What is growing on your patio?
The Royal Horticultural Society describes four kinds of growth on paving, and each needs a slightly different approach.
- Black spot (lichens). Small, hard, black or grey dots, often the size of a pinhead to a fingernail, firmly attached to the surface. The RHS notes that flat, crust-like lichens colonise even dry, bright paving. Natural sandstone, with its open texture, is especially prone.
- Green film (algae). A slimy green or dark coating, worst in shaded corners, along north-facing walls and wherever water lies. This is what makes a patio dangerous to walk on in winter.
- Moss. Green cushions in joints and along edges, spreading in from lawns and borders.
- Liverworts. Flat, leafy green growths in very damp spots, often a sign of poor drainage.
The RHS is clear that these growths do not damage the stone itself. The reasons to remove them are safety and appearance.
Why Middlesbrough patios grow so much
Teesside winters are long, grey and damp rather than very cold, so paving can stay wet for weeks at a time. Several local conditions add to that:
- Mature trees. Streets around Albert Park, The Avenue and the older parts of Acklam and Nunthorpe have big trees that shade gardens and drop leaves and sap.
- Clay ground. Much of the town sits on glacial till and lake clays that drain slowly, so the ground around a patio stays wet.
- Walled yards. Victorian terraces in Linthorpe often have yards enclosed on three sides, with little sun or air movement.
- Flat or back-falling patios. A patio laid without enough fall, or with a fall towards the house, holds puddles that feed algae.
Cleaning by material
Porcelain
Porcelain is dense and absorbs very little water, which is why it stays cleaner than natural stone. Most algae comes off with a stiff brush and warm water with a little washing-up liquid. A pressure washer is fine on porcelain slabs, but keep the jet off the joints, especially if they are a brush-in compound, which can be blasted out. Lichen is rare on porcelain; when it does appear, it is often growing on dirt sitting on the surface rather than in the slab.
Indian sandstone
Sandstone is porous and softer. A pressure washer on a narrow, high-pressure setting can etch the surface, open up the texture and strip the pointing, which then holds more dirt than before. If you use one, choose a wide fan nozzle, hold it well back and work at a low angle. For lichen, a cleaner sold specifically for natural stone, applied and left to work over days or weeks, is gentler than scrubbing. Avoid acid-based products on any stone unless the maker says it is safe for that stone, and never use them on limestone.
Concrete and textured slabs
Concrete flags are tough but can lose their surface finish to aggressive jet washing, exposing the aggregate underneath. Brush first, then wash at moderate pressure.
A safe cleaning routine
- Sweep and clear. Remove leaves, pots and furniture. Leaves left lying are a major cause of staining and algae.
- Scrape the joints. The RHS suggests a pointed weeding tool or an old knife to lift moss from cracks.
- Brush and wash. A stiff broom with warm water and a mild detergent removes most algae.
- Treat lichens separately. Use a product made for the job, follow the label and allow time.
- Rinse to the garden, not the drain. Let rinse water run onto a lawn or border where possible.
- Check the joints. If cleaning has opened them up, they need re-filling before winter or weeds and water will get in. See our guide to re-pointing patio joints.
The RHS adds two cautions about pressure washers: always wear goggles, and use them carefully where drainage is poor, because the extra water can make damp problems worse.
What not to wash into the drains
Many Middlesbrough homes have separate pipes for foul water and surface water. The Environment Agency explains that the surface water system carries rainwater from roofs and paved areas straight to rivers and lakes. Anything poured into a yard gully or a channel drain on a patio can end up in Marton West Beck, Middlesbrough Beck or the Tees without treatment. Patio cleaners, bleach and detergent should not be tipped down these drains, and rinse water carrying cleaner is best kept on the garden.
Keeping it cleaner for longer
- Improve the fall. A patio that sheds water dries faster. If puddles sit on yours after rain, the levels are worth looking at.
- Let in light and air. Thinning overhanging branches makes a big difference. In a conservation area, the council needs six weeks' notice before tree work.
- Brush often. The RHS points out that regular brushing stops growth taking hold.
- Choose materials carefully. If you are planning a new patio in a shaded garden, porcelain is far easier to keep clean than sandstone. Our porcelain patios and Indian sandstone pages compare them.
- Consider a sealer for sandstone. An impregnating sealer can reduce staining, but it changes how the stone looks and needs renewing.
Questions people ask
Does black spot ever go away on its own?
Rarely. Lichens grow slowly and hold on firmly. They fade after treatment with a dedicated cleaner, and regular brushing helps stop them coming back.
Is it safe to jet wash Indian sandstone?
It can be done carefully, with a wide fan nozzle, low pressure and the lance held well back. A narrow, high-pressure jet can damage the surface and strip the pointing.
Can I use bleach on my patio?
Bleach can mark some stone and harms plants and wildlife, and it should never go into a surface water drain. A cleaner made for patios, used as directed, is a better choice.
How often should a patio be cleaned?
A brush every few weeks and a thorough clean in spring and autumn is enough for most gardens. Shaded or tree-lined gardens need more attention.
Sources: RHS: algae, lichens, liverworts and mosses on hard surfaces; Environment Agency blog: wrong pipe, wrong place (misconnections); GOV.UK: pollution prevention guidance; Northumbrian Water: charges and surface water drainage.