How a roof is built to shed water
Tiles and slates overlap, each row sitting over the one below, so that rain runs down the surface and off the bottom edge into the gutter. The system relies on gravity. Water is meant to travel downwards over the surface, and it is not meant to meet any force that pushes it the other way.
Beneath the covering lie battens and underlay, the sheet that catches the odd drop that gets past. The underlay is a second line of defence behind the tiles or slates, not the main barrier against the weather.
A pressure washer is not the right tool for that surface. Marley, the tile and slate maker, puts it this way: "The use of high powered jet washes is not recommended as it can damage the surface of the slate or tile, thus reducing its expected life considerably."
Nothing about this is dramatic at the time. The roof looks washed and the owner is pleased, and the cost only shows in the years that follow, as the covering ages faster than it should.
What force does to the surface
The face of a tile is not a smooth patio slab. It is a finished surface, and a hard jet at close range can wear it down, leave it rougher and make it hold dirt and moss more readily afterwards.
On slate, the risks are different. Old slates can be brittle, and a strong jet can crack a slate or loosen one that is held by a single nail. Cleaning a slipped slate off the roof later is a worse outcome than leaving a little moss.
Risks of a hard jet include:
- stripping the surface of concrete or clay tiles;
- cracking or loosening slates and old tiles;
- lifting mortar at the ridge, hips or verge, where it is already weak;
- blasting moss and grit into the gutter in one lump, which blocks it;
- soaking the lofts and walls beneath where pointing or flashings are worn.
Roofs differ, and some coverings tolerate more than others. That variation is exactly why force is a poor default. Our page on whether roof cleaning can damage tiles goes through the surfaces one at a time.
The gentle method we use
Our own approach is brush, treat and rinse, and we use it because it is gentler on the covering and the roof beneath it. They go in this order:
- Loose moss, leaves and debris are lifted away with a soft-bristled brush, working down the slope.
- A treatment chosen for the covering is applied as its label directs, and left to work on what remains.
- The slope is rinsed with a gentle flow of water, from the ridge towards the gutter, so water travels with gravity.
- The gutters and downpipes are cleared, since the debris ends up there.
- A second look follows once the roof is bare, and any cracked or slipped tiles are noted.
This is slower than a jet, and a treated roof takes time to show its full result as the remaining growth dies back and washes away. The cost of that patience is small compared with the cost of water in the loft.
You will find the choices set out on roof moss removal methods. The thing to remember is that the gentle method is a decision about the roof, not a lack of equipment.
Questions to put to anyone quoting
If a firm offers to pressure-wash your roof, you do not have to refuse, but a few questions will tell you how they work:
- How close will the jet be held to the tiles, and in which direction will it run?
- What happens to the moss and grit that come off, and who clears the gutters?
- Will the roof be inspected once it is clean?
- If water gets in afterwards, who will put it right?
A confident answer to the last question matters. Section 49 of the Consumer Rights Act 2015 asks for reasonable care and skill in the work you pay for, which gives you something to stand on if a roof is damaged by the method used.
Beware of manufacturer claims for products, cleaners or coatings that you cannot check. A coating applied to a wet or poorly cleaned surface will not last, so the cleaning has to come first. Our page on roof cleaning and moss removal describes the full service, and one total price is written after the roof has been seen.
