Can You Lay Composite Decking Over Concrete? A UK Homeowner's Guide

If you have a tired concrete patio or an old paved area in the garden, one of the most common questions we hear is whether you can lay composite decking over concrete rather than digging the whole lot out. The good news is yes — and laying composite decking over concrete is often quicker, cleaner and cheaper than starting from bare ground. The catch is that “over concrete” should never mean “straight onto the slab”.

We install composite decking across Merseyside and Cheshire, and a fair share of those jobs sit on top of existing concrete, paving or an old patio base. Reusing a solid surface that is already there can save days of groundwork and skip the muck-away bill. But concrete brings its own considerations — ventilation, levels, drainage and the height of your back door — that decide whether the finished deck lasts for decades or starts causing problems within a couple of seasons.

This guide covers when an existing concrete base is worth using, why you still need a frame on top of it, the two ways we build that frame, and the practical pitfalls — door thresholds, damp and settlement — that catch people out.

The Short Answer: Yes, But Never Straight Onto the Slab

Composite boards can absolutely sit over an existing concrete or paved base. What you must never do is screw the boards directly down onto the concrete. Decking needs a gap underneath it for air to move and water to drain away. Lay boards flat onto a slab and you trap moisture against them, water pools instead of running off, and over time you get standing damp, algae, and — on any timber in the structure — rot. A deck that cannot breathe starts to fail within a few seasons.

So the real question is not “concrete or no concrete” — it is what goes between the concrete and the boards. That layer is the frame, and it is what creates the air gap, sheds water, and gives you a dead-level surface to fix to.

Why You Still Need a Subframe

It is tempting to think a flat concrete base lets you skip the structure. It does not. The frame does three jobs the slab on its own cannot.

First, ventilation: even a 20–40mm gap under the joists lets air circulate so moisture is not held against the boards or the frame. Even decking manufacturers like Trex stress that a deck built over a solid surface still needs that air gap. Second, drainage — patios are usually laid with a slight fall for run-off, and the frame lets that water escape beneath the deck rather than backing up. Third, a true level: almost no existing patio is perfectly level, and the frame is how you take out the dips and slopes so the finished deck is flat.

This is the same principle that decides every build — what is underneath the deck boards determines how long the structure lasts, which we go into in our guide to composite decking subframes.

Two Ways to Build the Frame Over Concrete

There are two sound ways to build a frame over a solid base. Which one we use comes down to how level the concrete already is and how much height you have to play with.

Adjustable pedestals

Pedestals are screw-jack supports that sit on the concrete and carry the joists above them. Their big advantage is that each one adjusts for height, so we can take a patio that slopes for drainage and bring the deck back to dead level without endless packing. They lift the frame clear of the slab for excellent airflow and drainage, and they are quick to fine-tune. They suit uneven or sloping bases, and any situation where a little extra height is not a problem.

Battens or joists laid on the base

Where the concrete is already flat and you want to keep the build as low as possible, we lay a frame of joists across the base, levelled and raised on packers so water and air still pass underneath. For this application we use recycled plastic or aluminium joists, never timber. Sitting low against a slab is exactly where timber is most exposed to trapped moisture, and it is the corner cheaper installers cut. Recycled plastic does not absorb water and cannot rot, and aluminium gives a dead-straight, dimensionally stable frame. Neither will wick moisture up from the concrete, which is the whole point of building this way.

Either way, the boards end up on a ventilated, level frame. The choice is about levels and height, not about quality.

The Height Problem Most People Forget

This is the single biggest thing homeowners overlook. Building over an existing slab raises the finished floor level — you are adding the frame plus the board thickness on top of whatever is already there. That can be anywhere from around 75mm to well over 100mm.

Why it matters: that extra height has to work with your back door and your home's damp-proof course (DPC) — the barrier that stops ground moisture rising into the walls. The deck must never trap moisture against the building, so we always hold the frame at least 30mm clear of the wall and leave a consistent 10–15mm gap around the perimeter of the boards. That gap lets air move, stops debris packing into the joint, and keeps water off the brickwork rather than channelling it into the house. Built that way, the deck does not need to sit a set distance below the DPC — the detailing is what protects the building, not raw height.

What the height does still govern is the threshold. The finished deck should not come up higher than your back door, or you create a trip point and risk water tracking in over the sill. So the real constraint is the door, and the frame is planned around it. Sometimes that means a low-profile build; sometimes it means stepping the deck down from the door. It is exactly the kind of detail we plan for on the jobs in our completed projects.

Is Your Concrete Actually Up to the Job?

Not every slab is a good candidate, and it pays to be honest about the base before you build on it. A few things to check:

Settlement. Fresh concrete can take up to a year to settle fully and may sink as it does. If a slab is newly poured, give it time — building on concrete that is still moving means a deck that goes out of level.

Condition. Hairline cracks are usually fine, but a slab that is heavily cracked, crumbling or visibly tipping has a problem of its own that a deck will not cure. Heaving or rocking sections need sorting first.

Slope and drainage. Most patios have a deliberate fall for run-off, which pedestals handle easily; a severe slope may mean more support and height. And make sure water leaving the deck still has somewhere to go — if the concrete sits in a dip that already puddles, that needs addressing so you are not trapping water under the new deck.

If the base is sound, reusing it is a genuine win. If it is not, it is far better to know before the boards go on than after.

Laying Composite Decking Over Concrete the Right Way

So, can you lay composite decking over concrete? Yes — and where the base is solid, it is one of the smartest ways to get a clean, low-maintenance deck without the cost and mess of fresh groundwork. The whole thing rests on what goes between the slab and the boards: a ventilated, level frame on pedestals or properly supported joists, the right height worked out around your door and DPC, and an honest look at the condition of the concrete first. Get those right, and a quality capped board — whether that is NewTechWood or Millboard — simply gets on with the job for decades.

If you have an old patio or concrete base and you are wondering whether it will make a good foundation for a new deck, we are always happy to take a look and tell you straight. Get in touch for practical advice tailored to your garden.