Decking on pedestals, on joists or on a slab: which structure to choose?

A timber deck is an outdoor floor: boards screwed onto joistsJoist: the horizontal timber the decking boards are screwed onto, known as a lambourde in French. It is the skeleton of the whole build: if it rots, everything has to be redone. Outdoors it must be class 4, the durability rating for wood that gets wet often., themselves laid on a support. That support is what changes everything, and it depends first on your ground. There are four main situations.

The gold-underlined words are explained on hover. All definitions are in the joinery glossary.

Four ground types, four solutions

You already have a sound concrete slab. This is the simplest and cheapest case: low pedestals or packers, the joists on top, and an air gap kept under the wood. The only requirement: the slab needs a slight fall to drain the water away, around 1.5%.

You are starting from a more or less level garden. The modern solution: adjustable polypropylene pedestals on a bed of compacted gravel with a geotextile membrane. Each pedestal adjusts to the millimetre, carries 500 kg, and the ground keeps draining naturally. It can be dismantled, corrected, and it goes in without a concrete lorry in the garden.

Your plot slopes or the deck sits high. We move to a genuine load-bearing structure: concrete pads founded below frost depth, or posts, with joisting sized like a floor. Above one metre high, it is no longer a deck in the eyes of the trade rules, it is carpentry, with a guardrail required. Here in the Pyrenees, sloping plots are not in short supply: it is a case I come across often.

You have nothing at all and someone suggests pouring a slab. That is where I put the brakes on, figures in hand, two paragraphs further down.

The comparison in figures

To put the whole picture in place: a complete timber deck, structure, boards and fitting, runs between €80 and €150 per m² in softwood, and climbs to €110 to €290 in tropical hardwood. The choice of the wood itself is another subject, which I covered in which wood for a deck in the mountains.

My honest opinion: pouring a new concrete slab to lay wood on top is paying for two decks. The slab costs three to five times more than a pedestal structure, it forces a month of drying before fitting, and the wood does not need it: it wants air, not concrete. I keep the slab for cases where it serves another purpose, a future conservatory for example. In every other case, that money is better placed in the quality of the boards and the screws.

The rules that make it last (and that nobody sees)

The fitting of timber decks is governed by a technical document, the DTU 51.4DTU 51.4: the Document Technique Unifié for exterior timber decking, part of France's codified trade standards, revised in 2018. It is the official rulebook: joist spacing, ventilation, fixings, gaps between boards. A serious craftsman works to it, even when nobody checks.. In practice, here is what it means on site:

• Joists every 40 to 50 cm, never more: beyond that, the boards turn into a trampoline and the fixings take the strain.
• Air under the decking: at least 5 cm below the joists and openings around the edges. An underside that cannot breathe is the number one cause of rotten decks.
• Class 4 structural timber: a class 2 joist outdoors, even sheltered under the boards, is doomed. A single rotten joist can condemn the whole deck.
• A2 stainless steel screws, two per bearing point: zinc-plated screws rust and stain the wood from the first wet season, and it never comes off.
• A protection strip on every joist: it stops water seeping in through the screw holes. This few-euro detail can double the lifespan of the structure.
• A gap between boards matched to their moisture content at fitting, from 2 to 7 mm: the wood is going to move, so we leave it room. I explain this phenomenon in why wood moves.

What changes in the mountains

In Gerde, in Bagnères or in the Campan valley, two extra constraints come into play. Frost first: a concrete pad founded too shallow gets lifted by the freezing ground, and the deck warps a little more every winter. At altitude, we found deep, down to a metre, or we choose adjustable pedestals on a draining bed, which frost cannot trap. Snow next: in mountain areas, the load to allow for quickly exceeds 200 kg per m², where a lowland deck is designed for much less. In practice, I tighten the pedestal spacing and the timber sections. It is invisible on the quote from next door, and that is exactly where the price difference hides. To see what this specification looks like in real life, have a look at my deck facing the Pyrenees: species chosen for the mountains, ventilated structure, invisible fixings.

A well-designed structure then needs no structural maintenance: a sweep with a broom twice a year, an eye on the ventilation grilles, and that is all. The rest, greying and saturateur (a penetrating decking oil), is a matter of looks: I cover it in looking after a wooden deck.