Soft washing a roof – handy tips

June 01, 2015

Softwashing a roof

The unsightly red, green, or black staining on a roof is likely due to the presence of algae, lichen, fungi, moss, or mould, or a combination of them all.

Black algae makes a roof look dirty and appears on a new roof after a few short years. How long exactly depends on the nature of the surrounding environment.

Lichen are the round white, brown, and black spots that power washing cannot remove. You’ll probably have noticed lichen on slate and concrete tiled roofs.

Lichen grow into, and attach themselves to, the surfaces of roofs. The only way to remove lichen is to kill it and wait for the growth to release itself from the surface. This can take anywhere from a few weeks to several months depending on the degree of colonisation and the local environment. A second application of Benz Bio Cleanze after one month will speed up the process.

Moss on roofs often grows in the joints between tiles. This allows water to penetrate where tiles overlap, damaging the underlying timber. Even small amounts of moss can impede the full drainage capacity of a roof.

Moss will normally start to appear around a property after about seven years. It particularly loves to grow on concrete roof tiles. On many buildings moss will start to grow on the roof, spread from the roof to the gutters, and then onto footpaths and drives. And it can spread very quickly

Moss holds water, which can add weight to a roof causing stress on the structure. It expands when frozen causing damage to gutters. Keeping a roof clear of moss helps protects property from the destructive effects of uncontrolled moss growth.

Preparing a roof for softwashing

First remove the worst of the moss and other growths. This is important because many roofs only get cleaned when the moss build-up has become extreme. Scrape the bulk of the moss off first. If the pitch of the roof is not too steep, and the tiles are sound, a trained roofer can do this.

But if the roof is too steep or fragile to walk on, carry out pre-spraying work from the ground using tools based on water fed pole technology.

Protect gutter outlets from becoming blocked before beginning work. And lay tarpaulin around the perimeter of the property to catch any falling moss and other debris.

Protecting plants

The active ingredient in Bio Cleanze and Benz Lightning Cleanze can discolour leaves and flowers. Thorough drenching, both before after treating the roof, with copious amounts of fresh water is an effective way to protect your customer's plant life.

Cllck here to learn how to protect your customer's plant life when softwashing

Solar panels

It’s best to treat a roof before fitting solar energy panels – with a strong dilution of Bio Cleanze – because afterwards it will be almost impossible to remove any growths living under the panels.

Can a moss and lichen covered roof be treated with Benz Bio Cleanze alone?

For a few, particularly challenging situations – those hard to reach with a pole and scraper, or to scrape from a roof ladder for example – treating a moss-covered roof with high-strength Bio Cleanze is an option. 

We suggest treating twice at 20:1. Thoroughly soaking the area of moss, then return to where you started and give the second treatment.

This will enable the chemical to soak more deeply into the moss and lichen, ensuring greater effectiveness.

How long it will take to fully show results depends on the local environment, prevailing weather and how steep the roof pitch is, as the steeper the pitch the better the self-cleaning action.

As a rule of thumb, moss will turn brown in 48 hours or so.

The cleaning effect of Bio Cleanze will continue for the next 6-12 months. And if the surface is porous, will provide a degree of ongoing protection against re-colonisation.

Natural weathering or rain, wind and sun will gradually remove the dead moss and lichen through time.

Quick tip: You will need to educate your customer in advance as to the long-term, ongoing cleaning effect of Bio Cleanze.

This is an unusual property of a cleaning chemical, but your customer will be happy when the situation is fully and clearly explained to them.

Softwashing equipment

Window cleaner’s water-fed poles (WFP) are fine for “brushing in” and avoid the use of ladders for the majority of properties.

Low-pressure application systems use telescopic poles to reach every part of a property. Using this softwash system means the operator stays on the ground, or on sturdy scaffolding, which is safer both for him and his customer’s property. Use a coarse spray of relatively large droplets to minimise spray drift.

Quick tip: Softwashing roofs with Bio Cleanze is a profitable extra service to the repair and replacement work of existing roofing contractors, who already have the equipment and skills for working at heights.

How long does a softwash treatment last?

Exposure to the sun, nearness of trees, humidity, porosity and quality of the tile’s surface roughness all influence how long it will be before re-treatment becomes necessary.

If the roof has previously been power washed, the porosity and surface characteristics of the tiles will likely have been damaged and so biological growths are likely to return faster. In the worst cases, it may not be possible to clean the tiles completely.

Most roofs will stay clean for three years before biological regrowth requires another treatment. Concrete tiled roofs last longer before regrowth will be visible again and another treatment becomes necessary.

Restoring and cleaning old roofs by soft washing them

Some people unknowingly perceive moss, lichen and algae growth on old buildings as a mark of character but in fact such growth can damage the roof and structure.

Bio Cleanze acts to gently clean and restore these ancient roofs to their former glory without causing the surface damage that high pressure washing will.

Apply  Bio Cleanze evenly across the surface area of the roof. Brushing-in should be done gently and with care. The microbes and algae will die within a few hours without aggressive action.

Sandstone can be safely treated without damaging it. Biocidal treatment will remove the black dots, which become so well ingrained that even high pressure washing cannot shift them.

Quick tip: If a surface is badly infested with black lichen a treatment with strong solution of Lightning Cleanze prior to treating with Bio Cleanze is often the most effective approach.

Non-aggressive soft washing is the only safe way to clean old slate and tile roofs without damaging the surface of the tiles and therefore reducing their lifespan.

Health & Safety

Before starting work always evaluate the site for Health & Safety considerations and establish safe working methods. Always wear appropriate and effective PPE and wash exposed skin with copious amounts of fresh water before eating, drinking, smoking and upon completion of a job.


We hope this information is helpful.

Click here to read more Trade Tips posts and watch more videos ...


Ben, Leo
 & Fiona

Click Here to explore the full range of Benz popular softwash cleaning products

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

Legal statement about using DDAC biocide to kill and control moss: For many years the active ingredient in Benz Bio Cleanze (DDAC) has been used to kill and control moss.

But now, as from June 2015, the politicians and bureaucrats have decreed that moss is to be considered a “plant” and therefore comes under a different set of labeling regulations.

As such the law states that now, only manufacturers of expensive herbicides (aka “weedkillers”), which are often quite unhealthy for humans and for our environment, can claim to kill and control moss.

From June 2015 no biocide manufacturer can state that a biocide such as Benz Bio Cleanze kills and controls moss, even though it was used to kill and control moss for many years ...

Click here to read a fuller explanation of the current legislative situation.

Leave a comment

Comments will be approved before showing up. Please revisit this Trade Tips post to read our response.

Also in Benz Trade Tips

Remove rust from stainless steel with Benz Oxy Cleanze
How to remove rust stains from stainless steel (video)

November 25, 2020

In this short video Ben demonstrates the treatment of a stainless steel door handle to remove rust stains – caused by contamination with carbon steel –by applying Benz Oxy Cleanze.

Click the link on this page to watch the video >>>

View full article →

Questions we are asked about soft washing – answered 03
Questions we are asked about soft washing – answered 03

November 18, 2020

Every week we receive a large number of questions about soft washing. And, as new contractors are coming into the business all the time, questions are often repeated.

So we thought it would be useful to put together the most frequently asked questions in this series of Trade Tips posts, for your easy reference.

Click the link on this page to read the full post ...

View full article →

Ben soft washing wall render with Benz Lightning Cleanze
Your soft washing questions answered - 02

November 11, 2020

Every week we receive a large number of questions about soft washing. And, as new contractors are coming into the soft washing business all the time, questions are often repeated.

So for your easy reference we thought it would be useful to put together some of the most frequently asked softwashing questions in this series of posts.

Click the link on this page to read the full Benz Trade Tips post ...

View full article →