Why Pressure Washing Can Destroy Roof Tiles

Mark Cave

Right, let's- let's just cut straight to the chase today. Every single week, I see some contractor on Facebook boasting about how fast they blasted a roof with a high-pressure lance. And honestly, it- it makes my blood boil because half of them have absolutely no idea what material they're actually standing on, let alone the damage they're leaving behind. We need to talk about roof tiles--specifically, clay, natural slate, concrete, and the absolute minefield that is old cement fibre. If you're treating every roof like a concrete driveway, you are a ticking liability clock, plain and simple.

Mark Cave

Let's- let's start with concrete tiles. Now, concrete is tough, yes, but when it's weathered, that factory-sealed top layer is- is already compromised. If you go up there with a high-pressure turbo nozzle, yeah, sure, the moss flies off in seconds and you look like a hero on camera. But what you've actually done is stripped away the remaining protective sand-face coating. You've made that tile incredibly porous, like a giant, thirsty sponge. And guess what? Organic spores--algae, lichen, moss--they thrive on damp, porous surfaces. By blasting it, you've literally created the perfect, rough-textured greenhouse for future growth. You've guaranteed that the moss will come back faster, thicker, and harder to clean next time. That's not professional cleaning; that's just creating repeat business through property damage.

Mark Cave

Now, clay tiles--clay is a different beast entirely. Especially older clay. They are incredibly brittle. The sheer mechanical force of a pressure washer--even if you think you've got the pressure dialed down--can cause micro-fractures. You might not see them on the day, but come winter, the water gets into those tiny cracks, freezes, expands, and bang, the face of the tile pops right off. That's spalling. And natural slate? Forget it. You should never, ever pressure wash natural slate. Slates are held by nails, they're layered, and they delaminate. High-pressure water will slide right up under those slates, split them apart, or worse, rip them completely off their fixings. If you're cleaning slate, it's a manual scrape and a low-pressure biocide treatment, end of story.

Mark Cave

But the real- the absolute massive danger that nobody wants to talk about is old cement-fibre tiles. If you're working on a property built before 1999 in the UK, and you see those thin, grey, artificial slate-look tiles... you need to pause. You need to stop immediately. A huge number of those older cement-fibre tiles contain white asbestos--chrysotile. If you take a pressure washer to an asbestos-cement roof, you are actively pulverising those fibres, aerosolising them into the air, and- and letting them drift onto the customer's garden, the neighbor's washing, and directly into your own lungs. It is illegal, it is highly dangerous, and the Health and Safety Executive will absolutely hammer you for it. If you cannot identify the material with 100% certainty, do not touch it with pressure. Get a sample tested, or stick to safe, non-abrasive softwashing methods.

Mark Cave

So, how do we actually approach a roof safely before we even think about chemicals or scrapers? It all starts with diagnostics. You don't just rock up, put your ladders up, and start washing. Your very first step--before you do anything else--is to ask the customer to look in their loft space. I know, I know, it sounds like extra work, but trust me, this one step will save you thousands of pounds in insurance claims. You need to go up there with a torch. Look for existing damp, look for daylight through the felt, look for rotten battens or sagging underlay. If the roof is already leaking, or if the sarking felt is completely rotted away, any water you put on that roof is going directly onto the plasterboard ceilings below. If you don't document those pre-existing defects first, guess who gets blamed for the wet ceiling in the master bedroom? Yep, you.

Mark Cave

And we have to understand the physics of how a roof actually works. A pitched roof is a brilliant piece of engineering, but it is designed to do one specific job: shed water downwards. Gravity does the work. The tiles overlap, the laps are designed to let rainwater run down the slope. But a pressure washer doesn't work with gravity. When you're standing on a roof, or on a scaffold, and you point that lance across the roof, or worse, upwards, you are driving high-pressure water sideways and upwards beneath those tile laps. You are forcing gallons of water directly under the weatherproofing system, bypassing the tiles entirely and soaking the timber battens and underlayment. It is a recipe for a structural disaster.

Mark Cave

Then you've got the physical mess to manage. If you're scraping or washing, where is all that heavy moss going? If you don't block the downpipes first, it's going straight into the gutters, down the pipes, and into the surface water drains. You'll block the gullies, flood the customer's driveway, and find yourself spent- spending three hours on your hands and knees clearing out black sludge. Professional contractors use gutter mesh, they block the downpipes with tennis balls or specialists plugs, and they collect every single piece of scraped moss in buckets as they go. You keep the site clean, you protect the drainage, and you show the customer that you actually care about their property.

Mark Cave

Once the heavy moss is manually scraped off with a profile-matched scraper--which is the absolute best practice, low-risk way to start--then, and only then, do you transition to your softwashing. We use a low-pressure delivery system, like the clever injector Dosatron, to apply a dedicated biocide like Soft Wash Pro 50, which is a DDAC-based product. You're applying it at a gentle, garden-hose pressure. No mechanical force, no lifting tiles, no stripping concrete coatings. The biocide sits on the surface, penetrates the porous structure of the tile, kills the micro-spores at the root, and let's- let's Mother Nature do the rest of the work over the coming weeks and months. It's safer for you, it's safer for the roof, and the results last significantly longer than just blasting the surface clean on the day. Let's start doing things right, professionals. Speak soon.