Short answer
Get rid of moss and green algae on a patio by scraping or brushing off thick growth, raking moss out of the joints, then treating the surface with a patio cleaner or moss and algae remover suitable for your paving. Scrub and rinse, then deal with the shade, damp and debris that let it grow, or it will come back.
Why patios turn green
Green algae and moss need moisture and somewhere to settle. Patios in shade, under trees, beside fences and walls, or on the north side of a house stay damp for longer and turn green first. Leaves, soil and other debris left on the surface hold water, and a leaking gutter or downpipe can keep one area permanently wet.
Algae usually shows as a thin green film that is slippery when wet. Moss forms soft green cushions, typically in the joints and corners first, and spreads across the slabs if left.
Safety first
Green, slimy paving is a slip hazard, especially when wet. Take care walking on it while you work, wear footwear with a good grip, and keep children and older visitors off it until it has been cleaned and has dried.
How to clear moss and algae
- Scrape off thick moss with a stiff broom, a plastic scraper or a flat spade held at a low angle.
- Rake moss and weeds out of the joints with a weeding knife or joint brush.
- Sweep the patio clean and bag the debris rather than putting it on beds, where it will spread.
- Wet the slabs and apply a moss and algae treatment or patio cleaner suitable for your stone, following the label, after testing it on a hidden area.
- Allow the contact time the manufacturer recommends, then scrub firmly with a stiff brush.
- Rinse thoroughly with a hose or, on robust slabs, a pressure washer used carefully.
- Once dry, re-point any joints that have lost mortar or top up sand joints.
Choosing a treatment
Some products work quickly when scrubbed in and rinsed; others are applied and left so that rain washes them in and the growth dies back gradually. Both can work. Check the label suits your paving, especially if it is natural stone or porcelain, and follow the instructions on weather and keeping pets away.
Never mix products, avoid bleach on natural stone and near plants, and keep run-off out of ponds and drains. Our guide to cleaning a patio without a pressure washer has more on choosing a cleaner if you are working by hand.
Pressure washing a green patio
A pressure washer quickly removes surface algae from hard concrete slabs. Use a fan nozzle or rotary surface cleaner, keep it moving and avoid aiming into joints. On sandstone and other natural stone, lower the pressure or rely on treatment and scrubbing; blasting green stone can roughen the surface, which then holds more moisture and turns green again sooner. For the full general method, see how to clean patio slabs.
Slowing the return
Moss and algae will come back in damp, shady places, but you can slow them down. Cut back overhanging branches and shrubs to let in light and air. Sweep regularly and clear leaves promptly in autumn. Fix gutters and downpipes that drip onto the patio, and make sure water drains away rather than pooling. Raise pots on feet. A lighter clean more often is easier than a full restoration.
If the problem is small black dots rather than green growth, that is probably lichen, which needs a different treatment; see our guide to removing black spots from a patio.
When to call a professional
It is worth calling a professional if the patio is large or very green, if it is natural stone you do not want to damage, or if the growth returns quickly despite your efforts.
When we clean patios, moss and algae are removed with rotary surface cleaners and pressure matched to the stone, weeds and moss are taken out of the joints, and a biocide treatment can be applied after cleaning to help stop algae returning as quickly, which is especially useful on shaded patios.
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Sandstone, porcelain, slabs and decking brought back to life. Family-run by Jason and Lucas Prior, covering south Essex.
Frequently asked questions
What kills moss on a patio?
A moss and algae treatment or patio cleaner suitable for your paving, applied as the label directs. Removing thick moss by hand first helps the treatment reach the surface.
Why does my patio keep going green?
Shade, damp, poor drainage and leaves left on the surface all let algae and moss return. Letting in more light, sweeping regularly and fixing drips from gutters slow it down.
Is it safe to pressure wash moss off sandstone?
Only with care. Use a low setting or rely on treatment and scrubbing, because high pressure can roughen sandstone so it holds more moisture and turns green again faster.
Related guides
- How to clean patio slabs: the general method for most patios, when a pressure washer helps and when it does more harm than good.
- How to clean a patio without a pressure washer: a hand-cleaning method that relies on the right cleaner, a good brush and a thorough rinse instead of water pressure.
- How to remove black spots and lichen from a patio: why black spots survive scrubbing and pressure washing, and how to treat lichen without damaging the stone.