Composite decking over concrete: sleepers, pedestals and drainage

You can lay composite decking over a concrete slab, and I think it's one of the jobs composite does better than timber. The makers' install guides agree on the main rule: the boards go on a frame, never straight onto the slab. The frame can be thin. Treated pine battens will do it, and so will aluminium joists or adjustable pedestals. My advice is to settle the frame and the door height on paper first, because the laying itself is the quick part.

Can composite decking be installed over concrete?

Yes, with a sub-frame. One maker's guide puts it plainly: don't apply the boards directly to a solid surface such as concrete³. Another says not to attach the decking to any solid surface or watertight system. A board fixed flat to a slab would trap water under it, and the clips need joists to fix into. On a frame, air can move under the boards and the slab keeps draining the way it did before.

Timber is the harder case here. Timber laid hard against a damp slab rots, and the Whirlpool thread further down is blunt about it. Composite boards are sold as rot-proof, and one maker says its capped boards won't split, rot or warp². The frame under them is often timber, though, so on a slab I'd choose the frame material before anything else.

What the slab needs before you start

The slab has to be sound, and it has to fall away from the house. The most detailed over-concrete sheet among these guides asks for concrete in good condition with enough fall to drain, battens that run in the direction of that fall, and a deck perimeter that is not enclosed¹. If the slab falls towards the house or ponds in the middle, I'd fix that first, because a deck on top hides the water without removing it.

The frame also has to be held down, and this is where forum advice and the makers part ways. The same sheet says the sub-frame must be attached to the slab rather than left floating¹. On Whirlpool, a member recommending pedestals described them as free-standing, held in place by the deck's weight or a bead of construction adhesive⁸. Where the two disagree I'd follow the board maker, because one maker warns that installing its boards incorrectly may void the warranty⁵.

Sleepers, aluminium frames or pedestals?

The height at the door usually decides the frame for you.

Battens or sleepers are the traditional answer. The over-concrete sheet asks for battens at least 50 mm high¹, and the same maker's fixing chart gives a minimum clearance over concrete of 50 mm. Other makers accept less. One asks for 40 mm of clearance where there is adequate drainage underneath and 90 mm where there isn't³. Another gives 40 mm over concrete or paving but 100 mm over grass or dirt⁴. A third allows a 25 mm batten over concrete as long as no water pools under the deck⁵. The numbers differ, but the idea is the same: the lower you go, the better the drainage has to be.

I'd treat a treated pine batten as the weak link, because pine sitting on a damp slab stays wet. If you use pine, I'd pack it up off the concrete so it can dry. The over-concrete sheet calls that good building practice, and I'd seal every cut end as well. The H3 or H4 guide covers which class goes where. Steel and aluminium battens are immune to rot, and the NCC lists steel, aluminium and concrete among the materials it treats as not subject to termite attack⁷. That's a real advantage for a low deck against the house. There is more on the options at aluminium framing and pedestal frames.

Adjustable pedestals are the answer when the slab is out of level or you want to set the fall exactly. A small plastic pedestal foot sold at hardware stores adjusts from 25 mm to 40 mm and is rated to carry 2.2 kN⁶. The joists sit on the feet, and each foot winds up or down until the frame is level. On a Whirlpool thread in March 2019, KoDer described a treated pine frame on adjustable pedestals sitting about 80–150 mm above the concrete with a 1 degree slope, and noted the feet could correct a slope of up to 5 degrees¹⁰.

Where the height is very tight, one maker's aluminium system pairs a 25 mm joist with low pedestals for patios that have no room for a stump, bearer and joist frame⁵. The aluminium deck framing guide covers when that is worth the higher tier.

The height problem at the door

This is the sum I'd do before buying anything. Board thickness plus joist depth plus pedestal height is your finished deck level, and it has to land below the door sill or at a sensible step. If the sill is only a hand's width above the slab, a standard timber joist won't fit under it.

A Bunnings Workshop member, abassi3, asked in November 2024 about a composite deck over concrete under a pergola with a finished height of 130–140 mm, planning on 70 x 45 treated pine with pedestal feet¹¹. The Workshop team replied that, going by a timber maker's span tables, 70 x 45 isn't recommended for deck framing and 90 x 45 H3 pine is the smallest section that can be used. A second reply added it up, and a 23 mm board on 90 mm joists with a 25 mm pedestal foot came to 138 mm, just inside the limit¹¹. The owner posted in December 2025 that the deck was finished. My advice from that thread is not to solve a height problem with a joist the span tables don't recommend. The pedestal, not the joist, is where you find the last few millimetres.

If the sum won't work, I see two honest options. Use a lower frame, such as one of the low aluminium systems, or accept a step at the door and build it properly, with the riser rules in the stairs guide. One Whirlpool poster, womble66, wrote in June 2015 about planning a deck over an existing slab under a new pergola, so the outdoor floor would sit level with the sliding doors instead of stepping down 200 mm⁹. That's a sensible aim, and it only works when the slab leaves you the height.

Fall, drainage and the gap between boards

Keep the slab's fall and let the deck follow it, or use pedestals to level the deck while water still runs underneath. Either way, water must be able to get out, so don't box in the edges with a fascia that touches the concrete. That over-concrete sheet also asks for a minimum 6 mm side gap between boards whenever the clearance over the slab is under 150 mm¹. The wider gap is ventilation, and the same sheet says raising the frame off the slab improves airflow and lets the frame dry.

Battens or joists should run with the fall rather than across it, so water isn't trapped behind them. If they have to run across it, one maker says to raise them or check them out underneath so water can't pool behind them⁵. The end gaps matter as much as the side gaps, and the guide on expansion gaps has the detail.

What owners ran into

A Whirlpool thread from January 2016 covers most of the mistakes. The owner, xfix, wanted to deck an old, cracked patio slab that fell away from the house and asked how low the frame could go⁸. One member suggested bolting sleepers to the slab. Another, plucka99, pushed back hard on timber in contact with the slab, writing "Do not put timber directly on the ground" and warning that moisture would be trapped with next to no ventilation⁸. That reply pointed to adjustable pedestals instead. The owner then worried that free-standing pedestals wouldn't suit a slab with a noticeable fall, and the answer was to adjust each foot to suit.

I'd take two things from that thread. Timber laid on concrete rots, and composite boards make a good low deck over a slab as long as the frame under them is chosen for the same damp conditions. If part of the deck runs off the slab onto soil, the ground-level deck guide covers that side.

Before you order

Measure the sill height above the slab at the lowest point of the fall, then add the board thickness to the height of the frame under it. I'd check the slab after rain rather than on a dry day, because standing water is what you're looking for. Decide whether the frame is timber on packers, steel, or aluminium on pedestals. Read the install guide for the boards you've chosen, because makers don't agree on clearances or gaps. Lay out the joists at the centres that guide asks for, and read the joist spacing guide if you want to know why composite needs closer centres than an old hardwood frame. If you want a materials list from the layout, the deck designer draws a concept plan you can hand to your builder.