How do I clean a patio?

How to remove algae, moss and dirt from paving without damaging it, and whether a pressure washer is the right tool.

Difficulty: beginner
Time: 3 hr
Read: 3 min
Safety: caution

Short answer

Sweep and clear the weeds first, then scrub with a stiff brush and a patio cleaner, leaving it to dwell before rinsing. A pressure washer is faster but will strip jointing sand, pit soft stone and force water under slabs — use it carefully and at a distance, or not at all on sandstone and limestone.

Patio cleaning is mostly about matching the method to the surface. Concrete and porcelain tolerate pressure washing; sandstone, limestone and old brick don't. The green film is algae, which needs a biocide rather than force, and returning within months usually means the surface stays damp rather than that the clean failed.

Safety

Pressure washers can cause serious injury — never point one at anyone, including your own feet, and wear eye protection and sturdy shoes. Wet patios are extremely slippery during and after cleaning. Keep patio chemicals away from ponds, and don't let runoff enter a watercourse.

What you'll need

Stiff brush
Patio cleaner or dilute bleach solution
Hose
Pressure washer (optional)
Jointing sand (optional)
Eye protection

Step by step

Identify the paving material before choosing a method.Concrete and porcelain are robust. Sandstone, limestone and old brick are soft and easily pitted by pressure washing.
Sweep thoroughly and pull weeds out of the joints.Removing loose material first means the cleaner reaches the surface rather than the debris.
Wet the surface, then apply a patio cleaner and leave it to dwell.Usually 15–30 minutes. Dwell time is what kills algae — scrubbing alone removes the visible film and leaves the spores.
Scrub with a stiff brush, working in sections.For most patios this is sufficient and is far gentler than a pressure washer.
Rinse thoroughly with a hose.Residual cleaner leaves a film that attracts dirt and can be slippery.
If using a pressure washer, use a fan nozzle, keep it moving and stay 20–30cm back.A pinpoint jet held close will carve lines into concrete and destroy soft stone within seconds.
Re-sand the joints afterwards if pressure washing stripped them.Empty joints let slabs move and weeds establish. Kiln-dried sand brushed in is the standard fix.
Deal with why it's green: shade, poor drainage or overhanging plants.Algae grows where surfaces stay damp. Cutting back overhanging growth often does more than any cleaner.

Tips

Autumn is a good time to clean a patio — you remove the summer's algae before the wet months when it's most slippery.
A patio that's green again within months has a moisture problem. Improving drainage or light does more than cleaning more often.
Never use a pressure washer on decking at close range — it raises the grain and leaves a permanently rough, splintery surface.
Sealing a patio after cleaning slows re-soiling and makes future cleans easier, but it needs a properly dry surface and the right sealer for the stone.

Common mistakes

Pressure washing soft stone at close range — It pits and erodes sandstone and limestone permanently. The damage is immediately visible and can't be undone.
Not re-sanding joints after pressure washing — Stripped joints let slabs shift and weeds colonise.
Skipping the dwell time — Algae needs a biocide with contact time. Scrubbing removes the visible layer and it returns quickly.
Letting cleaner run into a pond — Patio cleaners are toxic to fish and aquatic life.

Questions people ask

Is a pressure washer bad for patios?

It depends on the material. Concrete and porcelain generally cope; sandstone, limestone and old brick are damaged by close-range high pressure. Use a fan nozzle, keep it moving, and stay well back.

Why does my patio go green again so quickly?

Algae needs moisture, so a patio in shade or with poor drainage will keep greening. Cutting back overhanging plants and improving drainage does more than cleaning more often.