Indian sandstone looks superb when it is first laid, but many Stockport patios turn green, slippery and patchy within a couple of years. The good news is that it is almost always down to a handful of fixable causes, and a sensible routine keeps it looking clean without wrecking the stone.
Indian sandstone such as Kandla Grey, Raj Green and Autumn Brown is porous. It soaks up rainwater and holds it near the surface, which is exactly what algae, moss and lichen need to grow. Stockport sits on the wet side of Greater Manchester, close to the Pennines, so patios here rarely get a long enough dry spell to stop that growth.
Shade makes it much worse. North-facing gardens, patios squeezed between semis, and plots under mature trees in areas like Heaton Moor, Bramhall and Marple can stay damp for days after rain. Falling leaves then sit on the surface, rot down and feed the problem.
Sandstone is softer than it looks. A pressure washer on a narrow jet held close to the surface will strip the top layer, leaving the paving rough, faded and even quicker to go green next time. If you use one, keep to a wide fan nozzle, hold it at a low angle about 30cm away, and consider a rotary surface cleaner attachment for more even results.
A better approach for most people is a biocide. Choose a patio cleaner based on benzalkonium chloride, apply it to dry stone on a day with no rain forecast, and leave it to work. Some products need a scrub and rinse after 24 hours; others work slowly over several weeks as the weather washes the dead growth away. Always read the label, as dilution and dwell times vary.
Avoid bleach and acid-based cleaners such as brick acid. Bleach can leave patchy lighter areas and harm nearby plants and lawns, while acid can react with minerals in some sandstones and cause orange or rust-coloured staining that is very hard to remove.
Sealing does not make paving algae-proof, but a good impregnating sealer reduces how much water the stone absorbs, so it dries faster and stays cleaner for longer. It also makes spills from barbecues and plant pots easier to deal with.
The stone must be thoroughly cleaned and completely dry before sealing, which in Stockport usually means waiting for a settled spell in late spring or summer with at least 24 to 48 hours of dry weather. Expect to reapply every three to five years depending on shade and footfall. Avoid glossy film-forming sealers on sandstone, as they can go cloudy and make the surface slippery when wet.
Once the patio is clean, a little regular effort goes a long way. Most of the growth we see on neglected patios could have been kept in check with a stiff brush and two biocide treatments a year.
If puddles form in the same spots every time it rains, slabs rock underfoot, or the paving goes green within months of cleaning, the problem is probably in how it was laid rather than how it is maintained. Relaying sections with the correct fall and a full mortar bed is sometimes the only lasting fix. Professional cleaning and sealing costs depend on the size of the area, access and how much growth there is, so it is worth getting a site visit and a written quote rather than relying on a price per square metre found online.
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