A patio takes more abuse than most people realise: shade from the house for half the day, leaf fall in autumn, furniture standing in the same spots all summer and, in a wet county like Devon, months of damp that never quite dries out. The result is green film, black spot lichen and joints that crumble. Cleaning it well is mostly a matter of matching the method to the material and then dealing with the joints properly, because a beautifully clean patio with washed-out pointing is a job half done.
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Identify the material before you start
Indian sandstone, limestone, riven slate, porcelain, concrete slabs and reconstituted stone all look superficially similar once they are green, and they tolerate very different treatment. Sandstone is porous and softens at the surface. Limestone reacts to acidic cleaners. Slate delaminates. Porcelain is nearly inert but its joints are not. If you are not sure what you have, a discreet test patch in a corner is always the first move.
This guide sits under our Patios & Driveways work, covering 20-mile radius of Plymouth.
Black spot is lichen, not dirt
Those small, stubborn black marks that survive every wash are lichen bonded into the stone surface, most commonly on Indian sandstone. Pressure will not remove them without removing stone with them. They need a specific treatment left to work, sometimes repeated, and even then improvement can be gradual. Anyone promising instant total removal of black spot from sandstone is either going to damage the slabs or disappoint you.
Green algae and the slip risk
The green film is the easy part chemically and the important part practically, because it is what makes a patio lethal in winter. It responds well to treatment and gentle washing. On a shaded patio, the sensible approach is a proper clean followed by a light annual treatment, rather than waiting until it is slippery enough to be a hazard and then doing a heavy clean each time.
Pressure, distance and technique
Where pressure is appropriate it should be applied evenly, usually with a rotary cleaner, at a consistent height. Freehand lance work at close range leaves a permanent striped pattern in soft stone that is visible for years in low sun. Working in overlapping passes, keeping the fan angle constant and never dwelling in one place are what separate a clean patio from a scarred one.
Joints, pointing and what washing does to them
Cement pointing that is already tired will come out during cleaning, and it is better that it does than that it stays as a loose shell over a void. Sand-filled joints will need refilling. Resin jointing compounds can be reapplied. Budget for the jointing as part of the project, and if a large proportion of the pointing is failing, that is a repointing job with a clean attached rather than the other way round.
Steps, edges and adjacent surfaces
Steps are the part most likely to cause an accident and the part most often skipped, because they are fiddly. So are the narrow strips beside walls and under furniture. A patio clean should include the steps, the risers and the threshold, and should finish with the adjoining wall base rinsed so dirty splash-back does not dry on the render.
Sealing: useful, not magic
Sealing porous stone slows staining and makes future cleaning easier, and on sandstone it can reduce how quickly black spot re-establishes. It changes the appearance, usually darkening the stone, and it needs redoing periodically. It must be applied to a properly dry surface, which in Devon means planning around the weather rather than sealing the same day. It is a genuine benefit, not a permanent fix.
Keeping it clean between visits
Sweeping leaves off in autumn rather than letting them rot in place makes more difference than anything else. Moving furniture occasionally stops the pale rectangles. Keeping the gutter above the patio clear prevents the dark drip line along the house wall. None of this replaces a proper clean, but it means the proper clean can be a light one.
Questions we get on this
- Can black spot be removed completely?
- It can usually be reduced substantially with the right treatment, sometimes repeated, but on older sandstone complete removal is not always achievable without damaging the stone.
- Will cleaning wreck my pointing?
- Sound pointing survives correct cleaning. Pointing that is already failing will come away, which is better found now than left as a loose shell.
- Is sealing a patio worth it?
- It slows staining and makes later cleans easier, and it changes the look of the stone. It needs reapplying periodically, so treat it as maintenance not a one-off.
- How often should a shaded patio be cleaned?
- Usually annually for safety, since the green film that makes it slippery returns fastest where the sun does not reach.
Related reading
- Older and period properties: a conservative approach
- Garden walls, steps and paths
- Garages, sheds and outbuildings
- How exterior cleaning is actually priced