What chemicals are used for roof cleaning? Bleach / Biocide
You are about to hear a word that may make you pause, and that word is “chemicals".
There can naturally be some uncertainty or fear with the mixed information there is out there online when it comes to roof cleaning chemicals, but it's completely natural if you want to know what’s being used and why.
Whats the difference between bleach based and biocide roof cleaning products and which is “better”?
So asking what chemicals are used in roof cleaning and whether they’re actually necessary is a sensible and responsible question.
This article is here to explain that clearly, calmly, and honestly.
Not every roof clean has to involve chemicals
You always get to choose if you do or don’t want them used at your property.
And not all roof cleaning jobs have to involve chemicals.
In many cases, especially where low-pressure hot water cleaning is used, the majority of the work is done by water, temperature, and technique - not by chemicals.
So if you’re imagining strong solutions being sprayed everywhere as standard, that isn’t how a professional roof cleaning company works.
But that’s not to say that behaviour has never happened in our industry.
Why chemicals are typically used after roof cleaning
When chemicals are used, they’re usually not there to make the roof look clean on the day of the cleaning.
In the case of roof cleaning they’re there to eliminate organic matter that remains on the roof and to help control regrowth of organic matter on the roof.
Moss, algae, and lichen don’t just sit on the surface of roof tiles, they anchor themselves into tiny pores and imperfections. Even after a roof looks clean, microscopic spores can still remain.
This is where post clean chemical treatments come in.
Think of it less like “cleaning” and more like
Aftercare or
Maintenance
Common types of chemicals used in roof cleaning
There isn’t one single chemical used across the industry.
Different products exist for different purposes.
Here are the most common you’ll hear about.
Biocides (often BAC-based treatments)
Biocides are designed to kill organic growth such as moss, algae, and lichen and once applioed it doesn't have a particular odour or smell.
You may hear people refer to products like the generic term "biocide" or “BAC 50”, but it’s important to understand that this refers to a concentrate, not what’s actually applied to your roof.
In the real world, this is typically how its used by an exterior cleaner:
- it’s heavily diluted at roughly 20:1
- applied at low strength
Biocides don’t usually give instant visual results, they're designed to target organic growth and for it to have an effect over time.
They work gradually over time because they have a longer “persistence” which simply means they linger for longer. They actually bind with organic matter and cling to it for a longer life time.
They’re often applied
after cleaning, typically after manual roof scraping methods.
Sodium hypochlorite (bleach-based solutions)
Sodium hypochlorite is liquid bleach and they have a chlorine odour smell. Just like what you would remember from the swimming pools especially if you found yourself swimming length at Tudor Grange in the 80's and 90’s… and possibly earlier?
Sodium hypochloriteis very effective at:
- killing organic growth
- lightening staining
- delivering fast, instant visual results
Sodium hypochlorite is typically applied using equipment known as softwashing systems which are a higher priced product compared with the equipment levels needed to complete roof scraping. Softwash systems that are designed to handle and mix bespoke strenth softwash solutions for your roof in a safe and controlled way are typically seen among some of the more professional exterior cleaning businesses within the UK.
Sodium hypochlorite, just like biocide needs to be used carefully, and their a principles that must be followed when using either solution to treat a roof after cleaning, those are:
- correct dilution ratios
- surrounding areas must be protected
- runoff needs to be managed
- application should be controlled
When a soft washing approach is used properly, it can be effective.
Used carelessly, it can cause unnecessary damage , which is why softwashing isn’t suitable for every roof or every situation.
It's important to add a final reminder that the environmental care principles apply to both bleach based and biocidal products.
Environmental behaviour of different roof cleaning chemicals : BAC vs bleach
Using any roof cleaning solutions, the immediate effect of the chemical are not always instant on the day so it's important to understand what happens afterwards with each product
Different products behave very differently once they leave the roof surface.
Here’s a simple, plain English comparison.
| Question | Biocide (Bac50 diluted at 20:1) | Bleach based diluted to 4% strength) |
|---|---|---|
| Environmental persistence (half life effect) | Tends to persist longer if it reaches soil or sediment, as it can bind to surfaces and biodegrades more slowly | Breaks down relatively quickly once released, reacting with organic matter and sunlight |
| “Long-lasting effects” warnings | Often carries Aquatic Chronic hazard classifications due to potential long-term harm if released into watercourses | Can still be toxic to aquatic life, but active chlorine is short-lived |
| Practical “down to earth” meaning | More likely to linger and cause longer-term environmental harm if it reaches drains or waterways | More of a “hit hard, fade fast” product — still harmful if released, but doesn’t persist as long |
What this means when using chemicals for roof cleaning
This comparison isn’t here to say “one is safe and one is dangerous”
To be crystal clear : both products can cause harm if they’re misused or allowed to enter surface water.
The difference is mainly in how long their effects last once released.
That’s why professional roof cleaning isn’t just about which chemical is used, but about the correct dilution, controlled application, managing runoff, protecting drains, plants, and surrounding areas.
Really a contractor should be choosing the right product for the project
Used responsibly, both types of products can be part of a controlled, professional roof treatment process.
So where does Squeaky Solihull's roof cleaning process fit into all of this?
It’s worth being upfront about this: we’re not the right fit for every roof or every homeowner.
Different companies specialise in different methods for good reasons. What matters is understanding what approach is being used and why.
Our typical roof cleaning process is based around low-pressure hot water cleaning.
We use a pressure washer with the pressure turned right down, and we heat the water to somewhere between 60 and 100 degrees.
The heat does most of the work to sanatise the roof and remove moss and organic matter. We let the heat do the work, not force and that’s how we clean the majority of roofs we work on.
We fully appreciate that this method won’t be suitable for every roof, but for many properties it’s a very effective and controlled way of removing organic growth.
What does Squeaky Solihull do after the roof is cleaned?
Once the roof has been properly cleaned, we’ve:
- cleared the gutters
- removed as much organic matter as possible
- protected plants, drainage points, and surrounding areas
At that stage, some nearly all homeowners choose to have a post-clean sanitising treatment applied which is part of our cleaning process.
When we do apply a treatment, we typically use a sodium hypochlorite–based solution (bleach), metered precisely using our soft washing system equipment. This allows us to dial the finished solution down to around 3–4% which is less strength than you would find in a household bleach product.
The solution is applied carefully to what you might consider an
already cleaned roof. The soft wash solution is not just there to do the cleaning, although it does have a cleaning effect. Its role is simply to sanitise the surface and deal with any remaining microscopic organic matter on the roof tiles.
Why we choose Sodium Hypochlorite for the majority of roofs.
Despite the distinctive chlorine smell that emitted which is temporary following the treatment
One of the reasons we’re comfortable using this type of treatment after cleaning is how it behaves.
Within around 30 minutes, the active chlorine has reacted and oxidised, meaning it loses its potency relatively quickly once it’s done its job.
By comparison, some biocidal treatments are designed to linger for much longer. Biocide is essential after scraping a roof if the desired effect is for a clean look. But for us at Squeaky that is not the method we choose to use for the majority of homeowners we help.
Our process focuses on:
- removal of 95% of organic matter using low pressure hot water first
- then, lightly sanitising the roof to bring it back to a clean baseline using a bleach based soft wash solution.
Nothing extreme. Nothing unnecessary and for us a very usual procedure to follow.
Is chemical treatment for roof cleaning necessary?
No, and it’s important to say that clearly.
Not every roof needs a post clean treatment, and in some rare cases not every homeowner wants that aftercare option. It is always your choice.
That said it is in the very rare cases that clients choose not to have the soft wash treatment which comes as standard. Once the roof has been cleaned we recommend that once the roof has been properly cleaned, it makes sense to finish the job by ensuring the organic matter on the roof is sanitised to reduce early regrowth with a soft wash solution.
But the decision is yours. We offer our advice, but we understand that its not a one size fits all process.
Some final thoughts about chemicals at your home
Chemicals used for roof cleaning aren’t something to fear, but they shouldn’t be used blindly or in an uneducated way either.
Understanding what’s used, why it’s used, and whether it’s even necessary puts you back in control of the decision.
That’s the approach we take at Squeaky Solihull and it’s why we’re trying our best to be transparent at all costs.
So you can truly make the most informed decisions about your home's exterior cleaning.