Soft washing a roof from algae, lichen, moss & mould

June 01, 2015

Soft washing a roof from algae, lichen, mold & moss


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 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

Run-off from 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 these softwash systems means the operator stays on the ground, or on sturdy scaffolding, which is safer both for him and his customer’s property. Using a coarse spray of relatively large droplets will 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 can 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 and sincerely wish you great success in your soft washing business,

Ben, Leo
, Fiona, Stevie & Anthony

Click here to explore the full range of Benz softwash cleaning products

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

Leave a comment

Comments will be approved before showing up.

Also in Benz Trade Tips

How to safely soft wash asbestos roof slates, tiles & sheeting
How to safely soft wash asbestos roof slates, tiles & sheeting

September 15, 2021

Many soft wash contractors can be unsure of the safest way to soft wash asbestos roofs. So in this post we lay out the best practices we've learned from running a soft washing business since 2013.

Click the link on this page to learn how to treat asbestos ...

View full article →

How to soft wash natural slate roofs
How to soft wash natural slate roofs (video)

September 08, 2021

Ben demonstrates how to scrape moss and lichen from natural slate roof tiles and treat the biofilm to kill all the growths on this natural slate roof.

Click the link on this page to watch the how-to video ...

View full article →

Soft wash brush application
How to protect lead flashing & roofs when soft washing (video)

September 02, 2021 2 Comments

When soft washing it's vitally important to protect surrounding building materials. e.g. lead flashing. And not to create "clean streaks" – from overspray and run-off – on surfaces that are not part of your contract.

Click the link on this page to watch how we achieve this when soft washing a chimney ...

View full article →