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

For load-bearing members, the German standard DIN 68800-1 sets the minimum class for the heartwood: in UC 3, Class 1 to 3 is enough as long as water runs off quickly; where it can stand, it must be Class 1 or 2, and in UC 4 only Class 1 will do. Intermediate ratings such as “3–4” count as the lower class. For non-load-bearing parts this is a recommendation. If the species does not reach the required class, you need protection by design that keeps the member drier – or pressure treatment. With their rating of 3–4, larch and Douglas fir therefore count as Class 4: load-bearing, fully exposed members in these species need protection by design that keeps them drier – such as a cover on top, clearance from the ground and protected end grain.

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. In September 2026, a planed 12 × 12 cm KVH post cost around €11–12 per linear metre from online timber merchants in Germany.

Larch: the classic for weather-exposed components

Larch reaches durability Class 3–4 in the heartwood and is noticeably more resinous and harder than spruce. Siberian larch grows more slowly and is therefore denser, but in field tests with ground contact it did no better than other larches. 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. In the same sample, 12 × 12 cm sections ranged from around €11 to €40 per linear metre, depending on supplier, finish and cut length.

Douglas fir: the dimensionally stable alternative

Plantation-grown Douglas fir, as grown in Europe, sits at the same level with heartwood Class 3–4 (timber from natural stands: Class 3), 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 sits at the top end: 12 × 12 cm sections cost around €25–34 per linear metre in the sample. It makes sense where looks and lifespan count: exposed timber framing, sole plates, gate posts or showpiece carport posts.

Pressure-treated timber (KDI): for ground contact 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. It is not cheaper than untreated KVH, though: planed 12 × 12 cm cost around €17–18 per linear metre in the sample. 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

The prices are a sample from online timber merchants in Germany in September 2026 (single lengths including VAT, excluding delivery); they vary considerably by supplier, finish, region and quantity. Durability classes apply to the heartwood to EN 350.

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 – and with thorough protection by design.
• Fence and raised bed: pressure-treated pine for parts in ground contact; larch only for parts without ground contact, 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:

1. No direct ground contact: set posts on H-anchors or post feet; the underside of the timber sits at least 30 cm above ground level over hard surfaces, or 15 cm with a gravel strip or a surface with at least a 2 % fall.
2. Protect the end grain: fit post caps or bevel the top faces by about 15° so water runs off; avoid horizontal cut faces.
3. Mind the splash zone: allow around 30 cm between ground level and any unprotected timber.
4. 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, 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.

Sources and standards

• DIN Media – source for DIN and EN standards
• Informationsdienst Holz – German timber construction resources
• German Wood Protection Association – durability classes and use classes (German)
• Rapp/Augusta (BFH) – durability of larch and Douglas fir heartwood, DGfH conference 2000 (German)
• Informationsdienst Holz – timber protection for engineered timber buildings: clearance of exposed post bases above ground

Standards are not free to read and are named here for verification only. What applies is always the edition in force at the building location.