Can I build a low deck over my old concrete patio?

Why low decks are different

A deck that floats a few inches above a slab or the ground has no posts, no guard rail and no stairs, which makes it simpler in some ways and harder in others. There is very little room to frame, so joist depth is limited and spacing has to tighten to make up for it. There is very little room for air, so moisture trapped under the deck has nowhere to go. And there is no easy access later: once the boards are down, the frame is sealed in until someone pulls them up.

Done well, a low deck over an old patio is a clean plane that meets the house at the door, hides a cracked slab, and gives you the warmth of a deck with the stability of the concrete beneath it. The composite deck and pergola we built in North County sits just above a slab on a pressure-treated frame, with the pergola posts set on the same slab so the two line up.

Water first

Rain and hose water will go through the gaps between boards and land on the slab. The slab must shed it, ideally away from the house at the slope it was originally poured to. If the slab has settled toward the house, that has to be corrected before the deck goes on, with a drain, a cut and re-slope, or by building a drainage plane into the frame. Standing water under a deck is a mosquito farm and a rot farm.

Sleepers, the low joists that sit on the slab, should never sit in a film of water. We set them on composite shims or pads so there is a gap beneath, run them in the direction of the slab's drainage so they do not dam it, and tape their tops. Where the slab is badly broken, we often remove it and build on compacted gravel with pads or small footings instead.

Air second

Composite and PVC manufacturers nearly all require unobstructed airflow under the deck, and many set a minimum clearance under the joists, often on the order of an inch and a half or more, to keep the boards and the frame dry and to keep the warranty valid. Wood decking wants the same for the same reason. That means the perimeter of a low deck should be open or vented, not sealed with solid skirting, and the frame should not be packed against the slab.

On a very low deck the trade-off is between height and air. The board thickness plus the joist plus the clearance adds up quickly, and the total has to fit under the door threshold. This is where design matters: sometimes the right answer is a slightly thinner joist at tighter spacing, sometimes it is a step at the door, and sometimes it is removing an inch of slab.

The door threshold and the step

The residential code limits how far a landing outside an exterior door can drop below the threshold, and it governs whether a step is needed and how large a landing must be. A low deck that lands flush or a short step below the door is the goal, both for how it feels and for the code. Decking that runs tight against stucco or siding also needs a gap and flashing so it does not wick water into the wall. We draw this joint carefully on every low deck.

What a low-deck frame looks like

• Pressure-treated sleepers or shallow joists rated for ground contact, set on pads, oriented to the slab's drainage.
• Joists at 12 to 16 inches on center depending on the decking, with blocking and joist tape on every top.
• Fastening to the slab with corrosion-resistant anchors where the frame could shift, or a self-weighted frame where anchoring would crack a thin slab.
• Perimeter blocking for a picture-frame border and for fascia, with vent gaps or a vented skirt.
• A drainage check before the first board goes down: a hose run over the slab to watch where the water goes.

Related questions

Sources and further reading