Keeping Paving Clean Through Wet Weather

Why damp paving turns treacherous

British weather rarely gives paving a proper chance to dry out. From October through to March, many patios and paths stay damp for days at a time, and that constant moisture encourages a thin film of algae to form on the surface. The film is often invisible when the stone is dry, which is exactly what makes it so dangerous: a path that looks perfectly clean on a bright morning can feel like ice after a night of drizzle.

Natural stone such as sandstone, limestone and slate is porous, so it holds moisture and gives algae a foothold. Porcelain is far less absorbent, but a biofilm still forms on top of it, particularly in shaded corners and along north-facing walls. Add fallen leaves, which rot down into a slick paste, and moss creeping through the joints, and you have all the ingredients for a genuine fall risk.

The weekly sweep that prevents most problems

Sweeping is the least glamorous job in the garden and the most effective. Ten minutes a week through the damp months removes the organic debris that algae and moss feed on before it breaks down.

• Work when the surface is dry. Damp debris simply smears. If you must sweep wet paving, accept that you are moving mud rather than dust.
• Brush along the joints, then across them. Moss establishes in the gaps first, so disturb the joints rather than skimming the face of the stone.
• Sweep from the far end towards your collection point, so you avoid walking over what you have just cleaned.
• Deal with leaves weekly from September to December. A plastic rake or leaf blower first, then a broom for finer grit.
• Bin the sweepings. Leaf litter washed into a gully or channel drain is the fastest route to a blockage and standing water on the paving.

Washing: pressure, products and technique

Once or twice a year, most paving benefits from a proper wash. The mistake people make is reaching straight for the most powerful setting on a pressure washer. Natural stone can be etched and scarred by a narrow, high-pressure jet, and jointing sand is stripped out in seconds, leaving open gaps that collect water and weeds.

• Keep the pressure modest. Around 100–150 bar is plenty for domestic paving. Use a wide fan nozzle or a rotary surface cleaner rather than a pencil jet.
• Keep your distance. Hold the lance roughly 300 mm from the surface and keep it moving. Never dwell in one spot.
• Test on a hidden corner first, particularly with sandstone, limestone and any sealed or impregnated paving.
• For routine cleaning, skip the machine. Warm water with two or three drops of washing-up liquid, a stiff brush and a rinse will lift most surface grime without risk. Work in small sections and rinse as you go, so detergent never dries on the stone.

Tackling moss and algae properly

If green film or moss cushions have taken hold, cleaning products based on sodium hypochlorite will deal with them quickly and cheaply. They are also harsh, so treat them with respect.

Dilute according to the label, often somewhere between one part product to ten and one part to twenty parts water. Water surrounding plants thoroughly before you start, apply the solution to the paving only, and let it dwell for fifteen to twenty minutes before agitating with a stiff brush and rinsing well. Water the borders again afterwards. Never mix cleaning products together, and never let run-off reach a pond or water feature.

For a gentler approach, potassium-salt or fatty-acid based cleaners work more slowly but are kinder to planting and wildlife. Whichever you choose, repeat the treatment in spring and early autumn, and twice about a week apart on heavy growth. A pressure washer strips the green from the surface but does nothing about spores in the joints, so a chemical or biological clean is worth the extra effort.

Joints, drainage and other damp traps

Clean paving only stays safe if water can leave it. Check that the fall still runs away from the house, typically around 1:60 to 1:80, and that channel drains and gullies are clear of grit and leaves.

• Re-sand the joints once the paving is completely dry, using kiln-dried sand or a polymeric jointing compound. Open joints hold water and seed moss.
• Divert downpipes. A pipe discharging straight onto paving keeps that area permanently wet. Route it to a water butt or soakaway instead.
• Let light and air in. Thin overhanging shrubs and low branches. Shade and still air are what algae need most.
• Consider sealing. An impregnating sealer on natural stone slows water absorption, but apply it only to a thoroughly dry, clean surface and expect to renew it every few years.

A simple seasonal rhythm

Little and often beats a heroic annual blitz. Sweep weekly through autumn and winter, clear leaves after every storm, and keep an eye on shaded corners where the green film appears first. In spring, wash the whole area and treat any moss or algae. In autumn, repeat the treatment and top up jointing sand before the wet weather arrives.

Gritting deserves a mention too. Rock salt is corrosive to some natural stones and will damage planting if it washes into borders, so use it sparingly on paving and sweep up any excess once the ice has gone. A little routine care through the damp months keeps your paving looking its best and, more importantly, keeps it safe underfoot when the British weather does its worst.