Updated
Patio Slippery With Green Algae
A green or black film has built up on the paving and the surface is treacherous whenever it rains.
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What's happening?
The green film is algae, and the black staining that follows it is usually a mix of algae, lichen and dirt. All of it needs the same two things: moisture that lingers and light that is weak. That is why the slippery patch is on the north side, under the tree, in the shade of the fence, or wherever water sits after rain. The slab material matters as well — smooth, dense flags and sawn natural stone have very little texture to grip once a biofilm covers them, while riven and textured surfaces hold on longer. The important point is that a slippery patio is a wet patio. Cleaning it restores grip for a season; changing why the water lingers is what stops it coming back.
What usually causes it?
- Shade from trees, hedges, fences or the house keeping the surface damp for most of the day
- No fall or a low spot, so rain stands on the paving instead of running off
- Failed or open joints holding water in the surface between the slabs
- A smooth, dense or polished slab with little inherent slip resistance when wet
- Leaf litter left on the paving over autumn and winter, feeding the biofilm
- Overflowing or leaking guttering keeping one strip of paving permanently wet
- A sealant applied over damp paving or an unsuitable sealant that has become slick
Is it dangerous?
Yes, this is one of the genuinely risky garden faults, because wet algae on a smooth slab gives almost no grip and the surface looks the same as the dry paving next to it. Falls on slippery paving are common and the consequences are worst for older people and children. Treat it as urgent where the affected area is on the route to the door, on or beside steps, at a threshold, or next to a drop with no handrail. Ice on top of algae removes what little grip is left.
Can I fix it myself?
Clear the cause and the film together. Sweep leaves and debris off weekly through autumn, and keep the paving clear over winter rather than tidying it in spring. Clean the surface with a product intended for the slab material — a patio cleaner or a biocidal wash approved for the purpose — following the dilution and contact time on the label, and keep it off nearby planting. Check where water is standing after rain, and check the guttering above. A pressure washer will clear the film quickly but can etch soft stone, strip jointing and drive water under the slabs, so keep the pressure low and the lance moving. If the paving is smooth and shaded, cleaning will need repeating every year.
When should you call a driveway and patio installer?
Call a patio specialist when the slippery area is where people walk to the door or up steps, when standing water is the reason the film keeps returning, or when cleaning has stopped lasting a full year. Those are surface and falls problems rather than cleaning problems. It is also the moment to ask about the slab itself: replacing a smooth flag on a shaded route with a textured one solves permanently what cleaning solves temporarily.
What will the driveway and patio installer actually do?
- Check the falls and find the low spots where water is standing after rain
- Establish whether shade, drainage, joint failure or the slab surface is the dominant cause
- Clean the paving with a method suited to the material rather than the strongest available
- Re-lay or re-level areas that hold water, and re-point failed joints so water sheds instead of soaking in
- Replace paving on high-traffic routes with a textured unit where the existing slab has no wet grip
- Add or reposition drainage so runoff leaves the paved area, and correct rainwater discharge onto the surface
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