exterior
Render and exterior wall cleaning
Render cleaning costs £400 to £900 for a typical semi-detached house and £700 to £1,600 for a detached one, or roughly £4 to £9 a square metre. It is done with a low pressure chemical wash rather than a pressure washer, and that distinction is the whole trade: pressure takes the colour and the surface off modern render along with the algae.
What this covers
What it costs. Render cleaning costs £400 to £900 for a semi-detached house and £700 to £1,600 for a detached one, which works out at roughly £4 to £9 a square metre. Full cost breakdown.
What the job covers
- Softwashing silicone, K Rend and monocouche render
- Red and green algae, lichen and the black staining under sills
- Pebbledash and roughcast, which hold more and rinse slower
- Treatment left on to kill spores rather than rinsed straight off
- Render, soffits, fascias and gutter faces done together
What is involved
Modern render is a thin coloured coat over insulation or blockwork, often only a few millimetres of it, and it is the coat that carries the colour. A pressure washer strips it. The damage looks like patchy lightening at first and shows up properly when the wall dries, by which point the only fix is recoating the elevation.
So the method is a softwash: a hypochlorite solution applied at garden-hose pressure, left to work, and rinsed gently or in some cases not rinsed at all. What it is doing is killing the organism rather than scrubbing the stain, which is why a treated wall often carries on getting cleaner for a fortnight after the van has gone.
The staining under window sills and below copings is not usually algae at all. It is run-off, carrying dirt from a surface above onto the render below, and it comes back on the same schedule unless whatever is shedding it is dealt with.
Choosing someone for the job
Ask what pressure they use on render and be wary of any answer involving a pressure washer. Low pressure and chemistry is the method.
Ask what the treatment is and how long it stays on. A wash that is applied and rinsed within minutes has cleaned the surface without killing anything and the green returns within a season.
Ask how they protect planting. Hypochlorite kills what it lands on, so the beds under the elevation need covering and rinsing.
Common questions
Can it be pressure washed instead?
Not safely on modern render. Silicone, monocouche and K Rend carry their colour in a thin top coat and pressure removes it. Pebbledash tolerates more, but the mortar behind it does not.
How long does it last?
Three to five years on most elevations. North-facing walls and anything under trees green over sooner because they stay damp for longer.