How to clean a Cotswold stone patio without ruining it

Every spring, a homeowner walks onto their patio, looks at the green stuff growing in the joints and the slimy bit near the gutter, and has the same thought. "Right. I'll hire a pressure washer." Don't. Or if you do, read this first.

Why pressure washing Cotswold stone is risky

Cotswold stone is oolitic limestone. It's softer than granite, softer than sandstone, and it looks the way it does partly because of the gentle weathering over decades. A pressure washer on full blast doesn't just lift the dirt. It lifts the top layer of the stone. That warm, buttery surface you paid for when you bought the house? That's what comes off first.

It also blows all the jointing grout out, which you then have to put back, and it drives water deep into the stone which can cause spalling in the next frost. I've seen patios that were 80 years old turned into a flaky mess in an afternoon.

The order of escalation

Start with plain water and a stiff brush

Genuinely. Warm water, a long-handled stiff-bristled brush, and ten minutes of elbow grease will deal with 70 percent of what's on most patios. Algae, light moss, general muck. Takes longer than the pressure washer but doesn't cost you the stone.

Add a biological cleaner for algae and lichen

For the green slimy stuff, a biological patio cleaner (Wet and Forget, Algon, or the equivalent from the local garden centre) applied with a watering can, left for a few days, then rinsed. Costs about £15 for enough to do a decent patio. Non-caustic. Safe for plants and pets once it's dried down.

Weeds in the joints

Pull the big ones by hand. Smaller ones, a kitchen knife or an old screwdriver down the joint to lift them out. Weed killer if you must, but honestly, a hoe and patience gets you there.

The pressure washer, carefully used

If you absolutely must pressure wash, use the widest fan nozzle you've got (25 or 40 degree), keep the lance 12 inches off the stone, never closer, and work in long even strokes. Don't linger on one spot. Don't use the turbo nozzle. Stop immediately if you see the stone turning lighter in a patch, that's surface loss.

Grout top-up afterwards

Any cleaning will pull some old jointing out. Once the patio's dry, brush in fresh kiln-dried sand or a polymer joint filler. Water it down gently to set. Takes about an hour on a medium patio. Keeps the weeds down for 2 or 3 years.

What I use on client jobs

• A soft-bristled street broom for wet sweeping
• Biological cleaner, applied two weeks before the visit
• Warm water and a decent plastic pail
• A joint knife and fresh polymeric sand
• Only in the last resort, a pressure washer with a very wide nozzle

And a cup of tea from the client, which usually helps the middle stretch. A proper patio clean on a terrace around the size of a double garage is about half a day's work. Costs around £150 if I'm doing the grout at the same time.

One last thing. Sealing.

A lot of people ask about sealing the stone after cleaning. Cotswold limestone generally doesn't need sealing. It's meant to breathe. A sealer traps moisture inside the stone and over time can cause the same spalling you were trying to avoid. The only exception is if you're in a low-lying spot that gets standing water, in which case a breathable stone sealer can help. Do your homework. Don't use a generic paving sealer.