What matters with exterior timber
Outdoors, service life is not decided by strength but by durability — resistance to fungi and insects. And that sits in the heartwood. The sapwood, the outer zone of the trunk still carrying sap, is barely durable in practically every species. A decking board with sapwood does not deliver what the species promises.
Three routes lead to durable exterior timber. First, species with naturally durable heartwood — among native softwoods above all larch and Douglas fir, plus oak and overseas hardwoods. Classification follows EN 350, in durability classes 1 (very durable) to 5 (not durable). Second, modified timber, where resistance is raised by physical or chemical processes without biocides; thermally modified timber is the best-known example. Third, structural timber protection.
The third route is the most important. The best species will not last if the detailing traps water.
Use classes: the key to design
The central task in design is assigning every component a use class to DIN 68800-1. It describes the moisture exposure — and from that follows which species and which detailing are permissible at all.
Very different classes can occur within the same structure:
- Load-bearing timber decking normally sits in UC 3.2. If dirt accumulation is expected, it becomes UC 4.
- Decking under a roof can instead be classified UC 0 or UC 1 — which changes the material choice completely.
- Exposed main beams reach UC 3.1 with a capping and sufficient overhang, UC 3.2 without protection.
- Columns reach UC 3.1 where the end grain is at least 300 mm clear of the ground. With a gravel bed of 16/32 grading and at least 150 mm wide from the outer face, that clearance can be reduced to 150 mm.
Even directly weathered columns can be placed in UC 0 — but only under tight conditions: kiln-dried timber with a planed surface, sections limited to 16/16 cm in heart-separated solid timber and 20/20 cm in glulam, detailing free of standing water, capped end grain, and a column base free of standing and splash water.
A detail that often goes wrong: the fixings
For decking and cladding, stainless steel A2 is normally sufficient. With tannin-rich species such as oak or ekki it must be acid-resistant A4 — otherwise the fixing corrodes and the timber blackens around every screw head. In all other cases fixings must be at least galvanised.
Advantages
- Achievable without biocides. With the right species and clean detailing no chemical protection is needed. Where external cladding follows the trade rules, preventive treatment is required neither for the substructure nor for the cladding.
- The façade protects what lies behind it. It keeps driving rain off the structure and insulation, working for the whole building.
- Individually replaceable. A damaged board is exchanged, not the whole surface.
- Renewable material with a far better environmental profile than most alternatives.
- Comfortable underfoot. Timber heats up far less in the sun than stone or metal.
- Repairable. Sanding and re-oiling brings back a deck that other surfaces would need replacing.
- A look that ages rather than wears out. The silver-grey patina is a state, not damage.
Drawbacks
- It greys. Holding the original colour means regular maintenance — permanently, not once.
- The timber moves. Weathered components swell and shrink with every change; cracks, slight distortion and shifting joint lines are part of it.
- Slippery when wet and with algae growth, especially in shaded north-facing positions.
- Wide price range. Between native larch and overseas hardwood lie worlds.
- Sapwood content is a real risk and not always visible at first glance on rough sawn stock.
- Documentation duties for imported timber. Origin and legality must be evidenced, which adds effort and lead time.
- The substructure is often underestimated. It must be at least as durable as the surface — otherwise it fails first.
- Balconies require approval. Unlike terraces, which are exempt in most German states, balconies count as structures under state building law and must be designed to the relevant standards.
Applications
- Ventilated timber façades as lap siding, board-on-board or rhombus profiles
- Decking boards on a ventilated substructure
- Balcony surfaces and balustrades
- Privacy screens, fences and trellises
- Cladding of carports, garden houses and outbuildings
- Pergolas and roofed seating areas
- Canopies and ground-level outdoor structures
Where to avoid exterior timber or plan differently
- Ground contact without a suitable species. Posts in soil are the hardest case of all — use post anchors, not timber in the ground.
- Directly on the slab. Without clearance and ventilation from below nothing dries, and the board rots from the underside.
- Façade without a ventilation cavity. Cladding screwed tight onto insulation has no chance.
- Horizontal surfaces holding water. Handrails, balustrade cappings and post tops need a fall or a cover. End grain and joints must stay free of water.
- Places where dirt collects. Leaves and soil store water and keep the timber permanently damp — which is why a soiled deck drops from UC 3.2 to UC 4.
- Sapwood in weathered positions. A species durability rating applies to heartwood only.
- Permanently shaded, damp locations where the surface never dries between two rainfalls.
- Expecting a constant colour. Anyone who does not want patina and will not maintain will not be happy with timber.
The alternative: composite decking
That last point is exactly what leads many clients to WPC — wood-plastic composite. For some situations it is the more honest answer than timber. In detail: Trex composite decking.