Timber for Outdoor Use Compared: Spruce, Larch, Douglas Fir, Oak & Pressure-Treated (KDI)
Whether it is a pergola, carport or fence: the choice of species decides whether your structure lasts ten years or forty. This comparison walks you through durability classes, realistic price ranges and the typical uses of spruce/KVH, larch, Douglas fir, oak and pressure-treated timber.
What really matters outdoors
Outdoors, timber is under constant stress: rain, UV, temperature swings and, above all, persistent moisture. Once the moisture content stays above roughly 20% for any length of time, wood-destroying fungi get to work. How well a species resists this naturally is described by its durability class to EN 350 – from Class 1 (very durable) to Class 5 (not durable). Important: the rating only ever applies to the heartwood. In practically every species the sapwood is Class 5, larch and oak included.
The second key measure is the use class (UC) to EN 335. It describes how heavily a component is exposed to moisture:
- UC 1: interior and dry – any species works here
- UC 2: covered, only occasionally damp, such as rafters under a large roof overhang
- UC 3: fully weather-exposed but no ground contact – pergola rails, cladding, fence palings
- UC 4: permanent ground or water contact, such as buried posts or raised beds
As a rule of thumb: for UC 3 the timber should be at least durability Class 3, or protected by design. For UC 4 you need Class 1–2 – or pressure treatment.
The five candidates, up close
Spruce and KVH: the budget standard – but only when protected
Spruce is the bread-and-butter timber of structural carpentry. As structural solid timber (KVH) it comes kiln-dried (moisture content 15% ± 3%), dimensionally planed and finger-jointed in lengths up to 13 m. The downside: at durability Class 4, unprotected spruce is finished outdoors after just a few years. Under a solid roof, though – a carport, covered terrace or roof structure – it is the first choice, because it is cheap, light and easy to work. As a guide, a 12 × 12 cm post costs around €8–12 per linear metre, with cubic-metre prices of roughly €450–650.
Larch: the classic for weather-exposed components
European larch reaches durability Class 3–4 in the heartwood and is noticeably more resinous and harder than spruce. Siberian larch grows more slowly, is therefore denser and tends to sit at Class 3. Left untreated, larch weathers over the years to a silver-grey tone – a purely cosmetic effect, not damage. Typical uses are pergolas, decking boards, cladding and fences. As a guide, reckon on €12–18 per linear metre for 12 × 12 cm, or €700–1,000 per cubic metre.
Douglas fir: the dimensionally stable alternative
Douglas fir, at heartwood Class 3, sits at a similar level, but is a little lighter, more dimensionally stable and usually cheaper than Siberian larch. Its warm, reddish tone makes it especially popular for decking. A word of warning with both species: substances such as tannic acid and resin react with galvanised steel. Use stainless-steel screws (A2, or A4 near water), otherwise black streaks form around every fixing.
Oak: maximum durability, maximum demands
Oak heartwood reaches durability Class 2 and lasts many decades fully exposed – timber-framed houses have proved it for centuries. In return, oak is heavy (around 700 kg/m³), moves a great deal and its tannic acid attacks galvanised connectors. Pre-drilling is a must, otherwise even heavy screws will shear off. On price, oak is in a league of its own: €1,500–2,500 per cubic metre is a realistic guide for structural stock. It makes sense where looks and lifespan count: exposed timber framing, sole plates, gate posts or showpiece carport posts.
Pressure-treated timber (KDI): cheap when looks come second
In pressure treatment, pine (less often spruce) is impregnated with protective salts under pressure. Depending on the uptake, the result is suitable for UC 3 to UC 4 – so even for ground contact. Pressure-treated timber is by far the cheapest: 12 × 12 cm starts at around €7–10 per linear metre. The drawbacks: the greenish or brown colour, frequent drying cracks, and the need to re-treat every cut end with preservative, because the treatment only penetrates a few millimetres deep. Pressure-treated timber must be disposed of at a recycling centre – it should never go on a bonfire or into a wood burner.
Head-to-head: durability, price, use
All figures are a guide (planed structural stock, excluding delivery, as of 2026). Regional prices and grades can vary considerably.
| Wood species | Durability (heartwood) | Price €/m³ | 12 × 12 cm, €/lin. m | Typical use |
|---|---|---|---|---|
| Spruce / KVH | Class 4 | 450–650 | 8–12 | Anything under a roof: roof structure, carport, covered terrace |
| Pine, pressure-treated (KDI) | suitable for UC 3–4 through treatment | 400–600 | 7–10 | Fence posts, raised beds, components with ground contact |
| Douglas fir | Class 3 | 650–950 | 11–16 | Decking, pergola, cladding |
| Larch (European/Siberian) | Class 3–4 / 3 | 700–1,000 | 12–18 | Pergola, cladding, fence, decking |
| Oak (heartwood) | Class 2 | 1,500–2,500 | 30–50 | Timber framing, sole plates, exposed structures |
Which timber for which project?
- Carport or covered terrace: spruce/KVH for everything under the roof; run only the weather-exposed post bases and exposed edges in larch or Douglas fir.
- Pergola (fully exposed): larch or Douglas fir – posts 12 × 12 cm, beams 8 × 16 cm, mounted on post anchors, never in the ground.
- Fence and raised bed: pressure-treated pine if price matters; larch if the timber is allowed to weather grey untreated.
- Decking boards: Douglas fir (smooth, 28 × 145 mm) as an affordable entry point, Siberian larch for heavier use.
- Exposed timber framing, sole plates, entrance areas: oak – expensive, but lasts for generations.
Protection by design beats any species
Even the most durable species fails if water is left standing permanently – and even spruce lasts decades if it stays dry. The key rules:
- No direct ground contact: set posts on H-anchors or post feet, with at least 15 cm clearance from the ground.
- Protect the end grain: fit post caps or bevel the top faces by about 15° so water runs off; avoid horizontal cut faces.
- Mind the splash zone: allow around 30 cm between ground level and any unprotected timber.
- Decouple contact surfaces: provide a gap or an EPDM strip between joist and bearing so moisture pockets can dry out.
A note on structural design and permits: all cross-sections and prices in this article are a guide. Whether your project needs permission and which cross-sections will actually carry the load depend on your local building regulations and structural design – if in doubt, check with your local building authority and have load-bearing structures verified by a structural engineer.
Conclusion: settle the use class first, then ask your wallet
First work out how exposed your component will be, then pick the species: spruce/KVH under a roof, larch or Douglas fir for fully exposed structures, pressure-treated for ground contact on a tight budget, and oak for components meant to outlast generations. Mixing species is perfectly legitimate, too – many carpenters combine a KVH frame under the roof with larch on the weather sides. Tip: in a 3D planner like HolzBau 3D you can assign a cross-section and length to every component and get a cut list – so you can price the larch option against pressure-treated in a few minutes before you order.