Green patios: dealing with moss and algae

Why it happens here

Algae and moss need moisture, shade and something organic to feed on. A north-facing garden with a fence or hedge on the sunny side, plus a winter's worth of fallen leaves, supplies all three.

Porous surfaces hold moisture longer, which is why sandstone typically greens up faster than porcelain in the same position.

Cleaning without damage

Start with the least aggressive method: a stiff brush, warm water and a suitable patio cleaner, left to work rather than scrubbed hard immediately.

Pressure washing works but needs care on sandstone — too close or too concentrated and it erodes the softer surface and blasts out the jointing. Keep the nozzle wide, back off, and expect to re-point or re-joint afterwards.

• Brush and patio cleaner first; pressure wash second
• Wide fan nozzle, kept moving, especially on sandstone
• Re-joint after washing where the compound has been stripped
• Avoid strong bleach solutions near planting and lawns

Keeping it off

Clear leaves in autumn. Organic debris left over winter is the food source, and removing it is the single most effective preventative step.

Cutting back overhanging planting to let light and air onto the surface makes a real difference, and improving the fall so water does not linger helps more than any cleaner.

When it is a symptom of something else

Persistent green in one strip usually means water sits there — a fall problem, a settled slab or a blocked gully. Fixing the water fixes the moss.

Loose or rocking slabs alongside heavy moss growth suggest the bedding has washed out, which is worth looking at before another winter.