatory roofs and the awkward bits

Conservatory roofs are a common add-on and worth doing, because the green film on the glass or polycarbonate cuts a surprising amount of light out of the room below. Low pressure, from the ground or a tower, with the frames washed at the same time.

Chimney stacks I will treat if I can reach them safely from a tower. If it needs somebody actually on the roof, that requires proper access equipment and it changes the price, and I will tell you rather than improvise.

Access

Most two-storey houses are done from the ground or from a tower with an extended lance. That covers the majority of what I get asked for.

A steep roof, an unusually high one, or anything that needs a person physically on the tiles needs scaffolding or a mobile platform. That is a real cost and it goes in the quote as its own line so you can see what you are paying for. I will not free-climb a roof to save you the scaffolding, and I would be wary of anyone who offers to.

Timing

Autumn and spring are both fine. What matters more is that the biocide needs a few dry hours to work and it should not go on in freezing conditions.

Around the coast at Seaford, Newhaven and Peacehaven roofs green up faster than they do inland, because salt in the air holds moisture on the surface for longer and the growing season effectively never stops. A treated roof inland at Uckfield might go five or six years. On a clifftop at Peacehaven, plan on less.

Cost

Roof soft washing from £9.50 a square metre of roof area, which on a standard three-bedroom semi usually lands somewhere between £450 and £700 depending on pitch and access.

Render and cladding soft washing from £6.50 a square metre.

Both are quoted from photographs and a rough measurement. I will always tell you if I think a job needs access equipment before you have committed to anything, because that is the number most likely to change the answer.