Why Pressure Washing Roofs Can Be Dangerous

Mark Cave

You climb up the ladder, you look at a roof, and your- your absolute first instinct, if you're a standard contractor, is to just grab the lance and start blasting. But, uh, if you do that without a proper diagnostic process, you're basically playing Russian roulette with your customer's home. See, every single roof is a complex system, and before you even think about what chemical or tool to use, you have to run through a mental matrix of material, age, and pitch. You've got concrete tiles--like your classic Marley Moderns--which might look tough, but if they're forty years old, that top protective concrete slurry coat is completely gone. They are incredibly porous, and if you hit them with three thousand PSI, you're literally eroding the substrate and making them re-soil twice as fast. And- and then you've got clay. Clay tiles get extremely brittle with age. You step on them, or you hit them with a high-pressure jet, and they just shatter like glass. Natural slate? Forget about it. You use high pressure on slate and you'll delaminate the layers, crack the slates, or blow them right off their fixings. But the real, uh, the real nightmare--the absolute trap that catches out so many new guys--is cement-fibre tiles.

Mark Cave

Now, listen to me carefully on this. A lot of cement-fibre tiles manufactured in the UK before 1999--especially around the seventies and eighties--they contain chrysotile. That is white asbestos. Let that sink in. If you roll up to a job, don't identify the tile, and blast a cement-fibre roof with a pressure washer, you are releasing millions of hazardous asbestos fibres into the air, into the gutters, onto the customer's driveway, and into the neighbor's garden. You- you will face massive prosecution from the Health and Safety Executive, and quite frankly, you'll deserve it. Identifying the material is not optional. It is your legal and moral duty as a professional. You have to look for the stampings on the underside of the tile in the loft, or get a sample tested if you're in any doubt. Beyond the material itself, we have to look at the physics of how a roof actually works. A roof is designed to do one job: shed water downwards. The overlapping tiles, the laps, the underlay, the battens--they are all engineered for gravity to pull rain down the pitch. When you stand on a roof, or on a ladder, and you point a pressure washer lance upwards or sideways, you are driving high-pressure water under those laps. You are forcing gallons of water directly beneath the weatherproofing system, straight into the roof space, soaking the insulation, rotting the timber battens, and ruining the ceilings below. It's- it's just basic physics, and pressure washing completely ignores those physical dynamics.

Mark Cave

So, what is the alternative? How do we actually do this professionally, safely, and get a result that lasts twice as long as a high-pressure blast? Well, it comes down to a strict, eight-step alternative workflow, and it starts long before any chemical touches the tile. Step one is the comprehensive survey. I- I don't just mean looking up from the driveway. You need to get a drone up there, or get in the loft space with a torch. You are looking for existing defects, cracked tiles, failed pointing on the ridge caps, or evidence of current leaks. You take photos of everything. If there's a cracked tile and you didn't document it beforehand, guess who gets the blame when the clean is finished? Yeah, you do. Step two is preparation. You've got to block the downpipes. If you don't block the downpipes, all that scraped moss and debris is going to choke the soakaways and drains, and you'll be paying a drainage company hundreds of pounds to clear it out. Step three is safe access. We work under the Work at Height Regulations 2005. No balancing on fragile tiles. You use scaffold, tower systems, or a MEWP--a mobile elevated work platform--wherever possible. Safety is non-negotiable.

Mark Cave

Once access is secure, we move to step four: manual scraping. Instead of blasting, we use profile-matched roof scrapers on extendable carbon fibre poles. These are metal blades cut to the exact shape of the specific tile profile--whether it's a Redland Renown, a double Roman, or whatever. You gently scratch the bulk of the heavy moss off the surface. It's clean, dry, controlled, and doesn't damage the tile. Step five is clearing the gutters and valleys of all that dry debris you've just scraped down. Then we hit step six: applying the targeted biocidal softwash treatment under low pressure. We're talking low pressure here, like a garden hose flow, using a biocide like Soft Wash Pro fifty, which is a DDAC-based product, or a sodium hypochlorite mix with Clever Wash surfactant to help it cling. The key here is step seven: low-pressure delivery. We are not forcing water under the laps. We are gently wetting the tile, allowing the chemical to penetrate the organic root structure of the moss, lichen, and biofilm. And the final step, step eight, is letting weathering and dwell time do the rest. The biocide kills the microscopic spores, and over the coming weeks and months, the UK rain gently washes away the dead organic matter. It's a kinder, safer, and much more responsible way to clean. Alright, that's the professional way to handle a roof. No shortcuts, no cowboy blasting. I'm going to wrap it up there. Take care of yourselves on the tools, work safe, and I'll catch you on the next one. Bye-bye for now.