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Quick Answer
Patios go green and slippery because of algae, moss and lichen that thrive in damp, shaded conditions. The wet Chilterns winters and the shade from mature trees create ideal conditions for algae growth. Solutions: pressure wash annually, improve drainage, trim overhanging plants, apply an algae killer in autumn, and consider sealing the surface.
Why Patios Go Green
The green slime on your patio is algae, a simple plant that thrives in damp, shaded conditions. Algae reproduce by spores that are everywhere in the air. When the conditions are right (damp, shaded, with a rough surface to cling to), the spores germinate and form a green film. Within weeks, the patio is slippery and dangerous.
In the Chilterns, the conditions for algae growth are perfect. The wet winters (200 to 250mm of rain from November to February), the mature trees that shade many gardens, and the chalky dust that provides nutrients all encourage algae. Patios in Marlow, Henley and Beaconsfield, with their mature beech and oak trees, are particularly prone.
Moss and lichen also grow on patios, but they prefer slightly different conditions. Moss likes damp, shaded, compacted surfaces. Lichen likes sunny, exposed, porous surfaces. All three make the patio slippery and unsightly.
The Conditions That Cause Algae
- Damp: algae need moisture. Patios that stay wet for long periods are most affected.
- Shade: algae prefer shade. Patios under trees or on the north side of the house are most affected.
- Rough surface: algae cling to rough surfaces better than smooth. Concrete and sandstone are more affected than polished stone.
- Poor drainage: standing water encourages algae. Fix drainage to fix the algae.
- Organic matter: leaves, compost and bird droppings feed algae. Sweep regularly.
- Chalk dust: in the Chilterns, chalk dust provides calcium that algae love.
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How to Get Rid of Algae
1. Pressure Wash
The most effective way to remove algae. Use a pressure washer (110 to 150 bar) on a dry day in late spring or early autumn. See our pressure washing guide.
2. Chemical Treatment
If pressure washing is not enough, apply a patio cleaner with sodium hypochlorite or benzalkonium chloride. These chemicals kill algae and prevent regrowth for 6 to 12 months. Apply in spring or autumn, leave for 30 minutes, rinse off.
3. Vinegar Solution
For an organic approach, mix white vinegar with water 1:1 and spray on the algae. Leave for 15 minutes, scrub with a stiff brush, rinse off. Less effective than chemical cleaners but safer for pets and children.
4. Boiling Water
For small areas, boiling water kills algae on contact. Pour directly on the algae, scrub with a stiff brush, rinse off. Best for small patches, not large patios.
How to Prevent Algae Returning
1. Improve Drainage
Standing water encourages algae. Fix drainage problems by:
- Re-leveling sunken areas where water collects
- Installing a land drain to carry water away
- Creating a soakaway to absorb runoff
- Using permeable paving to allow water to drain through
See our garden flooding guide.
2. Trim Overhanging Plants
Shade encourages algae. Trim trees and shrubs that overhang the patio to let in more light and air. Even thinning the canopy by 20 percent can make a big difference.
3. Apply an Algae Killer in Autumn
Apply a preventative algae killer in October, before the wet winter weather sets in. This kills spores before they can establish. Reapply in spring if needed.
4. Seal the Surface
A sealant makes the surface less hospitable to algae. It fills the pores in the stone, leaving nowhere for spores to take hold. Apply every 2 to 3 years. Cost: 5 to 10 pounds per square metre.
5. Sweep Regularly
Leaves and organic matter feed algae. Sweep the patio weekly in autumn to remove leaves, and as needed through the year.
6. Use the Right Surface
If you are laying a new patio, choose a surface that resists algae:
- Polished stone is less prone to algae than rough stone
- Resin-bound gravel is less prone than block paving
- Composite decking is less prone than timber
- Avoid tarmac in shaded areas (it stays wet and grows algae fast)
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The Slippery Surface Test
To test if your patio is dangerously slippery, walk across it in wet shoes. If you slip or feel insecure, it needs cleaning. A safe patio should give good grip even when wet.
The Health and Safety Executive recommends a slip resistance of 36 or above (measured with a pendulum tester). Most patios with algae score below 20, which is dangerously slippery. A clean, dry patio scores 60 to 80.
Cost of Treating Algae
| Treatment | DIY cost | Professional cost |
|---|---|---|
| Pressure wash (40 sqm) | 50-70 (hire) | 200-350 pounds |
| Chemical treatment (40 sqm) | 20-40 (chemicals) | 100-200 pounds |
| Sealing (40 sqm) | 80-150 (sealant) | 200-500 pounds |
| Tree trimming (per tree) | 0 (DIY) | 100-300 pounds |
| Drainage improvement | 200-400 (materials) | 600-1,200 pounds |
Common Mistakes
- Not treating the cause. Cleaning algae without fixing drainage or shade means it comes back. Diagnose first.
- Using bleach on soft stone. Bleach etches sandstone and limestone. Use a dedicated patio cleaner instead.
- Pressure washing too often. Pressure washing wears the surface. Once a year is enough.
- Not applying a preventative treatment. An autumn algae killer prevents winter growth.
- Ignoring the slip risk. A slippery patio is a safety hazard, especially for elderly relatives. Clean it before someone falls.
Frequently Asked Questions
Why does my patio go green in winter?
Winter in the Chilterns is wet and shady, perfect conditions for algae. The algae spores are always in the air, but they only grow when conditions are right.
Will pressure washing damage my patio?
Not if done correctly. Use the right pressure for the surface (low for sandstone, medium for block paving, high for concrete) and keep the lance moving.
How do I stop my patio going green again?
Improve drainage, trim overhanging plants to let in light, apply an algae killer in autumn, and seal the surface every 2 to 3 years.
Get Help with a Slippery Patio
If your patio in Oxfordshire, Buckinghamshire, Berkshire or Bedfordshire has gone green and slippery, send us a WhatsApp with photos and we will aim to respond within 12 to 36 hours, Monday to Sunday.
Further Reading
In mature and established gardens, always assess the existing ecosystem before making changes. Small adjustments to timing and technique can preserve decades of growth and wildlife value.
Put this advice into practice
Use the relevant CGM tool or follow a related guide, then send us a few photos if you would like a tailored plan.
Sources and citations
This article draws on the following sources. Specific claims are cited inline where relevant.
- CGM field records (2024-05. Site observations from gardens across Oxfordshire, Buckinghamshire, Berkshire and Bedfordshire.
- Royal Horticultural Society (2024). RHS Gardening Advice and Plant Finder. Available at rhs.org.uk.
- Met Office (2024). Regional climate averages, South East England. Available at metoffice.gov.uk.
- British Geological Survey (2024). Soil parent material data for the Chilterns and Thames Valley. Available at bgs.ac.uk.