Roof Cleaning Myths

The Technical Truth: A Tradesman’s Guide to Roof Restoration

When it comes to roof cleaning in the UK, there is a lot of "expert" advice flying around that doesn't always match the reality of a 20+ year old roof.

At XTX Limited, we believe in looking at the technical facts and evidence.

Because I spent my early career as an electrician, I’m hard-wired to look at the "mechanics" of how things work. Here is the technical breakdown of what is actually happening on your roof and why different tiles need different treatments.

1. "Pressure washing a roof damages the protective coating"

You’ll often hear that pressure washing concrete tiles "destroys the protective coating." While this may be true for a newer roof that is being washed at too high a pressure, the reality for most Worcester homes is different.

The Fact: Most concrete tile coatings are designed to last roughly 15 to 20 years. If your home was built in the early 2000s or before, that factory seal has likely weathered away naturally years ago. At this stage, the tile is already porous, which is why the moss has taken hold and you are considering a roof clean.

The XTX Approach: On older roofs, the goal isn't "protecting a coating" that no longer exists; it’s about controlled cleaning to prevent further damage from moss and frost. We use specialist professional-grade equipment to remove moss without etching the tiles, which can happen if you use an aggressive technique or too high a pressure.

2. The "White Slate" Mystery (Fibre Cement Slates)

This is a specific issue we see a lot in and around Worcestershire. Many modern homes use "man-made" or artificial slates because natural slate tiles are expensive. Sometimes called Eternit. These are typically fibre cement - a mix of cement and fibres with a dark grey or black surface dye to make them look like slate. Tiles made before the year 2000 may well contain asbestos so testing the tiles is essential before any work can be done on these.

The Issue: Unlike natural stone, these slates are dyed on the surface. Years of UV exposure and rain cause the pigment to oxidise and fail. When these roofs are cleaned, even gently, the "biofilm" is removed, often revealing the white cement core underneath.

This can leave a roof looking patchy or completely white. This isn't necessarily "damage" from the clean; it is the cleaning process revealing that the tile's pigment has reached the end of its life. We always assess man-made slates carefully before we begin so you know exactly what to expect.

It is possible to apply a coating to your roof after it has been cleaned to recolour the tiles.

3. "Any roof can be pressure washed"

Not all roofs are suitable for pressure washing and some customers prefer the idea of scraping over pressure washing, which is why we also offer manual roof scraping as an option. Before we start any roof clean we survey your roof to assess whether it is suitable for pressure washing. If we feel tiles are too far gone or the ridges and valleys are in poor condition then we will only offer manual scraping and a biocide treatment. We also pay close attention to the age of your leadwork and flashing; pressure washing near degraded lead can lead to leaks, which is why our site survey is so critical. Some old roofs do not have any felt under the tiles, these should definitely not be pressure washed.

4. "You don't need a biocide treatment"

The Root Cause: Pressure washing or manual scraping alone is like "mowing the weeds." It clears the moss off the surface, but the microscopic spores stay deep in the pores of the concrete. If you don't apply a good biocide treatment the moss and algae will start returning straight away and your roof will be back to square one before long. The HSE approved biocide treatments that we apply after pressure washing or scraping a roof penetrate deep into the tile to sterilise it and kill off any remaining biofilm, ensuring the moss doesn't just come back twice as fast. A treatment of HSE approved biocide should keep your roof clear of moss or algae for around 3 years, less if your roof has overhanging trees.

Pro Tip: We recommend getting a maintenance treatment of biocide roughly every 3 years to keep your roof looking its best for many years to come. This is a fraction of the price of having it cleaned again.

Obviously you don't have to have a biocide treatment after a roof clean but it will mean your roof will need regular scraping or pressure washing if you want to keep it clean.

The Verdict: Experience Matters

Whether your roof tiles are concrete, clay, or man-made slate, the key is understanding the substrate. We don't use a "one-size-fits-all" approach. We assess the age and condition of your tiles and your overall roof to ensure we use the right pressure and the right chemicals to get the best result without compromising your home's "envelope."

Tile Type	Typical Age	Roof Structure / Condition	Recommended Method	Why?
Concrete Tile	20+ Years	Felted Underlay. Good ridges & valleys.	Controlled Pressure Wash	The original coating is gone. The tile is robust enough for professional pressure cleaning to remove deep-seated moss.
Concrete Tile	Any Age	NO FELT (common on pre-1960s homes) OR loose mortar.	Manual Scrape ONLY	⚠️ High Risk. Without felt, pressure washing will force water into the loft space.
Man-Made Slate (Fibre Cement)	15+ Years	Felted. Surface appears weathered/grey.	Manual Brush	Pressure washing will almost certainly reveal the white cement core. Scraping is gentler on the remaining pigment.
Clay or Natural Slate	Historic	Often unfelted, brittle, or delaminating.	Manual Scrape ONLY	These tiles are too fragile for high pressure. Gentle manual removal is required to preserve the asset.

Do you have an older roof that needs professional attention? Let’s take a look. Contact Darrin at XTX Ltd for a technical assessment and a free quote.