Why Do UK Roofs Get Dirty?
UK roofs are exposed to a lot over the years! In particular, a very damp climate for most of the year, which makes them the ideal environment for moss, algae and lichen to grow. Moss is the most common issue, particularly on north-facing or shaded roofs and tiles that have a rough surface it can latch onto.
Most people will consider cleaning their roods because:
• Moss can build up in gutters and block down pipes, causing them to overflow and potentially cause other building damage
• They want to improve the visual improvement of the roof
• It might conceal damage
• Moisture could be retained against the tiles, preventing them from drying out and leaving them susceptible to degradation through frost damage
While these are all understandable reasons, cleaning isn’t always essential to solve them. And how cleaning is done matters far more than whether it’s done at all.
Problems Caused by Incorrect Roof Cleaning
One of the biggest issues we see is the misuse of high-pressure washing equipment on tiled roofs. While it might seem like a good idea and an effective way to remove dirt and moss, using the wrong cleaning method can cause:
• Cracked or dislodged tiles
• Removal of the protective surface layer on the tiles (particularly on older tiles that are already weathered)
• Water being driven underneath the tiles, leading to felt damage or water ingress
• Damage to ridge pointing and flashings
• Potentially invalidating any product warranties
Sadly, we’re seeing more and more reports of unsafe working practices, with workers walking around on roofs, potentially causing damage to the tiles and risking their lives without any safe fall protection.
Understanding Roof Cleaning Methods
There are generally 3 methods that can be used to clean a roof.
High-Pressure Cleaning: Powerful water jets are used to blast moss and dirt from the roof surface. In our opinion, this option is never suitable. There’s a high risk of physical damage, particularly stripping protective coating from tiles. The strong jets can also force water beneath tiles and spray debris all over adjoining surfaces.
Low-Pressure (Soft Washing): This usually involves applying a cleaning solution or biocide, letting this work and then using water at low pressure to remove the debris. It’s claimed that this has a much lower risk of damage, and if it’s done correctly on tiles/slates in good condition, then it might be a good option. The biocide can also help prevent regrowth, so the results are longer-lasting. It does, however, take longer than jet washing the tiles orslates, and the tiles are still put under some pressure, which means they can be damaged if they’re already weathered. The main issue we see with this method is that the machines water pressure is still too high, even on low-pressure settings.
Steam Cleaning: Superheated water is used at low pressure to kill moss, algae and spores, avoiding the use of chemicals. This might be suitable for tiles in good condition with a smooth surface, but for older or weathered coverings, this could cause considerable damage and even force water under the tiles.
When is Roof Cleaning Actually Necessary?
Here’s the thing, roofs don’t generally need to be cleaned routinely. The moss that grows on them is often cosmetic and doesn’t usually cause any physical damage to the tiles. But, if you find yourself in any of these situations, then you might need to look at proper roof cleaning:
• Moss is blocking drainage points
• Excessive growth is lifting tiles
• A roof inspection is being obstructed
• Preparing for repair work
In many cases, simple moss removal from the gutters combines with regular inspections is enough to keep things clean and safe.
How Roof Cleaning Should be Done
If a professional has told you that roof cleaning is definitely needed, here are the next steps we recommend:
Professional inspection: Assess the condition of the roof, the type of tile, how old it is and any existing damage. They should also check with the manufacturer for suitable cleaning methods.
Manual moss removal: Moss should ideally be carefully scraped or brushed away by hand to avoid damage to the tiles.
Low-pressure treatment application: Apply an approved biocide or fungicide using low-pressure equipment.
Natural weathering: Any remaining organic matter is left to break down and wash away naturally over time.
Gutter clearance and final check: Clear any debris and ensure water is flowing correctly.
This approach prioritises the longevity of your roof over a short-term, temporary improvement in appearance.
Before you agree to any roof cleaning service for your home, make sure you do the following:
• Avoid contractors offering ‘pressure washing’ low-pressure method, as these can still cause damage to the tiles
• Ask what method is being used and why, and how they will make sure the work is done safely
• Get quotes from a few different reputable contractors. Don’t engage with cold callers
• Ask for copies of contractor’s insurance documents
• Be wary of discounts for cash payment
Roof cleaning should always be approached with caution. Making sure you’re only using low-pressure methods after professional assessments are essential to protecting your most valuable asset. When in doubt, consider professional advice, use roofing bodies like the National Federation of Roofing Contractors, and choose roof preservation over cosmetic shortcuts. For more information, or for our professional opinion, just get in touch with the team at Harrison Clarke today.
For more expert advice on surveying and property matters, check out our range of informative videos on our website or YouTube channel. Harrison Clarke Chartered Surveyors is here to guide you every step of the way!
At the time of writing, we have a total of 160 reviews across Trustpilot and Google. We are proud to say that the average rating is over 4.9/5.
