How to Clean a Patio Properly Without Damaging It

Porcelain is vitrified and essentially non-porous. Almost nothing penetrates it, so cleaning is mostly a matter of removing surface deposits. It tolerates most cleaners but the joints between slabs do not, so keep chemicals off resin jointing where you can.

Sandstone and limestone are porous natural stone. They stain, they hold algae in the surface texture, and they are sensitive to strong acids — acid-based cleaners can etch limestone and bleach the colour out of sandstone. Always test on a spare slab or a hidden corner.

Concrete slabs and blocks are moderately porous and are vulnerable to two specific things: high-pressure jets, which strip the surface layer and leave permanent lighter patches, and repeated salt exposure.

Whatever the surface, check the jointing first. Cleaning a patio with failing sand-and-cement pointing will finish the pointing off, and you will need to repoint afterwards. It is better to plan for that than to discover it halfway through.

Green film is algae, and it grows wherever the surface stays damp. It comes off easily with a stiff brush and warm soapy water on small areas, or with a proprietary patio cleaner on larger ones.

The important part is preventing its return. Algae needs moisture and shade, so improving the situation is more effective than cleaning more often: cut back overhanging planting, clear leaves promptly, and fix any spot where water ponds rather than running off.

A once-a-year treatment with a biocidal patio cleaner in spring keeps most patios clear. Applied properly on a dry day and left the full dwell time, it kills the growth at the root, and the residue weathers away over the following weeks rather than needing to be scrubbed off.

Black spot is not dirt. It is a lichen that bonds chemically to the surface of the stone, and it is the single most frustrating patio problem because pressure washing will not shift it. People commonly wash a patio for hours, damage the surface, and leave the black spots exactly where they were.

The only reliable treatment is a dedicated black spot remover, applied to a dry surface and left to work. Most need several hours and some require a second application. The spots typically do not vanish instantly — they fade over days or weeks as the killed lichen breaks down and washes away.

Black spot is most common on Indian sandstone and on patios that stay damp. Once treated, the same prevention applies: light, air movement and prompt leaf clearance.

Orange rust marks on sandstone usually come from iron pyrite inclusions in the stone itself weathering out, or from metal furniture left standing. They need an oxalic or specialist rust remover rather than a general cleaner, and they will recur if the source is still there.

Oil and barbecue grease should be treated with a poultice — an absorbent cleaner left to draw the oil back out of the stone — rather than washed, which pushes it deeper. On porcelain, surface degreaser is enough because nothing has penetrated.

White powdery bloom on new paving is efflorescence: soluble salts working out of the slab or the bedding mortar. It is not a fault and it is not permanent. It will weather off over several months, or can be removed with a dedicated efflorescence remover. Do not seal over it.

Red wine, leaf tannin and moss staining on porous stone all respond to an alkaline patio cleaner and patience. Bleach is a poor choice — it lightens the stone unevenly and kills adjacent planting.

If you use one, fit a fan nozzle or a rotary patio attachment rather than a pencil jet. Keep the lance at least 300mm from the surface and keep it moving in overlapping passes so you do not create stripes.

Never use a pressure washer on natural stone with soft bedding planes, on old or riven surfaces that are already flaking, or on any patio with sand-and-cement pointing you want to keep.

After washing block paving or any sand-jointed surface, re-sand once dry. After washing a patio with resin jointing, check for any areas that have lifted and repair them before winter.

Work away from the house and towards a drain or a border that can take the runoff. Cleaner-laden water should not go onto a lawn or into a pond.

Sealing is optional and it is not a cure for a dirty patio. It reduces porosity, so future spills are easier to lift and algae has less to grip, but it needs redoing every few years and it must be applied correctly.

The surface must be clean, fully dry through its depth and freshly jointed. Sealing damp stone traps moisture and produces a cloudy finish that has to be stripped off. In practice that means a settled dry spell — several rain-free days, not one sunny afternoon.

Porcelain does not need sealing. Concrete may benefit. Sandstone and limestone benefit most, particularly in shaded gardens, provided a breathable impregnating sealer is used rather than a surface film.