Soft washing Bristol & Bath. Chemistry-led, low-pressure exterior cleaning

Last updated: 2026-05-21 · Founder-led · £5m insured · 7-day rework

Pressure damages. Chemistry kills. Roofs, render, painted masonry, brick, fences, decking, headstones, Bath stone. Sodium hypochlorite at controlled dilution kills the algae, lichen and moss at root level. The substrate stays intact. The clean lasts years. £5m PL, EA Upper-Tier Waste Carrier, 7-day rework guarantee on every job.

What is soft washing?

Soft washing is low-pressure exterior cleaning where chemistry, not force, does the work. We apply dilute sodium hypochlorite with a surfactant at the pressure of a garden hose, let it dwell for fifteen to twenty-five minutes, then rinse gently. The biocide kills algae, lichen and moss at root level, so the surface stays clean for two to four years rather than the six to nine months you get from a pressure clean.

Pressure damages. Chemistry kills.

Most UK homeowners reach for a pressure washer when their roof greens over, the render goes black, or the fence turns grey. It is the wrong instinct. Pressure removes what you can see, but it leaves the spores, hyphae and rhizoids that produced the staining alive in the substrate. Within a season the colony returns.

Worse, water at 2,000 to 3,500 PSI can strip render keys, lift roof tile granules, drive water under coatings, and shatter the fragile crust on Bath stone. So we lead with chemistry, not force. Soft washing puts dilute biocide on at low pressure to kill the organism at root level, then rinses gently. The substrate stays intact. The result lasts years.

We used to run heavier dilutions in our second year because the visible result came faster. Stopped after a Westbury Park K-Rend gable came up faintly bleached six months later where the chemistry had ghosted on a south face. Now we run lighter mixes, longer dwells, and accept the slower visible curve. The wall stays the right colour and the kill is still complete.

What soft washing actually is

Soft washing is the deliberate inversion of the conventional pressure-cleaning model. A rotary nozzle removes biological matter mechanically, abrading it off the surface with shear force. Soft washing relies on chemistry to do the work, and uses water only to deliver the chemistry and to flush the dead organism away once it has lysed.

The active ingredient at the centre of professional soft washing is sodium hypochlorite (NaOCl). The same oxidising biocide that disinfects municipal drinking water and swimming pools, applied at a carefully controlled dilution (typically between 0.5% and 4% available chlorine, depending on surface, organism load and dwell time). To this we add a non-ionic surfactant that lowers surface tension, so the solution penetrates the porous matrix of a tile, render coat or timber fibre rather than beading off.

The third variable is dwell time. Usually ten to twenty-five minutes. During that window the hypochlorite oxidises the cell walls of the algae, the symbiotic fungal-algal partnership of lichen, and the rhizoidal root structure of moss.

Application is low-pressure. We use 12V or air-diaphragm pumps that deliver the solution at typically under 300 PSI. Closer to the pressure of a garden hose than a pressure washer. On the most delicate substrates we use pure spray application with no rinse pressure at all. The biocide does the cleaning. The water just carries it.

Surfaces we soft-wash

Soft washing is the right method for almost every porous, coated, or organically colonised exterior surface in the South West. We treat:

• Concrete roof tiles. Typical condition: heavy black algae, lichen rosettes. Chemistry approach: standard NaOCl, extended dwell.
• Clay roof tiles. Typical condition: moss in valleys, surface algae. Chemistry approach: reduced dilution, gentle rinse.
• Natural slate. Typical condition: algal greening, moss cushions. Chemistry approach: lowest dilution, no rinse pressure.
• Sand-and-cement render. Typical condition: black streaks, green sheeting. Chemistry approach: pre-wet, surfacted biocide.
• K-Rend & monocouche. Typical condition: lichen spotting, mould. Chemistry approach: manufacturer-compliant dilution.
• Silicone render. Typical condition: algal film, dirt entrapment. Chemistry approach: pH-buffered system.
• Painted masonry. Typical condition: mould, mildew, algal staining. Chemistry approach: coating-safe formulation.
• Brick (red, buff, engineering). Typical condition: green algal staining. Chemistry approach: standard, with iron-stain check.
• Treated softwood fences. Typical condition: grey algal patina. Chemistry approach: reduced dilution with brightener.
• Hardwood (oak, iroko). Typical condition: tannin grey, algae. Chemistry approach: tannin-aware mix.
• Decking (composite & timber). Typical condition: slip-risk biofilm. Chemistry approach: low dilution, anti-slip post-treat.
• Headstones & memorials. Typical condition: lichen, biological staining. Chemistry approach: heritage-grade, reversible.
• Bath stone & limestone. Typical condition: biological soiling, gypsum crust. Chemistry approach: specialist low-dilution protocol.

If it is porous, painted or organic, soft washing is almost certainly the right tool. If it is hard, dense, non-porous and load-bearing, see the next section.

Soft wash vs pressure wash. When each is right

This is the conversation we have most often on quotation visits, and it matters because the wrong method causes thousands of pounds of damage every year in the Bristol and Bath area.

Pressure washing is the right method for hard, dense, non-porous surfaces where mechanical removal is appropriate and where the substrate can take the force. Concrete forecourts. Granite or sandstone driveways laid as paving. Block paving (with a rotary surface-cleaner to avoid jet-track scarring). Brick paviors. Tarmac in good condition. These surfaces tolerate high-flow, controlled-pressure cleaning, particularly when delivered through a rotary surface cleaner that distributes force evenly.

Soft washing is the right method for everything else. Roofs of any tile type. Render of any system. Painted walls. Timber. Stonework with any age, friability or carved detail. Memorials. Statuary. Conservatory roofs. Anything north-facing that has gone green. Anything with a coating you want to keep.

The mistake is binary thinking. Most exterior cleaning jobs in Bristol and Bath involve a mix. A pressure-washed driveway with a soft-washed render elevation above it. Or a soft-washed roof above a pressure-cleaned patio. We carry the kit and the chemistry to do both, and we will tell you honestly which method belongs on which surface.

Stage one: pre-wet

Plants, lawn edges, downpipes and any organic perimeter are saturated with clean water before any chemistry leaves the van. Pre-wetting dilutes any incidental drift instantly and protects the vegetation. We also pre-wet the substrate itself when surface temperature is above 18°C, to slow evaporation and extend dwell.

Stage two: biocide apply and dwell

The mixed solution (sodium hypochlorite at the surface-appropriate dilution, blended with non-ionic surfactant, sometimes with a controlled-release additive for vertical surfaces) is applied at low pressure from the bottom upwards. We then let it dwell. Ten to twenty-five minutes is typical. Longer for lichen, shorter for soft tannin-bearing timber. During dwell, the hypochlorite oxidises cell walls. Lichen rosettes go from green-grey to bone-white.

Stage three: low-pressure rinse

A clean-water rinse at low pressure flushes the lysed biological material clear, neutralises residual biocide and leaves the substrate visibly different. On heritage stone and on silicone-rendered elevations we frequently leave the chemistry to weather off naturally over days, with no rinse. The gentlest option of all.

Why soft wash lasts

Algae, lichen and moss do not sit on a surface. They penetrate it. Pressure washing scalps the visible body but leaves the anchoring structures alive. They simply regrow. Sodium hypochlorite oxidises the cell walls wherever the solution penetrates. The kill is at root level. A competent soft wash typically gives 24 to 36 months of clean appearance against six to twelve months for a pressure clean alone.

What drives the cost

Soft wash pricing in our area ranges from a couple of hundred pounds for a small fence run to several thousand for a full elevation render and roof package. The drivers are predictable.

• Surface area. Square metres of substrate to be treated.
• Biocide quantity. Derived from area, organism load and required dilution. Heavy lichen on a north-facing concrete tile roof needs significantly more chemistry than a light algal film on a south-facing render.
• Dwell architecture. How many applications and re-wets are required before rinse. Mature lichen often needs two passes a week apart.
• Height and access. Ladder, tower scaffold, MEWP (cherry-picker), or roof-edge work-at-height kit. Access drives both labour and insurance loading.
• Waste-water capture complexity. Some sites are simple. Others require bunding, vacuum recovery or temporary drain isolation.
• Substrate sensitivity. Bath stone, listed structures and memorials carry a heritage-grade rate because the protocol is slower, gentler and more carefully documented.

Indicative range for 2026: a fence run from £120. A full-elevation soft-wash on a semi-detached render £450 to £750. A full roof and render combined £900 to £2,200 depending on access. See the pricing and guarantee detail for the full logic.

Honest hedge. Most of the time those ranges hold. Bath stone and listed properties sit higher because the heritage protocol takes longer per square metre. Every quote is itemised so you see what you are paying for.

Where we work

The Bristol and Bath area is, in microbiological terms, an excellent place to grow algae. The combination of a damp Atlantic-fed climate, a south-westerly prevailing wind that drives moisture onto west elevations, mature tree cover, a relatively mild winter and a long humid shoulder season either side of summer makes for one of the most biologically active exterior environments in the UK.

10 cities and towns we cover

• Bristol
• Bath
• Keynsham
• Saltford
• Portishead
• Clevedon
• Thornbury
• Nailsea
• Yate
• Chew Valley

Sub-areas and neighbourhoods we work in regularly

• Clifton
• Redland
• Cotham
• Bishopston
• Henleaze
• Westbury-on-Trym
• Stoke Bishop
• Sneyd Park
• Lansdown
• Bathwick
• Widcombe
• Combe Down

West-facing Bristol elevations. The BS3 to BS9 spine. Clifton, Redland, Henleaze, Westbury-on-Trym, Sea Mills, Stoke Bishop. They take the weather first and gather the heaviest biofilm. Render on these aspects often needs a soft wash every 30 to 36 months as a maintenance cycle. Bath stone is porous, friable, and gypsum-crusted. It is also listed across vast tracts of the city. Pressure washing Bath stone is an act of vandalism. Our heritage-grade soft-wash protocol uses the lowest viable hypochlorite dilution, no rinse pressure, and is preceded by a written method statement when conservation officers require it. See areas we cover for the full list.

Recent soft-wash work

BS9, Sneyd Park. Roof and render combined, March 2026.

Concrete tile roof with heavy lichen plus K-Rend silicone render on the rear gable. Two-day job. Roof soft-washed first day, render the second day after a 24-hour gap so the run-off had cleared the gutters. MEWP access from the driveway. Owner sent a six-week follow-up photo, lichen well into the weather-off curve.

BA1, Lansdown. Headstones at a churchyard, April 2026.

Diocesan permissions in place beforehand. Heritage-grade reversible biocide at the lowest viable concentration. No-rinse protocol. Three Victorian limestone headstones with dense lichen rosettes. Visible result builds up over twelve weeks as the chemistry weathers through. Family thanked us in writing.

BS31, Keynsham. 28 m fence run plus decking, February 2026.

Treated softwood fence with grey algal patina. Composite decking with slip-risk biofilm. Reduced dilution with brightener for the fence, low dilution with anti-slip post-treat for the decking. Half-day on site.

Frequently asked questions

Is soft washing safe for plants?

At our applied dilutions, with pre-wetting and post-rinse, yes. We pre-wet the vegetation, monitor drift, and rinse the foliage afterwards. We have not lost a plant in years of operation. The single rule we never break: never apply chemistry over dry, thirsty, sun-stressed plants in summer. We pre-wet, always.

Is soft washing safe for pets?

During application and dwell, pets must be indoors. Once the surface has been rinsed and dried (typically a few hours after we leave), sodium hypochlorite has broken down to salt, water and oxygen, and the surface is safe for dogs, cats and garden wildlife. Indoor cats are unaffected throughout. We brief you specifically before we start.

How long does a soft wash last?

Twenty-four to thirty-six months is the realistic window for most domestic substrates in the Bristol-Bath area. Some south-facing, well-ventilated render holds clean for four years. Some heavily shaded north-facing concrete tile under tree cover may want a refresh at twenty-four months. We tell you, at quotation, what to expect on your specific elevation.

Will it work in winter?

Between roughly April and October, yes, without caveat. Below 4°C surface temperature, no. Hypochlorite chemistry slows and the kill becomes unreliable. Through a typical Bristol or Bath winter we book ahead for spring rather than chase a marginal December slot. Honestly, cold weeks are not soft-wash weeks.

Will it stain my brick?

Properly mixed and properly rinsed, no. The risk on brick is iron-bearing or manganese-bearing brick reacting with high concentrations of hypochlorite to produce a transient yellow halo. We test-patch first on any unfamiliar brick and adjust dilution. In ten years we have not had a brick stain that did not fade within a fortnight of weathering.

Related services

Soft washing is the underlying method behind most of what we do. Common pairings.

• Driveway & patio cleaning
• Roof cleaning
• House render & wall cleaning
• Gutter, fascia & soffit cleaning
• K-Rend & silicone render cleaning
• Hot washing
• Conservatory cleaning
• Window cleaning
• Solar panel cleaning
• Commercial exterior cleaning

Get a soft-wash quote

Send a few phone photos of the surface you want cleaning. Three or four shots showing the whole area, plus one close-up of the worst patch. We come back inside 24 hours. Quotes are free, on-site where needed, itemised, and no pressure-sell.

• Phone. 07944 014 111
• Quote form. Send photos for a quote
• WhatsApp. +44 7763 741067
• Email. enquiries@bristolandbathexteriorsolutions.co.uk