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Why patios turn green
Algae and moss need moisture, shade and somewhere to grip. Patios beside a north-facing wall, under trees or next to a lawn stay damp for longer after rain, and that is where green growth starts. Porous slabs such as sandstone and concrete hold water in their surface, which helps algae settle in.
Poor drainage makes it worse. A patio laid with too little fall holds puddles, and wide joints full of soil give moss a root hold. Once established, moss traps more soil and moisture, which encourages further growth around it.
Moss and algae are not the same
Green algae is a thin film or powdery coating that sits on the surface and turns slick when wet. Moss is a small plant that forms cushions, usually in joints, along edges and in the texture of riven slabs. Algae can spread over a whole patio in a season; moss tends to build more slowly in particular spots.
Both are treated, but moss needs to be physically removed as well as killed, otherwise dead cushions stay put. Algae usually lifts once treated and rinsed.
What green spot algae is
Some patios show small round green spots rather than an even film. This is often called green spot algae: colonies that start at a point and spread outwards. They can appear on slabs that otherwise look clean and are often confused with lichen, which forms tougher, crusty patches that cling more firmly.
Black spots are a different problem, usually lichen that has turned dark and bonded tightly to the stone. The black spots on patio guide explains why those are harder to shift.
Is it harmful?
Green algae is not usually harmful to people in itself, and it will not poison pets or children. The real hazard is slipping. A green patio, especially steps and the area by a back door, can be very slippery in damp weather, and falls are the main risk.
Left for years, algae and moss can also slowly affect softer stone and hold moisture against the surface, which in frosty weather can contribute to surface spalling. It is better dealt with than ignored.
Clearing moss and algae from slabs
Professional cleaning usually combines a suitable treatment with pressure washing at a level the stone can take. Hard porcelain and concrete tolerate more pressure; sandstone and older slabs need a gentler approach so the face is not etched. Moss in joints is lifted, and any lost jointing can be refilled.
Home remedies such as bleach can harm plants and lawns nearby and may discolour some stone. Wire brushes can scratch porcelain. If in doubt, test any method on a hidden slab first.
Keeping it from coming back
Some regrowth is inevitable on a shaded patio. A biocide treatment after cleaning slows it, and simple habits help: sweep off leaves, keep joints filled so soil cannot build up, cut back overhanging plants to let in light and air, and clear any blocked drainage channels.
Block paving behaves differently, because weeds and moss root in the sand joints between blocks. The weeds and moss in block paving guide covers that problem specifically.
Need it sorted? Book patio cleaning
Ask Jordan Sartain at OOR External about patio cleaning. Use the quote form below with your postcode and a few photographs of the whole area.