Can I lay decking straight over an existing patio or concrete?

An old patio is a ready-made, drained, compacted base and it makes an excellent foundation for a deck. What kills the idea is height. A deck over a slab adds the pad, the joist and the board — often 130 to 180mm — and if the patio was already close to the damp proof course the sum does not work. Check the height before you plan anything else.

Safety

Step by step

1. Measure from the existing patio to the damp proof course.You need 150mm of clear brickwork remaining after the deck is built. Add up your intended build-up — pad, joist depth, board thickness — and see whether it fits. If it does not, the answer is no, or the patio comes up.
2. Check the existing surface for movement and drainage.Walk it. Rocking slabs get lifted and re-bedded, or removed, before anything goes over them. Pour a bucket of water on and watch: a patio that ponds will pond under the deck too, and you will not be able to get at it.
3. Plan for air under the joists.At least 50mm of clear ventilated air between the underside of the joists and the patio, with the perimeter open on at least two sides. A joist laid flat on a slab wicks moisture from the concrete and rots along its full length.
4. Use pads or bearers rather than fixing to the slab.Adjustable deck pedestals, treated bearers on DPC pads, or paving supports carry the joists and give the ventilation gap. If you do need to fix down, fix through pads with a DPC layer between timber and concrete, never timber straight onto concrete.
5. Keep the existing falls doing their job.The patio's fall is now the drainage layer under the deck. Keep the low edge open so water can leave, and build the deck's own fall in the same direction so nothing is fighting.
6. Work out the joist span from the pad spacing, not the slab.The slab supports the pads; the joists span between pads. Set the pads close enough that the joist section is comfortable — closer pads mean shallower joists, which is often exactly how you win back the height you needed.
7. Leave the perimeter open or vented.Slatted skirting, a ventilated fascia, or simply an open edge. A deck boxed in solid over an old patio is a sealed damp void and the frame rots from below.
8. Deal with what is living under there.Clear vegetation and debris off the patio first, and consider whether you are creating a rodent void against the house. A fine mesh behind ventilated skirting keeps air moving and animals out.

Common mistakes

• Laying joists flat on the slab — Concrete holds and releases moisture, and there is no airflow under a joist lying on it. The joist is wet on its underside permanently and rots from a face you cannot see.
• Boxing the deck in all round — It creates an unventilated damp void the full area of the deck. Everything inside it stays wet and the frame has a fraction of its expected life.
• Ignoring the damp proof course because the patio was already there — The existing patio may already be marginal. Adding 150mm of deck to it takes a borderline situation and makes it a certain one.
• Building over rocking slabs — They will still rock, now under the pads carrying your deck, and you cannot reach them. Lift and bed anything that moves first.

Alternatives

• Adjustable pedestals: Plastic screw-adjustable supports designed for raised floors and paving. They take out unevenness in the old patio quickly, need no cutting or packing, and keep everything off the concrete. The neatest solution where the height budget is tight.
• Treated bearers on DPC pads: The conventional approach: bearers running across the patio on offcuts of DPC over pads or slabs, joists on top. Cheap, robust, more work to level than pedestals.
• Take the patio up first: The right answer when the height does not work, when the slabs are unstable, or when the patio was itself laid too high against the wall. More work, but it lets you build the deck at the correct level with proper foundations.
• Lay new paving over the old instead: Where the height budget is very small, thin porcelain paving bonded to a sound existing slab uses far less height than a deck. Different look, different job, but worth pricing.

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