Why Pressure Washing Roofs Is Usually a Bad Idea

Mark Cave

Can you pressure wash roof tiles? Technically, yeah. You can. But the real question—the one that actually keeps you out of court and stops you from ruining a client's home—is-- is should you? And ninety-nine percent of the time on UK roofs, the answer is a flat-out no. You see, a roof is a weatherproofing system. It's designed to shed rainwater downward. One way. When you climb up there with a pressure washer, you're blasting water sideways, upwards, and directly under the tile laps. You're forcing water straight under the underlay, soaking the battens, and potentially causing hundreds of pounds of hidden rot before you've even packed up your van.

Mark Cave

And-- and then you've got to look at the materials. I- I- I see guys online saying, "Oh, it's fine, concrete is tough." Well, weathered concrete tiles lose their protective surface layer over time. You hit them with high pressure, and you just erode them further, making them more porous. And if they're porous, guess what? The moss comes back twice as fast. Don't even get me started on clay tiles or natural slate. They're brittle. They crack under foot traffic, and the reaction force from a heavy lance can snap them like biscuits. And here is the absolute massive red flag that too many new contractors completely ignore: old cement-fibre tiles. If you're looking at a roof from the seventies or eighties and those tiles are cement-fibre, there is a very high chance they contain asbestos. If you hit an asbestos roof with a pressure washer, you are breaking the law, you're contaminating the neighborhood, and you're looking at a massive prosecution from the Health and Safety Executive. You have to walk away unless you are hundred percent certain of the material.

Mark Cave

Now, are there exceptions? Yes, but they're tight. If you're dealing with relatively modern, completely sound concrete tiles with no defects, you might use a carefully controlled, low-pressure system. I'm talking fan jets, absolutely never turbo nozzles. And you only do it from safe access like a MEWP or a scaffold, never standing on the tiles themselves. But if you see cracked pointing on the ridges, loose leadwork, or if there's any sign of damp in the loft-- you just don't do it. You don't take the risk.

Mark Cave

Which brings us to why we do what we do. Softwashing didn't just appear out of nowhere because someone wanted to sell fancy chemicals. It actually emerged back in the 1980s and nineties, mostly over in the US first, precisely because contractors were destroying roofs with pressure washers. They realized that blasting the surface didn't actually solve the problem. The black stains, the moss, the yellow lichen-- they aren't just dirt. They're alive. It's a biological biofilm. If you just blast it with mechanical force, you leave the microscopic roots and spores deep inside the porous tile. It's like mowing weeds; they just grow back stronger.

Mark Cave

Softwashing swaps raw, aggressive force for controlled chemical action. We use low-pressure delivery—similar to a garden hose—combined with specialized solutions like sodium hypochlorite paired with a good surfactant like Clever Wash, or a high-quality DDAC biocide like Soft Wash Pro 50. The chemical does the hard work. It penetrates deep, kills the organic growth at the root, and sanitizes the surface. And because it's low pressure, there's zero risk of lifting tiles, forcing water into the loft, or stripping the tile's protective coating.

Mark Cave

If you want to do this professionally in the UK, you need to follow a safe, structured workflow. We teach an eight-step standard. First, you do a full roof survey—and yes, that includes poking your head up into the loft to check for existing leaks. Second, you plan your safe access and fall protection, keeping inline with the Work at Height Regulations. Third, you manually scrape the heavy moss first using controlled, roof-safe scrapers on extendable poles. Fourth, you bag up and collect all that debris. Fifth, you clear out the gutters and block the downpipes so no chemical runoff gets into the drainage system. Sixth, you wet down all surrounding grass and plants to protect them from any overspray. Seventh, you apply your softwash solution evenly using low pressure. And finally, eighth, you let the treatment dwell and let Mother Nature do the weathering over the coming weeks. It's a safer, longer-lasting clean, and it's what separates the professional contractors from the cowboys.

Mark Cave

If you're looking to upgrade your setup or you just need the right compliance documents, SDS sheets, or training to get your business sorted properly, head over to softwashing.uk. Alright, that's enough waffling from me today. Get out there, stay safe, and I'll catch you on the next one. Bye-bye for now.