How long does a larch deck last in Geneva?

The honest answer is: between 15 and 30 years. The dishonest answer is whatever a salesperson tells you it'll be — and the difference between the two ends of that range is almost entirely in the details of how it was built and looked after.

A larch deck built badly in Geneva can fail in eight years. A larch deck built properly can still be in service in your kids' generation. Same wood, same climate, same suppliers. The variable is the work — and there are five or six decisions, made before the first board is cut, that determine which end of that range yours will land on.

The headline numbers

In the Geneva climate, sitting on a properly built substructure, you can reasonably expect:

• European larch (Alpine): 20–30 years before any major work
• Siberian larch: 25–35 years (denser, slower-grown — costs a bit more)
• Pressure-treated softwood (sometimes sold as 'Class 4 larch'): 12–18 years — not the same product, despite the name

That's the boards. The substructure underneath should outlast the boards by 5–10 years if it's done right, which means a typical larch deck in Geneva is usually one or two board replacements through its life, not a full rebuild.

What kills a deck early

1. Joists too close to the ground

The single biggest predictor of an early deck failure. If the joists can't breathe, they rot from below — and you don't see it until a board feels soft underfoot. Ten years can become five.

2. Boards laid tight together

Wood swells in winter rain. If there's no gap between boards, they push against each other, cup, split, or pop their fixings. A 5–7 mm gap is the difference between a deck that ages and one that fights itself for a decade.

3. The wrong fixings

Stainless A2 is the cheapest reasonable option for Geneva; A4 is required if the deck is anywhere near a pool or salt-spray. Anything zinc-plated will leave black streaks down each board within two years and rust through the wood by year five. The screws cost less than the wood, but they decide the lifespan.

4. No annual sweep

Leaves and pine needles trapped between boards keep the joists damp through the whole winter. A ten-minute sweep in November buys years. It's the cheapest piece of deck maintenance there is, and the most consistently skipped.

What lengthens a deck

Same logic, mirrored:

• Joists raised at least 80 mm off the ground, on adjustable pads, with airflow underneath
• A geotextile membrane below the deck so weeds don't push up against the joists
• Stainless fixings, pre-drilled, recessed just enough that water doesn't pool around the heads
• A spring sweep, a wash, and a re-oil every 2–3 years (not every year — over-oiling is its own problem)
• Boards laid bark-side-up so cupping pushes water off rather than holding it

The honest range

A larch deck I'd build today, on a properly raised substructure, with stainless fixings, in a Geneva garden — I'd expect to look at it again in 22 to 28 years, and I'd be a little disappointed if it needed real work before then.

So when someone tells you a deck will last fifteen years, ask them how it's being built. The answer to that question is what determines the actual number — not the wood, not the wood, not the wood.