Can I lay a patio on top of an existing concrete base?

An old concrete slab is a ready-made rigid base, which is exactly what paving wants. The reasons not to use one are all specific and checkable: it may be too high, it may be broken, it may hold water, or it may be moving. Work through those four before deciding, because each one has a different answer.

Safety

Step by step

1. Measure the height you have against the damp proof course.Add the bed thickness and the slab thickness to the existing concrete level. That total must still leave 150mm of clear wall below the damp proof course. Thin porcelain on adhesive uses far less height than a slab on mortar, and that difference often decides the method.
2. Test the slab for soundness.Tap over the whole area with a hammer and listen. A ringing sound is solid; a dull hollow sound is a debonded or cracked area. Chip a corner — the concrete should be hard and dense, not soft or dusty.
3. Check whether it moves.Look for stepped cracks, rocking sections and level differences either side of a crack. A slab that has broken and settled will keep moving, and new paving over it cracks along the same lines. That slab has to come out or be broken up and used as sub-base.
4. Check the fall and where water goes.Pour a bucket of water on. It should run away from the building and off the slab. A concrete base that ponds will hold water under the new paving, which freezes and lifts it. You can build a fall into the bed thickness if the concrete is flat, but not if it falls the wrong way.
5. Prepare the surface for a bond.Clean off moss, algae, oil, paint and any laitance or dust. A stiff brush and a degreaser for oil; a scabbler or a grinder for a smooth power-floated surface, which is far too polished to bond to as it is.
6. Prime it if the product or the substrate requires it.A cement slurry or a bonding primer, depending on the system. On a dense, smooth slab this is what gives the bed something to hold. On a porous, textured slab, a good clean and a dampened surface may be enough for a mortar bed — check the mortar's instructions.
7. Lay on a full bed, not spots or sand.A continuous mortar bed across the whole slab footprint. Spot bedding over concrete leaves voids that fill with water, and it has nowhere to drain because the concrete is beneath.
8. Deal with existing joints and cracks.A movement joint in the concrete will move, and it will telegraph through the new paving. Carry a matching joint straight up through the new surface at the same position rather than paving over it.
9. Sort the drainage at the edges.Water can no longer soak through anywhere, so the surface has to shed it to an edge that can take it — a gravel margin, a channel drain, a gully or a border. Decide where before you lay.

Common mistakes

• Laying slabs on a sand bed over concrete — Sand over an impermeable slab is a permanently saturated layer. It washes out at the edges, the slabs sink unevenly and in frost it lifts them.
• Bonding to a power-floated surface without preparation — A trowelled concrete surface is glass-smooth and often carries laitance — a weak dusty skin. Adhesive bonds to the laitance and the laitance lets go.
• Paving over a moving crack — The crack is the slab telling you it is in two pieces. Whatever you lay across it will crack in the same line, usually within a year.
• Not checking the damp proof course before buying materials — It is the one constraint that can make the whole plan impossible, and it is a five-minute check.

Alternatives

• Thin porcelain bonded with exterior tile adhesive: The best answer when height is tight and the slab is sound. Uses perhaps 30mm total, gives a rigid, fully bonded surface and needs no sub-base work at all. The slab must be clean, sound and correctly falling.
• Slabs on a full mortar bed: The conventional method. Around 30 to 50mm of bed plus the slab thickness. Forgiving of an uneven concrete surface because the bed takes up the variation, at the cost of height.
• Break the slab up and use it as sub-base: The right answer if the concrete is cracked, moving, too high or falling the wrong way. Broken concrete makes excellent hardcore, which saves both a skip and a delivery.
• Pedestal-mounted paving: Slabs on adjustable supports over the concrete, with an open drainage void underneath. Takes out level problems instantly and drains freely, but it uses more height than bonded methods and the slabs must be rated for pedestal use.

Questions people ask