Does pressure washing damage a roof?

How does a pressure washer damage a roof?

A pressure washer cleans by force, and roof tiles aren’t made to take that force. The damage isn’t always visible from the ground, but it can shorten the life of the roof. The main risks are:

• A worn tile surface. Concrete tiles have a surface finish that helps water run off. High pressure can strip it away, leaving the tile rougher and more porous. A worn tile holds more water, fades in colour and gives moss more to grip, so it can come back faster. Each jet wash takes off a little more: red tiles can fade to pink, and in time bare concrete shows through.
• Cracked and broken tiles. Tiles in good condition usually cope, but tiles that are already old or weakened can crack, under a high-pressure jet or under the weight of someone standing on them to aim it. Clay tiles and slates get more brittle with age, and frost and years of heating and cooling weaken them further.
• Water forced under the tiles. Tiles overlap so that rain runs down and off. A jet aimed up the roof can drive water underneath, into the underlay and the timber battens, which can lead to damp and leaks.
• Ridge and verge mortar washed out. The mortar that holds the ridge tiles and edges in place can be weakened or blasted out.
• Mess and blockages. Moss and grit are sprayed over the walls, windows and garden, and washed into the gutters and drains.

Which roofs are most at risk?

Any tiled roof can be damaged, but some are more vulnerable than others:

• Older or weakened tiles, whose surface has already worn, or which have small cracks from frost and age.
• Clay tiles and natural slate, which are brittle and crack more easily.
• Roofs with weak mortar on the ridge or along the edges.
• Roofs with solar panels, where the tiles around the panel mounts are hard to reach and easily disturbed.

What do roof tile manufacturers say?

Leading UK roof tile manufacturers advise against pressure washing clay and concrete roof tiles. Their warnings cover damage to the tiles, water getting in, changes to the tiles’ appearance and loss of the surface finish.

If your roof is fairly new, it may still be covered by a manufacturer’s guarantee. Cleaning it against their guidance could affect that, so check the care advice for your tiles first.

What is soft washing, and how is it different?

Soft washing cleans a roof with a cleaning solution rather than force. The solution is applied at low pressure, left to work on the moss, algae and lichen, and then rinsed off gently. A biocide finishes the job by killing the spores and roots, so growth is slower to return.

At Washmates, we match the method to your roof, and never use a high-pressure jet wash. Every roof is then treated with a biocide, and we don’t walk on fragile tiles. Every roof clean also includes the gutters, fascias, soffits and any solar panels on the roof. Tell us about your roof and we’ll send you a fixed price, usually without a site visit.

Can a roof damaged by pressure washing be repaired?

Some damage can be repaired, and some can’t. A roofer can replace cracked or slipped tiles and re-point the ridge. A tile surface that has been stripped can’t be put back, though; the tile stays more porous for the rest of its life.

If your roof has been pressure washed in the past, look out for damp patches in the loft, slipped tiles and grit building up in the gutters. If you see any of these, get the roof checked by a roofer before it is cleaned again. Some companies offer a roof coating to protect worn tiles. It costs considerably more than a clean, so treat it as a separate decision.

What should you ask a roof cleaner before they start?

A few questions tell you how your roof will be treated:

• How will the roof be cleaned? Look for hand scraping, steam at low or very low pressure, a low-pressure soft wash, or a dry air wash with compressed air. Avoid a high-pressure jet wash.
• Will a biocide be applied? Without one, moss comes back sooner.
• Will anyone walk on the tiles? Fragile tiles should be worked from roof ladders, scaffold towers or a cherry picker.
• What happens to the gutters? The moss that comes off the roof often ends up in them.
• Are you insured? Public liability insurance covers any damage while the work is done.

The answers also explain why quotes differ so much. You can read more about why roof cleaning costs what it does, and see typical roof cleaning prices by type of house.