Why we never pressure wash a roof
Pressure washing a roof looks effective for a moment, but it strips the protective granules off tiles, drives water under the laps, and shortens the roof's life. We scrape moss off by hand and treat what's left with biocide instead, it's the correct method, even though it takes longer.
It looks like it works, that's the problem
A pressure washer will strip moss and grime off a roof fast, and straight afterwards the tiles can look genuinely clean. That's exactly why it's tempting, and exactly why it's the wrong method. The force needed to blast growth off a tile also strips away the surface of the tile itself, and the damage doesn't show up as a problem until later, once the roof's protective layer is already gone.
What pressure actually does to a tile
Concrete roof tiles have a protective surface layer and, on many, a granule coating that sheds water and slows how much moisture the tile absorbs. A pressure washer's jet is strong enough to blast that layer off along with the moss, leaving the tile more porous underneath. A more porous tile holds more water, and a tile holding more water is far more vulnerable to frost damage over a Teesside winter, cracking and flaking faster than a tile that's kept its protective surface intact.
Water forced where it shouldn't go
It's not just the tile surface. High-pressure water aimed upward or across a roof pitch can force its way under tile laps and overlaps that are only designed to shed water running downward, not to resist a jet. That can push moisture into the roof space itself, and problems that start this way often aren't obvious until damp shows up on a ceiling months later.
The correct method: scrape, then treat
Proper roof cleaning is a two-stage job. First, moss is scraped off by hand, tile by tile, which removes the bulk of the growth without putting any pressure on the tile surface itself. Then a biocide treatment is applied at low pressure, closer to garden hose pressure than a pressure washer, which kills the remaining growth and spores at the root rather than just blasting the visible mess away. The treatment keeps working for months afterwards, with lichen and staining continuing to fade as the biocide does its job.
Why this takes longer, and is worth it
Scraping by hand is slower than running a pressure lance across a roof, and that's reflected in the time a proper job takes. But the outcome is completely different: a roof cleaned this way keeps its protective surface, sheds water the way it was designed to, and doesn't come with the hidden long-term damage that pressure washing leaves behind. If a contractor offers to pressure wash your roof, it's worth asking what they'll do about the granules they're about to strip off.