It's a tempting shortcut, and plenty of people reach for a jet washer thinking it's the fastest way to a clean roof. In most cases, though, it does more harm than good.
Why high pressure isn't the right tool for tiles
A pressure washer blasts water at a force well beyond what roof tiles are designed to handle, and it strips away the granule coating on concrete tiles that helps protect them from weathering. On slate and older clay tiles the risk is even greater, since the force can crack or dislodge tiles outright, occasionally forcing water underneath the roof covering rather than just cleaning the surface.
Even where it doesn't cause immediate visible damage, repeated pressure washing shortens the working life of the tiles and can void certain manufacturer guarantees, which is an expensive trade-off for a job that was meant to save money.
What to use instead
A soft wash approach, low-pressure water combined with a moss and algae treatment, does the same cleaning job without the structural risk. It's slower, but it treats moss at the root rather than just blasting the visible growth off the surface, which also means it tends to come back more slowly afterwards.
Brunel Roofing Bristol uses this gentler method as standard, tailoring it to the specific tile or slate type on your roof rather than a one-size-fits-all blast.
Where pressure washing does work fine
It's worth saying that pressure washing isn't a bad idea everywhere, it's just the wrong tool for roof tiles specifically. Driveways, patios and paved areas are designed to take that kind of force and clean up well with it, which is probably why the idea of using the same approach on a roof seems logical at first glance.
The difference is that roof tiles are thinner, often older, and held in place in a way that relies on overlapping layers staying intact, so the same pressure that lifts grime off a slab can crack a tile or drive water underneath the roof covering instead of off it.
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