Cleaning a patio without stripping the pointing

A pressure washer will lift years of green film in an afternoon. Used carelessly it will also empty every joint and leave you re-pointing.

Start gentler than you think

The instinct is to fit the most aggressive nozzle and get on with it. Stone, and especially the mortar between it, is softer than it looks - and sandstone, limestone and old lime pointing are all easily damaged.

Begin with a 25-degree fan at moderate pressure, held around 30 cm from the surface, and close in only if it is not shifting. Test in a corner that furniture normally covers.

Technique that avoids stripes

Stripes come from an inconsistent gap between nozzle and stone, and from stopping mid-pass.

• Keep the lance at a constant height and angle - practise the arc before you pull the trigger.
• Work in overlapping passes, each covering roughly a third of the last.
• Keep moving. A lance held still for two seconds leaves a visible mark that will not blend out.
• Work along the joints rather than across them wherever you can.

Protecting the joints

Sand-filled and weak mortar joints are the real casualty. Once emptied, the slabs lose lateral support and start to rock.

• Angle the lance away from joints, never straight down into them.
• Keep a rotary or turbo nozzle away from jointing entirely - it will excavate.
• Expect to re-sand block paving afterwards. Kiln-dried sand, brushed in dry, on a dry day.

Timing

Early autumn is the sweet spot. Algae growth has peaked, and there is still enough warmth and dry weather for the surface to dry properly and for fresh jointing to cure before frost.

Avoid cleaning immediately before a hard frost. Saturated stone that freezes can spall - the surface flakes away - and that damage is permanent.

When not to use a pressure washer at all

Some surfaces should simply be washed rather than blasted:

• Old or soft brick paving
• Lime-mortared stone
• Anything already loose, cracked or hollow-sounding underfoot

For these, a stiff brush, a patio cleaner solution and patience will do the job without taking the surface with it.

Afterwards

Green film returns fastest where a surface stays damp and shaded. Cutting back overhanging growth to let light and air in does more for the next two years than the cleaning itself did.