How to Mix Softwash Solutions Correctly
Mixing a softwash solution correctly means calculating the finished strength you need, adding clean water to the tank first, measuring the correct amount of sodium hypochlorite or biocide, adding compatible surfactant where required, and applying the mix safely within the product label, COSHH assessment and site risk controls. The aim is not to make the strongest possible solution; it is to make the mildest effective solution for the surface, contamination level and working conditions.
For UK exterior cleaning contractors, facilities teams and serious DIY users, correct mixing is one of the most important skills in softwashing. A well-mixed solution improves cleaning results, reduces staining and plant damage risk, protects equipment, helps with legal compliance and gives more consistent pricing from job to job.
This guide explains how to mix softwash solutions in practical terms, including dilution calculations, surface strength guidance, chemical compatibility, common mistakes and safe working practice. It is written from the perspective of professional exterior cleaning, where accuracy, documentation and repeatable processes matter.
What Is a Softwash Solution?
A softwash solution is a controlled chemical cleaning mix used to treat organic growth such as algae, mould, mildew, bacteria, biofilm, lichen and moss on exterior surfaces. Unlike pressure washing, softwashing relies primarily on chemistry and dwell time rather than force.
The most common active ingredients used in professional softwashing are sodium hypochlorite and biocides such as DDAC-based products. These are not the same thing and should not be treated as interchangeable. If you are unsure which chemical is appropriate for a surface or type of growth, SoftWash UK’s guide to softwashing chemicals is a useful starting point.
The Main Components of a Softwash Mix
- Water: Usually the main carrier and dilution component.
- Sodium hypochlorite: A fast-acting oxidising cleaner often used for algae, black staining and organic soiling.
- Surfactant: Helps the solution cling, wet out the surface and work more evenly.
- Biocide: Often used for longer-term residual treatment, especially after moss removal or as a maintenance treatment.
- Masking scent: Used by some contractors to reduce odour perception, not to make the chemical safer.
Professional products such as Sodium Hypochlorite for Soft Washing 14% to 15% and a softwash-compatible surfactant such as Clever Wash Surfactant are designed to give contractors more predictable results than unlabelled or household-grade alternatives.
Before You Mix: Assess the Job Properly
Good softwash mixing starts before you open a chemical drum. In professional work, the correct mix is dictated by the site conditions, surface type, contamination and risk profile.
Check the Surface
Different surfaces react differently to softwash chemicals. Painted render, K-rend style finishes, concrete, clay roof tiles, UPVC, natural stone, timber, tarmac and metal trims all require different levels of caution.
Before mixing, ask:
- Is the surface porous or non-porous?
- Is it painted, coated, sealed or weathered?
- Are there metals present, especially aluminium, lead, copper or galvanised fixings?
- Is the surface acid-sensitive or bleach-sensitive?
- Is the staining organic, mineral, rust, oil, tannin or pollution-related?
One of the most common on-site mistakes is assuming every stain is algae or black mould. Rust stains, lead carbonate staining, tannin bleed, oil marks and mineral deposits need different chemistry. Using sodium hypochlorite on the wrong stain can waste time and sometimes make the problem worse.
Identify the Growth Level
A light green algae film on render does not need the same strength as heavy black organic staining on old concrete. In most cases, experienced operators start with the mildest realistic solution, allow proper dwell time, then re-apply if needed rather than mixing too strong from the outset.
Understanding Sodium Hypochlorite Strength
Sodium hypochlorite is commonly supplied to the exterior cleaning industry at around 14% to 15% available chlorine when fresh. It degrades over time, especially when stored warm, exposed to sunlight, contaminated or kept for long periods.
This matters because your dilution calculation assumes a starting strength. If your drum has degraded from 15% to 12%, a mix calculated as 3% will not actually be 3%. For most contractors, this is why stock rotation, cool storage and buying from a specialist supplier are important.
If you want a deeper technical explanation, SoftWash UK’s guide on how sodium hypochlorite works explains the chemistry behind oxidation, organic growth breakdown and why pH and contact time matter.
The Basic Softwash Mixing Formula
The simplest way to calculate a sodium hypochlorite softwash mix is:
Required hypo volume = final tank volume × target strength ÷ stock strength
For example, if you want 20 litres of 3% softwash solution using 15% sodium hypochlorite:
20 litres × 3 ÷ 15 = 4 litres of sodium hypochlorite
You would then add enough water and compatible additives to make the total finished volume up to 20 litres.
Important Mixing Note
Always calculate based on the final volume in the tank, not simply “parts” thrown together by eye. Guesswork is one of the main reasons contractors get inconsistent results, overuse chemicals or under-quote jobs.
Example Dilution Table for 15% Sodium Hypochlorite
The table below gives practical examples for common finished strengths using fresh 15% sodium hypochlorite. These are general calculation examples, not universal recommendations. Always follow product labels, assess the site and test first.
| Final Mix Volume | Target Strength | 15% Sodium Hypochlorite Required | Approximate Water/Additive Balance |
|---|---|---|---|
| 10 litres | 1% | 0.67 litres | 9.33 litres |
| 10 litres | 2% | 1.33 litres | 8.67 litres |
| 20 litres | 3% | 4 litres | 16 litres |
| 20 litres | 4% | 5.33 litres | 14.67 litres |
| 50 litres | 2% | 6.67 litres | 43.33 litres |
| 50 litres | 3% | 10 litres | 40 litres |
Where surfactant is used, it is normally included within the final volume rather than added on top without adjustment. In practical terms, small surfactant amounts will not usually change the calculation dramatically, but disciplined operators still measure accurately.
Typical Strength Ranges by Application
The right softwash strength depends on the surface, growth level, weather and required outcome. The following table gives broad professional guidance, but it must not replace product labels, training, test patches or site-specific risk assessment.
| Application | Typical Approach | Key Cautions |
|---|---|---|
| Light algae on render or painted masonry | Often treated with a lower-strength sodium hypochlorite mix and surfactant | Pre-wet sensitive areas, avoid drying on glass or metals, test painted surfaces |
| Heavier black organic staining on masonry | May require a stronger mix, longer dwell or repeat application | Do not assume stronger is always better; manage run-off carefully |
| Roof tiles after moss removal | Often treated with sodium hypochlorite for rapid visual improvement or biocide for residual effect | Control run-off, protect gutters, water butts, plants and neighbouring property |
| Maintenance biocide treatment | Use a correctly diluted biocide according to label instructions | Do not mix biocides with sodium hypochlorite unless the manufacturer specifically allows it |
| Timber and delicate natural stone | Use specialist products or very cautious testing | High-strength oxidisers can lighten, damage or alter appearance |
For understanding when to use oxidising chemistry versus residual biocide treatments, the SoftWash UK article on sodium hypochlorite vs biocide is particularly relevant.
Step-by-Step: How to Mix a Softwash Solution Correctly
1. Read the Label and Safety Data Sheet
Before any mixing, read the product label and Safety Data Sheet. This applies to sodium hypochlorite, surfactants, biocides, masking scents and any specialist stain removers. Professional users should keep relevant SDS information accessible and ensure staff understand the hazards.
If you need organised documentation for products used on site, SoftWash UK provides Softwash SDS Sheets to help contractors manage safety information properly.
2. Complete a Site Risk Assessment
Identify people, pets, plants, ponds, drains, vehicles, neighbouring properties, electrical points, fragile surfaces and public access. For commercial sites and facilities work, this should be part of a formal RAMS process.
A Risk Assessment and Method Statement pack for exterior cleaning can help contractors structure their documentation, especially when working for managing agents, schools, councils, facilities managers or commercial clients.
3. Wear Appropriate PPE
At a minimum, consider chemical-resistant gloves, eye and face protection, suitable footwear, long sleeves and clothing appropriate to the task. Depending on application method and exposure risk, respiratory protection may also be required. PPE must be selected from the risk assessment, not guessed.
4. Use a Clean, Compatible Mixing Container
Use chemical-resistant tanks, drums and fittings compatible with sodium hypochlorite or the chemical being used. Avoid contaminated containers, metal vessels and unknown residues. Even small residues of incompatible chemicals can create dangerous reactions.
5. Add Clean Water First
As a practical rule, add water to the tank first, then add sodium hypochlorite or other concentrate. This reduces splash risk and helps the chemical disperse more evenly. Never lean directly over the tank when pouring.
6. Add the Measured Sodium Hypochlorite
Measure accurately using a suitable jug, dosing system or calibrated tank marks. Avoid “glug” measuring. On larger setups, professional softwash systems and metering equipment from the Soft Washing Equipment range can help improve consistency and reduce manual handling.
7. Add Surfactant Last Where Required
Softwash surfactant is usually added after the main dilution has been made. This helps avoid excessive foaming in the tank and improves control. Use a surfactant designed for softwashing, not washing-up liquid or unknown detergents.
The role of a surfactant is to improve surface contact and cling. It is particularly useful on vertical render, cladding, fascias and textured surfaces where run-off would otherwise reduce dwell time.
8. Mix Gently
Agitate gently by circulation, careful stirring or controlled movement of the tank. Avoid aggressive agitation that causes excessive foam or splash. Never use unsuitable pumps or tools that may corrode or introduce contamination.
9. Test a Small Area
Always test an inconspicuous area before full application, especially on painted coatings, coloured render, timber, natural stone, older brickwork, metal trims and surfaces with unknown previous treatments.
10. Apply, Monitor and Rinse Where Required
Apply evenly from the bottom up on many vertical surfaces to reduce streaking, then manage dwell time carefully. Do not allow sodium hypochlorite solutions to dry on sensitive surfaces. Rinse where appropriate and protect plants before, during and after the work with clean water.
What Not to Mix with Sodium Hypochlorite
This section is critical. Sodium hypochlorite must never be mixed with acids, ammonia, vinegar, acidic brick cleaners, descalers, toilet cleaners or unknown chemicals. Dangerous gases can be released, including chlorine gas or chloramines.
Do not mix sodium hypochlorite with:
- Acid-based cleaners
- Vinegar or citric acid products
- Ammonia-based cleaners
- Household detergents of unknown composition
- Rust removers unless the manufacturer specifically confirms compatibility
- Biocides unless the label specifically allows it
- Solvent cleaners or degreasers
If a surface needs multiple types of treatment, separate the processes fully. Rinse thoroughly, allow suitable time between products and follow manufacturer instructions.
Mixing Sodium Hypochlorite and Surfactant
A compatible softwash surfactant can make a significant difference to cleaning performance. On vertical render, for example, water alone can run off before the sodium hypochlorite has had enough dwell time. A good surfactant improves wetting, helps the solution cling and can reduce patchy results.
However, more surfactant is not automatically better. Too much can create excessive foam, make rinsing more difficult, increase residue risk and waste product. Follow the manufacturer’s dosing guidance and adjust based on surface texture and application method.
Mixing Biocide Solutions Correctly
Biocides are commonly used for residual treatment, particularly where the aim is to inhibit biological regrowth over time. Products such as DDAC-based biocides are usually diluted according to label instructions and applied to the surface after preparation.
A product such as Soft Wash Pro 50 DDAC may be appropriate where a professional residual biocide treatment is required, but it should be used as directed and not casually combined with oxidising softwash mixes.
The key point is simple: sodium hypochlorite is often selected for rapid cleaning and visual improvement; biocide is often selected for longer-term biological control. The correct choice depends on the job, the surface and client expectations.
UK Legal and Compliance Considerations
Professional softwashing in the UK is not just about whether a mix cleans well. Contractors need to consider COSHH, product labelling, environmental controls, PPE, staff training, waste and run-off management.
There is often confusion around the legal status of sodium hypochlorite. If you need a focused explanation, SoftWash UK’s guide to sodium hypochlorite softwashing UK law explains the subject in more detail.
As a professional minimum, you should be able to show:
- Product labels and Safety Data Sheets
- COSHH assessments
- Site-specific risk assessment
- Method statement for the task
- Evidence of staff competence and training
- Controls for plants, watercourses, drains and public access
Common Softwash Mixing Mistakes
Using One Mix for Every Job
A one-size-fits-all approach is unprofessional. A mix suitable for stained concrete may be too strong for painted render or delicate stone. Adjust the chemistry to the job.
Making the Mix Too Strong
Stronger solutions can increase risk without improving the result. Over-strength mixes may damage plants, lighten surfaces, corrode metals, increase fumes and shorten equipment life. Proper dwell time and repeat treatment are often better than excessive strength.
Ignoring Chemical Degradation
Sodium hypochlorite weakens during storage. Contractors who buy too much stock and store it badly may find their “3% mix” performs like a much weaker one. Store cool, shaded, sealed and away from contamination.
Adding Surfactant First
Adding surfactant before water or sodium hypochlorite can cause foaming and poor mixing. Water first, chemical second, surfactant last is a more controlled process.
Using Washing-Up Liquid
Household detergents are not designed for softwashing. They may contain ingredients that are unsuitable, incompatible or inconsistent. Use a proper softwash surfactant with known compatibility.
Mixing Incompatible Chemicals
This is the most dangerous mistake. Never mix sodium hypochlorite with acids, ammonia or unknown cleaners. If in doubt, do not mix.
Not Measuring Accurately
Professional results require repeatability. If you cannot reproduce your mix, you cannot properly diagnose problems, train staff, cost jobs or defend your method if something goes wrong.
Practical Tips from Site Work
- Label your containers: Do not rely on memory, especially on busy sites.
- Mix only what you need: Fresh solutions perform better and reduce storage problems.
- Keep a job log: Record surface type, mix strength, dwell time, weather and result.
- Protect plants early: Pre-wet before application and rinse again afterwards.
- Watch the weather: Bright sun, high wind and freezing conditions all affect safety and performance.
- Check neighbouring property: Cars, windows, washing lines, ponds and garden furniture are common claim risks.
- Train staff properly: Mixing should not be left to untrained operatives.
Example: Mixing 20 Litres of 3% Softwash Solution
This is a common calculation example for contractors and facilities teams. Assume you are using fresh 15% sodium hypochlorite and want 20 litres of finished 3% solution.
- Calculate sodium hypochlorite required: 20 × 3 ÷ 15 = 4 litres.
- Add approximately 15 to 16 litres of clean water to the tank, allowing space for chemical and surfactant.
- Carefully add 4 litres of sodium hypochlorite.
- Add the correct dose of compatible softwash surfactant according to the product instructions.
- Top up to the final 20 litre mark if required.
- Mix gently and apply according to your method statement.
This example is deliberately simple. On a real job you would also factor in surface sensitivity, access method, weather, run-off controls, dwell time and rinsing requirements.
Should You Batch Mix or Use a Dosing System?
Both methods are used professionally. The right choice depends on the type of work you do, your team size and how often you change mix strengths.
| Method | Advantages | Limitations | Best Suited To |
|---|---|---|---|
| Batch mixing | Simple, low equipment cost, easy to understand | More manual handling, risk of inconsistent measuring | Small jobs, occasional users, controlled site work |
| Proportioning or dosing system | More consistent, faster changes, reduced handling | Requires correct setup, calibration and maintenance | Professional contractors, larger sites, repeat work |
Whichever method you use, calibration matters. A dosing system that has not been checked can be just as inaccurate as a badly measured bucket mix.
Want To Improve Your Softwashing Knowledge?
If this is an area you want to understand properly, you can start by accessing the free SoftWash UK course preview. It includes selected lessons from the full training course and is designed to help contractors see whether professional softwashing training is the right next step.
Frequently Asked Questions
What order should I mix a softwash solution in?
In most practical softwashing situations, add clean water first, then the measured sodium hypochlorite or concentrate, then compatible surfactant last. This reduces splash risk, improves mixing and helps avoid excessive foaming. Always follow the product label if it gives a specific instruction.
Can I mix sodium hypochlorite and biocide together?
Do not mix sodium hypochlorite and biocide together unless the manufacturer specifically confirms they are compatible and the label allows it. In professional softwashing, they are normally used as separate treatments for different purposes.
What strength softwash solution should I use?
There is no single correct strength for every job. Light algae may only require a low-strength mix, while heavier organic staining may need a stronger treatment or repeated application. The correct strength depends on the surface, growth level, weather, dwell time and risk assessment.
Can I use household bleach for softwashing?
Household bleach is not ideal for professional softwashing. It is usually weaker, may contain additives and is not supplied for exterior cleaning use in the same way as professional sodium hypochlorite. Contractors should use appropriate professional products with proper labelling and SDS information.
How long does a mixed softwash solution last?
Freshly mixed sodium hypochlorite solutions are best used promptly. Strength declines over time, especially in heat, sunlight or contaminated containers. For consistent results, mix what you reasonably expect to use and avoid leaving active mixes sitting around unnecessarily.
Do I always need surfactant?
No, but surfactant is very useful on vertical, textured or water-repellent surfaces where cling and wetting matter. It can improve dwell time and reduce patchy cleaning. Use a softwash-compatible surfactant and follow dosing guidance.
Conclusion: Accurate Mixing Produces Safer, Better Softwashing
Correct softwash mixing is a professional discipline, not a guessing game. The best results come from understanding the surface, choosing the right chemistry, calculating dilution accurately, using compatible additives, controlling dwell time and working safely within UK best practice.
For contractors and facilities teams, consistent mixing also protects margins. When you know exactly what you used, how it performed and how long the job took, you can quote more accurately, train staff more effectively and reduce costly mistakes.
SoftWash UK supports the industry with professional softwashing chemicals, equipment, documentation and training resources for those who want to work safely and confidently. To continue building your knowledge, explore the Softwashing Knowledge Hub, review suitable soft wash chemicals, or consider the full Soft Wash Training Course if you are ready to develop a more structured professional approach.
