Soft washing roofs

Softwashing Biocides UK roof cleaning

We advocate three softwash treatment systems to clean a roof.

Your choice of softwash system will depend on the nature and materials of a particular roof and the requirements of your customer.

Quick Tip: We suggest you position soft washing your customer's roofs primarily as a "treatment", not just as "roof cleaning". By doing this your customer will be prepared to wait for the full results to show, just like taking a medicine for a body ailment.
Cleaning a roof using soft washing with Benz Blackwash
Above right is a roof softwashed with Benz Bio Cleanze

1) The first softwash system

This is the system we recommend for most softwash roof treatment contracts

Use Benz Bio Cleanze – after manually removing the worst of the growths – for cleaning most roof surfaces. We've found this to be the simplest, safest and most profitable way to clean roofs by softwashing.
Bio Cleanze has the advantage of being able to get deep into the laps by capillary action more effectively than other cleaning products (it's a better "wetting agent" even than water).

Because it does not need rinsing, using Bio Cleanze is also the most profitable way to soft wash roofs.

And Bio Cleanze is the perfect product for use in the Benz "Clean & Maintain" softwash systems, which give customers a perpetually clean property.

Quick tips:
1) Brush application, without spraying first, will not allow Bio Cleanze to fully penetrate between the slates. The air bubbles in the suds can give a false impression that the surface has been fully saturated.
2) Brushing-in first can also result in some moss/lichen/algae remaining untreated in the gaps between the slates, which in turn leads to their rapid regrowth.

The correct method of applying Bio Cleanze to roof slates and tiles:
  1. Block the downpipes (a tennis ball often works well)
  2. Manually scrape moss and lichen from the surface of the roof. Scraping and removing moss and lichen from a roof before giving a chemical treatment minimises the possibility of:
    • Birds ingesting treated moss
    • Softwash chemicals entering the food chain
  3. Remove the debris from the roof, gutters and then from the site. Dispose of safely and with respect for our environment.
  4. Spray Bio Cleanze, diluted 20-25:1, up towards the apex of the roof, so the chemical flows under the lap joints, which is where spores like to hide. If the underside of the laps are not treated, this will lead to rapid recolonisation of the rest of the roof.
  5. Ensure the gaps between the slates are thoroughly saturated, as this is another place where spores, as well as live moss/lichen/algae, often remain.
  6. Now brush Bio Cleanze into the roof surface, gently agitating to ensure the chemical soaks deeply into any remaining moss, lichen and algae.
  7. Do not rinse.

2) The second softwash system

For most roofing materials we recommend treating with Bio Cleanze – and prefer it even for non-porous natural and man-made slate because of the way the product gets deep under the laps and does not require rinsing.

Benz Lightning Cleanze can be used to give a more "rapidly clean" effect when needed. It can work particularly well on smooth, non-porous roof surfaces such as natural and man-made slate, especially if your customer wants a quick clean (before moving house, for example).

As when treating a roof with Bio Cleanze, spray up from the gutters towards the apex of the roof. Spraying up means that some product will get under the laps and kill the spores that hide there. Then, while the roof is still wet, gently brush in to help dislodge the biofilm.

Quick Tips:

1) Manually scrape moss and lichen from the surface of the roof, following the instructions for the scraping and removal of moss and lichen detailed above (steps 1-3).

2) Bear in mind that when Lightning Cleanze soaks into porous surfaces it can take considerable time to rinse thoroughly. Rinsing is easy and quick on walls and horizontal surfaces but much more time-consuming on a roof. Remember to factor this rinsing time into your pricing if you decide to use Lightning Cleanze on a roof.

3) Set your pump flow rate such that there's only minimal run-off into the gutter. Treat sections of the roof at a time, allowing Lightning Cleanze to dry on one secion while you treat the next, then return to rinse the first section. This approach helps reduce run-off.

Ongoing, we suggest maintaining all roofs with Bio Cleanze, even those initially cleaned with Lightning Cleanze, by applying our "Clean & Maintain" softwash systems.

Because Bio Cleanze's highly effective capillary action (i.e. its 
"wetting" ability) is greater than water it easily penetrates the laps and cracks. And because it does not require rinsing, ongoing maintenance of roofs is fast and simple.

3) The third softwash system

Gently power (aka pressure or jet) wash the roof – using a low pressure setting if you have a variable pressure washer – and/or use a fan jet and keep the nozzle some distance from the surface if your power washer does not have a variable pressure control.

Do not use very high pressure because this can damage the surface of roof tiles and significantly reduce their life expectancy.

Work from the apex down (never – when using a pressure washer – work from the gutters up, because of the dangers created by the pressurised water spray getting under the tiles).

Quick tips:

1) Block downpipes before pressure washing.

2) After pressure washing, remove debris from the site and dispose of it safely and ecologically.

3) Spraying from the roof apex down is the opposite to when soft washing with Benz biocides because then we want the biocide to get under the laps and kill the spores that hide there. Because of the low-pressure delivery system, no damage will ensue.

4) When the roof looks clean from power washing, remember that spores will not have been killed, so post-treating with Bio Cleanze is essential.

How long will a treated roof stay clean?

The time a roof treated with Bio Cleanze stays clean will depend on multiple factors. For example, the severity of the colonisation, the prevailing weather, the type of surface and the local environment.

A reasonable general expectation for a porous roof surface would be around 18-36 months on average, and up to 5 years in optimum conditions.

Treat with Bio Cleanze at these dilutions:

  • Treat a steam cleaned roof at 50:1
  • Treat a pressure washed roof at 30:1.
  • Treat a scraped roof at 20-30:1.

How to soft wash a roof on a hot day

Whichever softwash system you use to clean a roof, on a very hot day, especially on an elevation facing the Sun, it can be a good idea to first hose down the surface to cool it down. This reduces the possibility of the softwash biocide "boiling-off" before it has a chance to work. 

Back to top

The importance of rinsing Lightning Cleanze

If you use Lightning Cleanze to clean a roof, always thoroughly rinse it from the surface after allowing 30-60 minutes for the cleaning action to take effect. This ensures that the small quantity of salt left behind by the break down of chlorine is removed.

Also, we advise rinsing between applications of Lightning Cleanze to remove the debris cleared from the surface by the first treatment, allowing the next coat to penetrate the biofilm to an even greater depth.

On many jobs a single application will be sufficient. But if the colonisation is very heavy, two treatments – and exceptionally three – may be required.

We also recommend gently brushing each application of Lightning Cleanze into the surface, so that it penetrates the biological growth to the maximum depth and thus increases its effectiveness.

Quick tip: Rinsing Lightning Cleanze thoroughly is particularly important if you are going to post-treat with Bio Cleanze to ensure the surfactant in Lightning Cleanze is removed from the surface. 

Treating yellow lichen and moss balls on a roof with Benz Bio Cleanze

Yellow lichen on roof: It can take 7-12 months for yellow lichen to fully disappear from porous roof tiles. Althought depending on the local environment and how long the yellow lichen has been allowed to grow into the surface, the timescale may be shorter. On non-porous slate the time-scale time-scale could also be shorter.

Moss balls on a roof: Any moss balls over 10mm should be scraped if at all possible. Small moss balls will die after treatment with Bio Cleanze and fall into the gutter. Bigger than this and it’s down to the roof pitch – the steeper the pitch, the more likely the moss balls will fall off.

Biocide roof UK Softwashing

The above photo shows heavy moss being scraped from a man-made slate roof using a Benz roof scraper with nylon blade.

Click here to learn about out roof moss scrapers

If a roof cannot be scraped

If the moss balls are heavy and/or on a shallow pitch roof, they may stay there and eventually recolonise. Allow up to a year for full results to be seen after treating with Bio Cleanze.

If the moss is still there after 12 months, re-evaluate what to do. If scraping is not an option then treat with Lightning Cleanze at 5:1 (two treatments one after the other can be particularly effective, as the product will soak in more deeply). The moss will turn to white mush after a few hours. The mush should be be sucked up and removed from the site.

Quick tip: This is not the best practice – as the mush can block gutters and downpipes after rinsing (downpipes must be sealed prior to treating) and may contain a certain amount of sodium hypochlorite. This is the reason for sucking up the mush and removing it from the site.

Important note regarding offers of "instant cleaning"

To get the near instant results claimed by some contractors requires the application of extremely strong sodium hypochlorite (ie bleach).

We generally recommend using Benz Lightning Cleanze at 5:1 dilution (2% available chlorine), or exceptionally, 4:1 (2.5% available chlorine).

At 5:1 dilution Lightning Cleanze is classified as non-corrosive to metal and rubber fittings. Even at 4:1 it is only mildly corrosive, which can be offset by pre and post rinsing of vulnerable fittings.

But contractors applying raw, industrial sodium hypochlorite at 3-6% available chlorine are spraying a highly corrosive chemical onto their customers roofs.

Unfortunately this means:

  • They are endangering their own, customers and the general public's health through breathing chlorine fumes and being exposed to a highly toxic chemical that can inflict serious chemical burns.
  • They are endangering the health of animals and children who may stray into the working area.
  • The run-off and overspray will be highly toxic to plant life.
  • Metal and rubber fitting on the roof, guttering and windows fitted into the roof or exposed to spray drift will be damaged.
  • Roofing materials themselves can be permanently discoloured.
  • If run-off is allowed to enter surface drains, which discharge into waterways, it will damage our environment and is illegal.
  • Spraying unapproved sodium hypochlorite for purposes other than its "intended use" (usually disinfecting swimming pools and milking parlours) is also illegal.

We are not sure of the impact to a contractors professional liability insurance policy in the event of an accident, or to their customer's household insurance policy, as this will likely vary from company to company. What we are certain of though is the the consequences will not be good ...

In contrast Benz Bio Cleanze and Benz Lightning Cleanze are fully legal and have been approved by the UK government (HSE) and Irish government (PRCD) for the cleaning of exterior hard surfaces.

So the best way to avoid serious negative consequences is to "Buy Benz and Be Safe".

Evaluate the area to be treated by softwashing and calculate a price for the job

Click here to download your “Benz Professional Softwash Contractors Pricing Guide, which explains clearly and simply how to calculate the price per square metre for any given softwashing contract. (comes free with Benz Trade Tips)

Determine how to access the roof

softwash cleaning chemicals Ireland England

Check to see if all elevations can be sprayed from the ground. If not, use a podium ladder, lightweight scaffold tower as in the above photo, cherry picker, or scissors lift.

Walking on roofs

This was standard practice for years before H&S came in. Nowadays best practice is to use a roofing ladder and scaffolding (a tower is often sufficient). If you don't, it is possible your insurance would be invalid in the event of an accident.

Also, using WFP it's often possible to treat the entire roof without stepping onto the roof at all. The exception can be the need to scrape moss and lichen before treating, although even for this a scraper attached to the end of a pole usually works fine.

Determine the extent of moss & lichen growth

Correct evaluation will help you work out the time the job will take and so price correctly.

Quick tips: 

  1. To do a decent job it is usually necessary to scrape the worst of moss and lichen from the roof before applying biocides.

    Even if you manage to kill the moss with a high concentration of Bio Cleanze, perhaps with multiple treatments, the moss will fall into the gutters and block them. And your customer will not be happy.

  2. Ben also tried treating moss with a strong dilution of Lightning Cleanze, which can kill the moss. But it turns to a white mush and washes into the gutters, blocking them.

    You will then left with the horrible job of cleaning chemically treated mush from the gutters. This turned out to be a highly ineffective and time-consuming approach compared with scraping and treating with Bio Cleanze. We do not recommend it except for exceptional circumstances.

  3. Brushing-in, without thoroughly spraying first, does not allow Bio Cleanze to fully penetrate between the slates or tiles. The air bubbles in the suds can give a false impression that the surface has been fully saturated.

    Brushing-in before spraying can also result in some moss/lichen/algae remaining untreated in the laps and in the gaps between the slates, which in turn leads to their rapid recolonisation.

The correct, and highly effective, method to treat roof slates and tiles with Benz Bio Cleanze:

  1. Block the downpipes.

  2. Manually – taking care not to damage the surface of the slates or tiles – scrape the worst moss and lichen from the surface of the slates.

  3. Remove the debris from the gutters and then from the site.

  4. Note that on artificial slates in particular, scraping must be done gently, so as not to damage the relatively fragile surface.

  5. Spray Bio Cleanze, diluted 25:1, up, towards the apex of the roof, so the chemical flows under the lap joints, which is where spores like to hide. If the underside of the laps are not treated, this will lead to rapid recolonisation of the rest of the roof.

  6. Ensure the gaps between the slates are thoroughly saturated, as this is another place where spores, as well as live moss/lichen/algae, often remain.

  7. Now brush Bio Cleanze into the roof surface, gently agitating to ensure the chemical soaks deeply into any remaining moss/lichen/algae.

Protect your customer's plant life

If there’s any danger of serious spray drift, cover your customer’s plants with a lightweight sheet, ensuring that the plants remain undamaged by the covering.

Always spray all plants and lawns in the vincinity of the treatment area, or likely to receive spray drift, with copious quantities of fresh water immediately before and immediately after applying a softwash treatment. 

Pre-drenching plant life helps dilute any biocide that may reach the leaves and soil. And post-drenching washes any spray drift from the leaves and further dilutes any that reached the soil.

Pre and post-drenching is a remarkably simple but highly effective way to care for your customer's plants, trees and lawns.

Quick tip: In our experience pre and post-drenching with fresh water, and minimising spray drift and run-off with skillful application techniques and Benz Softwash spray nozzles, works just fine.

We've found it to be the simplest, least time-consuming and most inexpensive way of protecting your customer's plants.

We've found absolutely no need to post-treat your customer's plants with any other product such as a liquid fertiliser (which in any case could be the wrong type of nutrition for your customer's particular plants and therefore could be harmful to them). 

Click here to learn how to protect your customer's plant life


Important general notes about softwashing a roof

  • If there’s a light breeze, angle the spray into the wind to reduce spray drift.
  • Never spray if the wind strength will cause unacceptable drift.
  • Use a projection nozzle only in the rare situations when part of the roof is out of reach of telescopic poles and cannot be sprayed any other way (in our experience this is almost never the case). Avoid moving your pole over unprotected grass or plants when spray is being emitted

What if access to a roof is blocked by a conservatory?

Quick tips: 

  • Scaffold over the conservatory, then work on the roof from the scaffolding.
  • Use a scaffold tower, and work on the roof from either side of the conservatory.
  • Factor the cost of hiring scaffolding into your quote, explaining the reason to your customer so they understand. When finances allow invest in your own scaffold tower.


Back to top

Setting up your soft washing equipment

Choose an area of hard standing to set up your application equipment. If at all possible do not set up your equipment on your customer's grass. If the grass is the only viable option, cover it with a strong tarpaulin first.

Prepare the roof for softwashing

Remove the worst of moss, lichen and liverworts with Benz Roof Moss Scrapers. It’s especially important to remove as much moss as possible from low-pitched roofs situated in sheltered or shady locations.

Watch the video below to learn about Benz roof moss scrapers for removing moss from roofs prior to soft washing with Benz Bio Cleanze

Softwashing application procedure

Begin spraying up from the eves, ensuring good penetration into the lap of the tile or slate. Pay particular attention to features such as a dormer back gutter, valleys, and so on as missed spots will start to show regrowth quite soon.

After spraying the whole roof up to the apex, follow with one final treatment down from the apex to ensure complete coverage of the roof.

Quick tips:

    1. Brush or spray application?

      As much as possible we prefer to spray roofs.
      The spray nozzle can direct the product against the raised lap of the tile giving the best chance of the product being able to creep under the lap and killing any spores. The product will creep under the tiles through capillary action, and directing your nozzle at the joint promotes this further.

      Brushing can give a false sense of saturating the biofilm as the suds it generates can create a blanket of foam which is product and air and may not actually saturate the biofilm.

      That said, on a steep or long roof in height, a brush can advantageous as the brush can rest on the roof so you are not having to hold up the pole to spray.

      In this case just bear in mind the above point and ensure that the biofilm is saturated and not just got a coating of foam.

      Having adequate flow rates normally achieves this and to aid this we use pencil jets in the brush.

    2. How do you achieve a straight line between adjacent properties? 

      Use the nozzle to “cut in” the line between the properties. The small amount of overspray will not have any noticeable effect on the other roof. Simply spray with the nozzle pointing towards the roof you are cleaning. The run-off will create a line. Or use a brush and drag it down to create the line.

    3. Why spray up to the apex, and not down from the apex?

      • Because you can see more clearly where you are spraying.

      • Because the slate or tile lap is where there’s the greatest build up of moss and spores. So it’s important to aim the nozzle upwards, thus achieving deep penetration into the lap joint.

      • Because by spraying up you get more product onto the roof and a more even coverage without the mix running down into the gutters.

      • Because if you were to spray down, the mix would just run over the lap without getting into the lap.

    4. How to soft wash a roof prior to painting or sealing?

      To clean a roof ready for painting, the most effective process is to take the following steps:

      1. Scrape all heavy moss. If very heavy lichens are present, which can be on old, heavily colonised roofs, then treat with Bio Cleanze and leave to naturally cleanse for 7-12 months before continuing.

      2. Treat with Lightning Cleanze at 5:1 to kill the worst of the growths, leave for 60 minutes then use a pressure washer with just enough pressure to get the roof clean.

      3. Treat with Bio Cleanze at 25:1 to kill any spores remaining in the slate laps and provide a degree of residual protection against re-colonisation.

      4) Leave for a couple of rain showers to rinse Bio Cleanze from the surface of slates, or give a gentle rinse with a pressure washer after a couple of days. Rinsing Bio Cleanze from the surface prior to painting is important because it allows the paint to adhere more strongly.

      Quick tip: This step is often skipped and not seen as necessary by everyone but is the only way we know to ensure that all spores have been treated.

      4. Paint roof.

      Quick tip: If there is a necessity, or demand by customers, for more rapid results, it is common in the industry to use the following process instead:

      1. Scrape all heavy moss
      2. Treat with Benz Lighting Cleanze at a dilution of 5:1
      3. Power wash
      4. Paint

      Important: There is a drawback to the above process in that there will inevitably be some algae spores left under and between the laps of the slates.

      The method chosen needs to be discussed and agreed upon with the customer in advance of starting work, because there's a danger that, after painting, green streaks may start to form from the spores left under the laps.

    5. How to prepare a roof ready for sealing?

      1) Follow the steps above for soft washing a roof prior to painting.

      2) We suggest using a transparent, breathable, roof sealer that protects the tiles and extends their life but does not leave a "paint-like" coating on the surface of the tiles.

      3) Final results will depend on the type of roof sealer used and the quality of the application.

    6. How to rinse a roof that cannot be fully seen?

      We favour applying soft wash biocides and rinsing high roofs from ladders secured by Tetra level 2 or 3, a scaffold tower, or a Cherry Picker.

      Our preference is for a scaffold tower. It is safer than working from ladders and you can do a better job when working from the eves.

    7. How do I use your softwashing products to softwash roofs in high summer heat?

      Cool the surface by spraying it with water first and keep it damped down whilst treating the roof, using another operator to deliver the cooling spray if necessary. If this is not practical, do the spraying in the cooler times of early morning and late afternoon.

Back to top

Drainage systems

It is important to protect our environment by minimising run-off into sewers and surface drains. Run-off can be kept to minimum or zero by using Benz softwash spray nozzles along with skillful spray and brush control techniques. If some run-off to drains is unavoidable, block the drains and/or gutter downpipes until application has been concluded.

Click here to learn the best practices for working with run-off and drains

Watch the video below to see how Ben applies Benz Bio Cleanze using a water-fed pole fitted with a Benz softwash spray nozzle 


Notes on the video above: On this particular roof, with the ridge running down the hip, Ben wanted to make sure the biocide did not run down the roof tiles and into the gutters.

Obviously all the tiles have to be treated at some point, but if the biocide is running down the roof – and starting to overtake where you are spraying – it's better to stop, treat something else, then go back to where you were spraying and continue from where you left off.

On a standard ridge Ben first cuts in, then does a few courses of tile, then sprays the top of the ridge, then goes back down and sprays a few more courses of tile until it starts running down. Then he works from the bottom up for a few more courses, then back up to the top. (you'll see Ben doing this in the video)

On a roof that has long gutters this process of spray control may not be needed – as by the time you get to one end the other end will be starting to dry.

Spraying techniques like these help keep as much biocide on the roof as possible and prevent it just flowing into the gutters, which is wasteful.


Before soft washing ...

After soft washing a roof with Benz Pro-Soft Wash biocides


After soft washing

After soft washing a roof with Benz Pro-Soft Wash biocides


Soft washing health & safety

Always wear full PPE (Personal Protective Equipment), including impermeable gloves, hair protection, chemical-resistant clothing and a full face shield.

Barrier off the working area and keep customers, the general public, children and animals away from chemicals and treated surfaces. Only allow them to re-enter when the area has been made safe.

On an area of hard standing, rinse your application and safety equipment with copious amounts of fresh water, being careful not to allow rinse water to flow into flower or vegetable beds, lawns, or surface drains.


Click here to download your – free – professional softwash contractors pricing guide

The Benz Softwash range of softwash cleaning products 


Click here to get more practical soft washing how-to tips

How to find answers to most soft washing questions: Simply enter a relevant "keyword" or short phrase (e.g. keyword for above link = "how to") into our in-site Search Engine (the magnifying glass icon on main menu) 

You can order Benz softwash chemicals & equipment online or by phone:

GB (England, Scotland, Wales)

Phone: 0800 70 74 222 


Ireland and other countries

Phone: 00353 214 622 978


Lines open Monday - Friday, 9am – 5pm

Please note: The above numbers are for orders only (click here for tech support)


Click here to explore the full range of Benz Softwash cleaning chemicals & equipment

Benz Softwash Bio Cleanze DDAC biocide & Benz Softwash Lightning Cleanze SH sodium hypochlorite biocide