Is it wise to pressure wash a roof?

Pressure washing gets moss off a roof quickly, and takes a slice of the roof's life with it.

Whether you need roof cleaning or roof cleaning in Godalming, we cover the whole area, with the work done by our own roofers and a 25-year written workmanship guarantee on every job.

Call Scott on 07428 300474 for a free survey and an honest fixed price in writing.

Is it wise to pressure wash a roof?

No. Pressure washing a roof is not wise, because the water pressure that strips moss also strips the protective granules off concrete tiles, drives water under laps and loosens bedding and pointing. A low-pressure soft wash with moss taken off by hand gets the same result without shortening the roof's life.

What a pressure washer actually does to a tiled roof

A jet washer works by force. On a patio that is fine, because a paving slab is a solid lump of concrete with nothing underneath it that matters. A roof is the opposite: it is a lot of small overlapping pieces, held down by nails or clips, sitting over a membrane, with cement or mortar holding the ridge and the verge in place. Force is exactly the wrong tool.

Concrete interlocking tiles leave the factory with a fine layer of surface granules and a coating over them. That layer is what keeps the tile shedding water quickly and keeps the colour. A lance held close enough to shift moss will take that layer off with it. You end up with a roof that looks scrubbed for a season, then goes patchy and porous, holds water for longer after rain and grows moss back faster than it did before, because the surface it is growing on is now rough.

Clay plain tiles and natural slate handle water better, but the fixings do not. Slate is held by two nails or a hook, and a jet aimed uphill lifts the tail, works at the nail hole and can crack the slate outright. Old nibs snap. Verge mortar cracks and drops off in lumps. None of that is visible from a driveway, which is why the damage from a jet wash usually turns up as a leak months later, long after the person with the machine has moved on.

Whether you need roof cleaning or roof cleaning services Guildford, we cover the whole area, and the answer we give is the same either way: the moss comes off, the roof stays as strong as it was when we arrived.

Where the water goes when it is forced uphill

A pitched roof is not waterproof in the way a bath is waterproof. It is weatherproof, which means it is designed to shed water that is falling downwards under gravity, using the overlap between one course and the next. Put water under 100 bar of pressure and aim it up the slope and you are asking the roof to do something it was never built to do. Water goes over the head lap, past the side lap and onto the underlay.

If the underlay is a modern breathable membrane in good order, it may cope. If it is 1960s bitumen felt, it is brittle, it has probably already split over the rafters, and the water finds the split. From there it goes onto the ceiling, into the insulation or down a wall, and by the time a stain appears nobody connects it to the cleaning that happened in the spring.

The same applies to anything that penetrates the roof. Lead flashings, soakers around a chimney, valley linings and vent tiles are all detailed to shed downward flow. A jet lifts lead upstand, forces grit behind it and can push out the pointing that holds a flashing in its chase. Repointing a chimney and redressing lead costs considerably more than the cleaning ever saved.

Whether you need roof cleaning or roof cleaning in Woking, we cover the whole area, and every survey starts with what the roof is made of and how old the underlay is, because that decides what is safe to do to it.

How we clean a roof instead, and why moss comes off by hand

Soft washing means low pressure. The water is applied at a pressure closer to a garden hose than a jet, with a treatment that kills off the moss, algae and lichen so it dies back and weathers away rather than being blasted off in one go. It is slower and it takes more of a person's time, which is precisely why some outfits reach for a jet washer instead.

The heavy moss growth, the thick green cushions that build up along the bottom course and in the valleys, comes off by hand with a scraper. Doing it that way means the person removing it is looking at each tile as they go. Cracked tiles, slipped slates, a nail-sick batten, a valley that has silted up, a gutter that has never drained properly: all of that gets found while the moss is coming off, rather than being hidden under it for another five years.

Then the debris has to go somewhere. Loose moss washes straight into the gutters and the downpipes and blocks them, so gutters and outlets get cleared as part of the job. Rainwater goods are sized to carry a calculated flow, and a downpipe half full of dead moss carries nothing. Our roof cleaning services cover the soft wash, the hand scraping and clearing the gutters out afterwards, because leaving the last bit undone creates the next problem.

When cleaning is the wrong answer altogether

Sometimes a roof does not need cleaning. If tiles are delaminating, if slates are shedding at the nail holes across the whole slope, if the underlay has gone and daylight shows through in a dozen places, then washing it is spending money on a roof that needs covering work instead. Scott has spent more than 36 years on Surrey roofs and will say so on the survey rather than sell a clean that changes nothing.

Equally, a moss problem is often a symptom. A slope that never dries out because of overhanging trees, a valley that traps debris, or a gutter line sitting full of water will grow moss back whatever you do to the surface. Cutting back the branches or fixing the fall on the guttering does more than any cleaning schedule.

We also do not push roof coatings on the back of a clean. A coating applied over a tired roof hides the condition of the tiles, can trap moisture and does not fix fixings or underlay. If the roof is sound, cleaning is cosmetic and that is a perfectly reasonable thing to want. If it is not sound, the honest answer is repair or re-roof, given as a fixed price in writing with no obligation.

• A jet wash removes the surface layer from concrete tiles and moss returns faster afterwards.
• Pressure driven uphill defeats the laps and lands water on ageing underlay.
• Lead, flashings and verge mortar are the first things a lance loosens.
• Hand scraping finds cracked and slipped tiles that a jet would simply hide.

Every job we finish carries a 25-year written workmanship guarantee, handed over before the team leaves. That guarantee is a good part of the reason we do not own a roof jet washer.