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What pressure washing does to a tile
Concrete tiles have a surface layer that sheds water. A high-pressure jet can erode that layer, exposing the coarser material beneath. The roof looks clean on the day, but the rougher surface then holds moisture and gives new spores an easier grip. Clay tiles can be marked or chipped, and fragile or older tiles may crack.
There is also the water itself. A jet aimed up the slope pushes water under the laps between courses, where it was never meant to go. On a roof with a membrane in poor condition, that can show up as damp in the loft.
Jet washing roof tiles and safety
A pressure washer lance pulls against the user, and wet moss on tiles is slippery. Working on a pitched roof with that equipment is a fall risk. Walking on tiles can crack them, especially older concrete and clay. None of this suggests homeowners should attempt roof cleaning from a ladder.
Debris also needs managing. A jet sends moss and grit across neighbouring roofs, windows and cars. Gutters fill quickly and outlets can block during the job.
Why soft wash and biocide are used instead
OOR External assesses every roof for covering, pitch, condition and access before a method is agreed. On tiled roofs, loose growth is removed with care and biocide is offered afterwards as a separate line. Soft washing applies a cleaning treatment at low pressure, gives it dwell time and rinses gently. Slate is cleaned by soft wash only.
The pressure washing versus soft washing guide compares the two methods across render, cladding and paving as well. On roofs, the conclusion is consistent: let the treatment do the work and keep force off the covering.
Signs a roof has been jet washed before
If you have moved into a property and the roof grows moss unusually fast, it may have been pressure washed in the past. Look from the ground for tiles that appear rougher or more faded than those on neighbouring roofs, patchy colour across a slope and granules collecting in the gutters. These signs do not prove anything on their own, but they are worth mentioning in an enquiry, because a roughened surface affects how the roof should be treated next time.
Older roofs with a history of hard cleaning are often better served by a gentle treatment and patience than another aggressive clean. The assessment decides that, based on the covering and its condition rather than on how dramatic a before and after photograph might look.
Is roof cleaning by jet wash ever used?
Some hard surfaces around a property, such as yards and certain paving, are cleaned with pressure set for the surface. Roofs are treated differently because of the risks above. If a contractor offers roof cleaning by jet wash, ask how they will protect the tile surface, where the water will go and whether a treatment follows.
What affects the cost of a roof clean
People searching for the cost of pressure washing a roof or the cost of jet washing a roof are usually comparing quotes. The figure depends on roof area, pitch, height, covering, the amount of growth, access and whether biocide is included. A cheaper jet wash may cost more later if tiles need replacing or growth returns quickly on a roughened surface.
The roof cleaning cost guide explains those factors in more detail. OOR External gives free written quotes based on photographs and access notes.