How to clean patio slabs without damaging them

A patio that looked crisp when it was laid can turn green, black-spotted and slippery after a couple of Irish winters. Shade, damp and fallen leaves give algae, moss and lichen everything they need. The good news is that most patios can be brought back, but the right method depends on what the slabs are made of. A technique that is fine for concrete can ruin natural stone.

This guide explains how to identify your paving, how to clean each type, how to deal with moss and black spots, and how to finish the joints so the patio stays cleaner.

First, identify your paving

Paving	How to recognise it	Care points
Concrete slabs	Uniform, often grey or buff, sometimes with a textured or imitation-stone face.	Robust; tolerates moderate pressure washing.
Sandstone (including Indian sandstone)	Natural colour variation, layered or riven surface, often buff, grey, pink or brown tones.	Porous and can be eroded by high pressure; avoid acid cleaners unless the product says it is suitable.
Limestone	Fine, even texture; grey-blue or black, often honed or flamed.	Etched by acids, including vinegar. Use pH-neutral or stone-specific cleaners.
Porcelain	Very regular, thin, almost non-porous; often looks like stone or timber.	Easy to clean; stains sit on the surface rather than soaking in.
Granite	Hard, speckled crystals.	Durable; still avoid harsh acids on polished faces.

If you are unsure, test any cleaner on a spare slab or a hidden corner first.

What you need

• A stiff yard brush and a hand scrubbing brush
• A weeding knife or patio knife for the joints
• A patio cleaner suited to your paving, and a moss and algae treatment
• A garden hose, and optionally a pressure washer with a wide fan nozzle or surface cleaner
• Gloves and eye protection
• Jointing material for re-pointing, if the joints are in poor condition

Step-by-step: how to clean patio slabs

1. Clear the patio of furniture and pots and sweep off leaves and loose dirt.
2. Clean out the joints. Scrape moss and weeds from between the slabs with a patio knife. This is slow, but it removes much of the growth that would otherwise re-seed the surface.
3. Treat the growth. Apply a moss, algae and lichen treatment according to the label. Many work slowly over days or weeks, so this step is often best done ahead of the main clean. Keep it off lawns and planting unless the product says otherwise.
4. Wash and scrub. Wet the surface, apply a patio cleaner suitable for your paving, and scrub with a stiff brush. Work in sections.
5. Rinse. Rinse thoroughly with a hose, working away from the house.
6. Let it dry, then check for remaining stains and treat them individually.

Should you power wash a patio?

Power washing is quick and very satisfying, but it is not always the best option.

• Concrete and porcelain generally cope well with a pressure washer used with a wide fan nozzle or a rotary surface cleaner, held at a shallow angle.
• Natural sandstone can be damaged by high pressure. The jet erodes the softer layers, roughening the surface so it holds dirt and algae more easily, and it can lift the riven face. Keep the pressure low and the nozzle well back, or stick to scrubbing and rinsing.
• Joints suffer most. A jet aimed along the joints will wash out sand and break up weak mortar. Keep the spray moving across the slabs, not down the gaps.

Whatever the paving, start with low pressure on a small area, wear eye protection, and use an RCD-protected socket for electric machines. Our guide to pressure washing a driveway covers technique and safety in more detail.

Black spots on patio slabs

Small black dots that do not scrub off are usually lichen, which roots into the surface of the stone. They are common on sandstone and concrete in shady, damp gardens. Pressure washing seldom removes them and often damages the stone in the attempt. Specialist black spot or lichen removers are available; follow the instructions closely, check they are safe for your type of stone, and be patient, because the spots often fade over days or weeks rather than vanishing at once.

Other common stains

• Leaf and tannin stains: brown marks left by leaves often fade in sunlight; a patio cleaner and scrubbing helps.
• Rust marks from furniture or metal pots: use a rust remover that is safe for your paving. Acidic products can etch limestone and some sandstones.
• Grease and barbecue fat: soak up fresh spills with an absorbent, then use a degreaser suitable for stone or concrete.
• White bloom (efflorescence) on new paving: salts coming out of the slabs or mortar. Brush it off dry; it usually stops on its own as the paving matures. Do not wash it, which can draw more salts to the surface.

Re-pointing the joints

Crumbling or missing joints let weeds and moss establish and allow water under the slabs. Once the patio is clean and dry, rake out loose material and refill the joints. Options include traditional sand and cement mortar and ready-mixed jointing compounds that are brushed into the joints. Follow the product's instructions on preparation and weather, as some need dry conditions and others need the paving to be damp. Good, full joints make a big difference to how quickly a patio goes green again.

Keeping a patio clean in a wet climate

• Sweep regularly, especially in autumn, and do not leave leaves lying on the slabs over winter.
• Move pots now and then, or raise them on feet, to avoid stains and damp patches underneath.
• Cut back overhanging shrubs and hedges to let in light and air, within the legal hedge cutting season.
• Check that the patio falls slightly away from the house and that water is not pooling.
• Consider a sealer suitable for your paving, which can make cleaning easier. Test it on a spare slab first, as some sealers darken stone.
• Apply a light moss and algae treatment once or twice a year.

Patios and decks often get cleaned together; our guide to cleaning and oiling decking covers the timber side. Alpine Home Improvements lists power washing among its handyman services in Tralee and County Kerry if you would rather not do it yourself.