A sound concrete patio can be an excellent starting point for a new deck. If you are asking whether decking can go over concrete, the short answer is yes - provided the slab is stable, drains properly, and the new deck is built with enough clearance for airflow. Covering tired concrete with timber or composite decking can transform a garden without the cost, mess and delay of breaking up and removing the existing base.
The important part is not simply laying boards on top. Concrete holds moisture, and decking needs a well-supported, ventilated subframe to stay level and last. A little planning at the start will help avoid standing water, movement and premature timber decay later on.
Can decking go over concrete?
Decking can go over concrete safely when the concrete is structurally sound. Check for major cracks, loose or crumbling areas, significant sinking and uneven sections before ordering materials. Fine surface cracks are not usually a problem, but a slab that is moving, sinking, or breaking up should be repaired first.
The concrete also needs to fall away from the house. A gentle slope allows rainwater to drain away rather than collect beneath the deck. If water already pools on the patio after a shower, fitting decking over it will not solve the issue. In fact, standing water underneath decking can create ideal conditions for the wood-destroying fungi that cause wood rot.
Height is another essential check. Measure from the concrete surface to door thresholds, air bricks, and damp-proof courses. The finished deck should sit below the damp-proof course, normally by at least 150mm where possible, and it must not block air bricks or ventilation to the property. If there is very little room below a door, a low-profile decking support frame may work, but it can be much harder to maintain good air circulation with this style of frame.
Why a raised subframe matters
Install a treated timber subframe above the concrete, then fix the decking boards to the joists. This separates the boards from the slab and creates a gap for airflow so moisture can escape. It also gives you a level structure to work from where the concrete has minor imperfections.
For a straightforward domestic patio, you can position treated timber framing on durable rubber or plastic support pads. These raise the timber slightly off the concrete, reducing direct contact with moisture. Adjustable decking pedestals are another option, although they are usually too high to fit under a deck frame fitted under a house door. They also cost considerably more than a simple timber-and-pad arrangement.
The joists should run in the opposite direction to the deck boards. Joist spacing depends on the decking board material, thickness and, most importantly, the manufacturer’s guidance. Never be tempted to save money by opening up joist spacing wider than the maximum the manufacturer recommends. Traditional timber boards commonly need supports at closer centres than thicker, high-quality composite boards; however, this is not always true, so always work from the product specification rather than guessing.
Preparing the concrete before you build
Start by clearing the slab thoroughly. Remove planters, old paint flakes, moss, soil and anything that could hold moisture against the new structure. Sweep it clean, then inspect the surface after rainfall if possible. This is the easiest way to see where water travels and where it settles.
You can repair small holes and shallow damaged patches with a suitable exterior concrete repair product. You may need to grind down raised lumps if they prevent the subframe from sitting level. For larger cracks or a slab that has dropped in one area, get the base assessed before proceeding. A deck can bridge minor flaws, but you should never build on a failing foundation.
It is worth considering weeds too. Concrete usually provides a good barrier, yet weeds can emerge through cracks and around the slab edge. Treat existing growth and use weed membrane as required. A membrane is not normally needed across an intact concrete base, but it can be useful around exposed edges where soil meets the decking.
Fixing a decking frame to concrete
Whether the frame needs fixing down depends on the deck design. A small, low-level deck that is fully contained and well weighted by its structure may remain stable without numerous fixings, particularly when it sits on pads. A deck that is on a concrete patio adjoining a house can be fixed in position with a sole plate attached to the house. Ensure that the sole plate is held clear of the the wall with spacers to stop moisture being transferred to the wall. Three or four galvanised large square washers is usually sufficient.
Use suitable concrete screws, masonry anchors or frame fixings at sensible intervals. Pre-drill the concrete accurately, and use galvanised or coated fixings for outdoor work. Ordinary interior screws and fixings will rust quickly and can stain boards or fail under load.
Timber or composite decking over a patio?
You can install both timber and composite decking over concrete, but they suit different enduser priorities.
Pressure-treated softwood decking is a popular choice where value, a natural look and the ability to cut materials on site matter most. It needs a quality treated subframe, appropriate deck screws and routine care. Cleaning, reapplying a suitable treatment and checking fixings will help keep it looking good and reduce surface weathering.
Composite decking has a more consistent finish and generally requires less ongoing maintenance. It is not maintenance-free, though. Leaves, dirt and algea still need clearing, and the boards expand and contract with temperature changes. Correct end gaps, side gaps and fixing clips are essential for composite boards. Follow the manufacturers recommended joist centres and expansion clearances requirements closely.
For either option, leave gaps between boards for good ventilation. The gap size will vary by board type and season, especially with timber, which can shrink as it dries. Avoid enclosing the sides of a deck laid over concrete. It is important that air is free to flow all around the deck support frame.
Common problems to avoid when decking over concrete
Most failures come from moisture, poor levels or ignoring the final deck height. Before starting, make sure you are not creating any of the following issues:
- A finished surface that sits too close to a door threshold, damp-proof course or air brick.
- A frame laid in a puddle or over concrete with no clear drainage fall.
- Unsuitable packing placed directly on the slab.
- Deck boards installed without the required expansion and drainage gaps.
- A loose frame that can shift, rock or lift.
Also consider whether a practical access point is needed. Once a deck is in place, repairs to drainage channels, external pipework or inspection covers can be more awkward. If an inspection cover falls within the patio area, plan a removable hatch into the deck rather than concealing it permanently.
When concrete should be removed instead
In some gardens, removing the concrete is the better long-term option. This is usually the case where the slab is badly cracked, too high against the house, directing water towards the building or preventing you from achieving enough clearance below doors and vents. Removal may also be preferable if you want to reshape the garden, improve ground drainage or build a properly elevated deck with posts and footings.
It involves more labour and disposal, but it can give you a cleaner result where the existing patio is the source of the problem. For a level, well-drained slab, however, building a ventilated deck over the top is often a sensible and cost efficient route.
A few careful measurements and the right subframe materials make all the difference. If you are unsure about board spacing, joist sizes or choosing timber for a concrete patio, Crestala Fencing Centre can help you select practical materials that suit the job - and the way your garden is built.
