BPW Module 10 — Soft Washing
Soft Washing — Low-Pressure Cleaning for Delicate Surfaces
Not every surface can take pressure. Render, painted masonry, conservatory roofs, and fascias all require a different approach — low pressure, the right biocide, and the patience to let chemistry do the work. This module covers when soft washing is the correct method and how to do it properly.
Learning Outcomes — Module 10
- Explain what soft washing is and how it differs from pressure washing
- Identify the surfaces that require soft washing rather than pressure washing
- Select the correct biocide concentration and application method for each surface type
- Explain the role of surfactants in soft wash solutions and when to use them
- Apply soft wash solution safely from ground level and at height
- Manage dwell time correctly across different surfaces and weather conditions
- Rinse safely without causing pressure damage to delicate surfaces
- Protect adjacent surfaces, plants, and client property during soft wash application
What Soft Washing Is — and What It Is Not
Soft washing is the application of a diluted biocide or cleaning solution at low pressure — typically below 100 PSI — to kill biological growth on surfaces that cannot tolerate the mechanical force of a standard pressure washer. The cleaning is done by the chemistry, not the pressure. The rinse that follows simply removes what the biocide has already killed and loosened.
Soft washing is not a weaker version of pressure washing. It is a different method with a different mechanism of action, used on surfaces where pressure washing would cause damage. Understanding which method is correct for which surface is one of the most commercially important skills in this trade — because offering soft washing alongside pressure washing significantly expands the range of jobs you can take on safely.
The commercial case for soft washing: Render cleaning, roof cleaning, conservatory roof cleaning, and fascia cleaning are all soft wash jobs. These are higher-margin, less competitive services than standard driveway cleaning. An operator who can soft wash safely can quote for the whole exterior of a property — not just the hard surfaces at ground level.
Surfaces That Require Soft Washing
| Surface | Why Pressure Washing Is Wrong | Correct Approach |
|---|---|---|
| Painted or rendered masonry | Pressure will strip paint, blow render off the wall, and force water into the substrate behind | Soft wash biocide application, extended dwell, low-pressure rinse |
| Monocouche and through-coloured render | High pressure damages the render surface and causes colour loss | Soft wash only — check render type and age before quoting |
| Conservatory polycarbonate roofs | Pressure will crack or craze polycarbonate panels | Soft wash application by pump sprayer or dedicated soft wash lance, rinse by low-pressure garden hose |
| UPVC fascias, soffits, and guttering | Direct high pressure can crack, discolour, or force water behind UPVC profiles | Soft wash solution applied by pump sprayer, rinse by low-pressure feed |
| Roof tiles (concrete and clay) | Pressure dislodges moss and lichen but also damages tile surface, strips protective coatings, and can lift tiles | Soft wash biocide application — biological growth dies and falls off naturally over weeks |
| Timber cladding and painted fencing | Pressure raises grain, strips paint, and forces water into the timber | Soft wash biocide at low concentration, minimal dwell, very low-pressure rinse |
Soft Wash Equipment
Dedicated soft wash pump systems
A dedicated soft wash system uses a 12V or 24V electric pump — not a pressure washer pump — to deliver diluted biocide at very low pressure through a hose and lance. These systems are purpose-built for the application rates and chemical compatibility requirements of soft washing. They can be configured to deliver solution at specific flow rates and can handle the chemical concentrations used in soft washing without the corrosion risk that would damage a standard pressure washer pump.
Using a pressure washer for soft washing
A pressure washer can be used for soft washing via a downstream injector with the pressure reduced to minimum. This is an acceptable approach for some applications but has limitations — the dilution ratio is fixed by the injector, the pressure reduction available on many machines is limited, and some biocide chemicals can damage pressure washer seals and pump components over time. Check chemical compatibility before using a pressure washer for biocide application.
Pump sprayer for small areas
For small areas — a single conservatory roof panel, a patch of render, fascia sections — a standard pump sprayer is the simplest and most controllable application method. It requires no specialist equipment beyond what you should already carry, and it gives you precise control over coverage and volume.
Biocide Selection for Soft Washing
The biocide is the active agent in soft washing. Product selection depends on the surface type, the contamination type, and the environmental conditions on site.
Sodium hypochlorite (SH) based products
Sodium hypochlorite is the most effective broad-spectrum biocide for soft washing biological growth — algae, moss, lichen, and bacteria. At the correct dilution, it kills biological organisms rapidly and produces a fast, visible result. It is the industry standard for roof and render cleaning in markets where it is widely used.
However, SH carries significant risks that must be understood before use. It is corrosive at high concentrations. It damages plants, grass, and vegetation on contact. It can bleach painted surfaces, strip render coatings, and damage metals. Runoff containing SH must not enter surface water drains. These risks are manageable with correct dilution, correct PPE, and correct site protection — but they cannot be ignored.
Quaternary ammonium compound (quat) based products
Quat-based biocides are less aggressive than SH-based products — lower toxicity, lower environmental impact, less risk to vegetation and adjacent surfaces. They work more slowly and are less effective on established lichen than SH. For many soft washing applications on residential properties — UPVC cleaning, lightly contaminated render, conservatory roofs — a quat-based product is a safer and more appropriate choice than SH.
Choosing the right product
Match the product to the job. Use SH-based products where rapid biological kill is required and site protection can be managed. Use quat-based products where vegetation proximity, surface sensitivity, or client preference makes a lower-risk option more appropriate. Read the SDS for every product before use and follow the manufacturer's dilution recommendations precisely.
The Role of Surfactants in Soft Washing
Surfactants serve two critical roles in a soft wash solution that biocide alone cannot achieve. First, they reduce the surface tension of the solution, improving contact with the surface being treated — particularly important on hydrophobic surfaces such as weathered UPVC, certain render coatings, and painted masonry where water beads rather than wets. A biocide solution that beads and runs off a surface is not in contact long enough to work effectively.
Second, surfactants slow the run-off of solution down vertical surfaces by increasing viscosity and improving adhesion. On a rendered wall or a fascia board, a biocide solution without surfactant will run straight down before the dwell time is complete. The surfactant keeps the solution in contact with the surface for long enough for the biocide to do its job.
Proprietary products vs mixing your own: Most proprietary soft wash products already have surfactant formulated in at the correct ratio. If you are mixing your own solution from SH concentrate, add a small quantity of a compatible non-ionic surfactant — typically 0.1–0.5% of the total solution volume. Do not use washing-up liquid or household detergents as a substitute. They foam excessively under application pressure, leave residue on the surface, and are not formulated for field use. Use a product specifically designed as a soft wash surfactant or wetting agent.
Dilution Rates for Soft Washing
Always follow the manufacturer's dilution guidance. The rates below are indicative only. Product formulations vary significantly — a 10% SH product requires a very different dilution to a 3% SH product. Using an undiluted or incorrectly diluted biocide on a surface can cause bleaching, etching, or coating damage that is irreversible. When in doubt, start weaker and test.
| Surface | Indicative SH Dilution Range | Notes |
|---|---|---|
| Concrete and clay roof tiles | 2–4% SH solution | Higher end for heavy lichen. Allow to dwell and rinse. Do not walk on treated roof. |
| Rendered masonry — standard | 1–2% SH solution | Test on inconspicuous area first. Some renders are sensitive to bleach. |
| UPVC fascias and soffits | 0.5–1% SH solution or quat-based product | Low concentration — UPVC is bleach-tolerant but strong SH can yellow older UPVC. |
| Conservatory polycarbonate | Quat-based product preferred | Avoid SH on polycarbonate — risk of crazing and accelerated UV degradation. |
| Timber cladding | 0.5% SH maximum or quat-based product | SH can bleach and raise grain on timber. Very low concentration or avoid entirely. |
Site Protection Before Application
Biocide overspray and runoff will damage plants, grass, paving, vehicles, and painted surfaces. Site protection before you apply anything is not optional — it is part of the job.
- Pre-wet all vegetation within the potential drift or runoff zone with clean water before applying any biocide. This does not make plants immune — it dilutes any chemical contact and reduces damage.
- Cover or move vulnerable plants where possible. A polythene sheet over a border or a potted plant takes thirty seconds and prevents a damage claim.
- Protect vehicles — soft wash overspray on vehicle paintwork causes damage. Move the vehicle or cover it.
- Block surface water drains in the runoff path before applying biocide. Remove the blocker and flush with clean water after rinsing is complete.
- Warn neighbours if the work area is close to a boundary — biocide drift does not stop at a fence line.
Application Technique
Working from the bottom up
Apply soft wash solution from the bottom of the surface upwards. This prevents fresh chemical running down over already-treated areas and causing uneven dwell time, and it prevents chemical running down onto areas you have not yet protected or pre-wetted.
Even coverage
Apply in overlapping passes to ensure complete coverage. Missed areas will not be treated — the biological growth will remain and the result will be patchy. Take your time on the application phase; the rinse is quick, the application is where the result is determined.
Working at height
Soft washing often involves working at height — conservatory roofs, first-floor render, fascias. A dedicated soft wash lance with an extension pole allows ground-level application for most first-floor work. For higher surfaces, working from a ladder or scaffold adds significant risk — ensure you have the correct access equipment and that your public liability insurance covers work at height before proceeding.
Dwell Time and the Rinse
Dwell time for soft washing is typically longer than for standard chemical pre-treatment — 15–30 minutes for most biocides on render and masonry, longer on heavy lichen colonisation. The biological material needs time to die before rinsing will remove it cleanly.
Rinse with the lowest pressure that will effectively remove the treated material — for most soft wash applications this means a garden hose flow rate rather than a pressure washer. On rendered surfaces, use a very low-pressure rinse — the water volume does the work, not the pressure. On roof tiles, rainfall over the following weeks will complete the rinse naturally after the biocide has killed the growth.
Roof tile soft washing — the slow result: When soft washing a roof, do not promise the client that the roof will look clean immediately after treatment. The biocide kills the biological growth, but the dead material remains on the roof until rainfall washes it off over the following weeks. Explain this before you start. A client who expects an immediate transformation and sees no visible change on the day will question whether you did anything at all.
Post-Application — Vegetation and Drain Care
After rinsing is complete, flush all vegetation in the affected area with clean water. Remove drain blockers and flush the drain with clean water. Remove polythene sheeting carefully — fold inwards to avoid spreading chemical residue onto adjacent surfaces.
Check that no biocide residue has pooled on hard surfaces, particularly near vehicles or painted features. If any chemical has made unwanted contact with a surface during the job, rinse it immediately — do not leave it until you have finished the rest of the work.