Pressure washing damage
| Pressure washing damage | |
| Question | How high-pressure cleaning damages a roof |
|---|---|
| Principal mechanisms | Surface removal; water driven under the covering; access loading |
| Visibility | Often delayed by seasons |
| Sourcing | Material behaviour; no statute or standard defines it |
Damage from pressure washing a roof is the most common complaint in roof cleaning, and its characteristic feature is that it is not visible when the work is finished. A pressure-washed roof looks better on the day than it has for years; the consequences appear over the following seasons, which makes the connection easy to miss and easy to deny. No statute or published standard addresses this; what follows is how the materials behave.
Surface removal[edit]
The first and largest mechanism is that the surface goes with the growth. A concrete tile's applied finish is what keeps water out of its body,[1] and a clay tile's fired surface is formed in manufacture and cannot be put back.[2] Pressure sufficient to remove adhering lichen is close to pressure sufficient to remove either.
The result is a covering that absorbs more water than it did, holds it longer, and offers a better surface for growth to re-establish on. The roof is cleaner and more prone to the problem than before, and both facts are true at once.
Water where it should not be[edit]
A roof covering keeps water out by shedding it down the slope over laps. A jet directed up the slope, or held close at an angle, drives water the other way — under the tile, past the lap and onto whatever is beneath.
Where there is felt or membrane in reasonable condition this may go no further. Where the underlay is old and brittle, which on many roofs it is, the water reaches the structure. A roof that leaks for the first time in the weeks after being cleaned is a common enough sequence to be worth stating plainly.
Access damage attributed to the water[edit]
Pressure washing generally puts the operative on the roof for hours. Foot traffic cracks tiles under point loading and shifts slates whose nails are already tired.
Damage of that kind is frequently attributed to the jet, and the distinction matters because it changes what a different method would have avoided: a method that keeps the operative off the roof addresses it, and one that merely lowers the pressure does not. The Work at Height Regulations bear on the same choice for a different reason.[3]
Why it is hard to attribute[edit]
The gap between the work and the consequence is the whole difficulty. Surface removal shows as faster regrowth and a duller covering over one or two seasons; water past a lap shows when the underlay finally fails; a cracked tile shows when it rains hard from the right direction.
By then the cleaning is months or years back, the contractor has gone, and the owner has little means of establishing what caused what. That asymmetry, rather than any dispute about the physics, is why the practice persists.
See also
References
- ^BS EN 490: Concrete roofing tiles and fittings. Product specifications. BSI. Retrieved 2026-09-05
- ^BS EN 1304:2013: Clay roofing tiles and fittings. Product definitions and specifications. BSI. Retrieved 2026-09-05
- ^The Work at Height Regulations 2005. legislation.gov.uk. Retrieved 2026-09-05