Timber decking, planking, fencing and pergolas are classic landscape engineering. Here are the pressure-impregnation classes (NTR A/AB/B), frost-free post foundation, beam and joist spacing, ventilated assembly, and choice of fasteners and fittings in the correct corrosion class.
Introduction
Timber decks, boardwork, fences, pergolas and carports are a regular part of the landscaper's work. Wood exposed to weather and wind rots if chosen and built incorrectly — therefore good timber work is about choosing the right durability class, founding posts frost-free and dry, building ventilated and using screws and fixings that don't rust.
Pressure impregnation and NTR classes
Pressure-treated timber is pressed with wood protection, so it can withstand moisture and decay. The Nordic marking NTR (Nordic Wood Preservation Council) indicates the class, which corresponds to a use class to DS/EN 335. Choose the class according to how exposed the timber is:
| NTR-class | Use class (EN 335) | Application |
|---|---|---|
| NTR M | Use class 5 | Sea water / marine environment (wharf, bathing pier in salt water) |
| NTR A | Use class 4 | Soil and freshwater contact, load-bearing outdoors — posts, sills, bedding on ground |
| NTR AB | Use class 3 | Above ground, weather-exposed without ground contact — deck boards, cladding, fence boards |
| NTR B | — | Windows and external joinery (cladding, not bearing externally) |
Penetration and uptake of wood preservative are specified in DS/EN 351. Alternatives to impregnation are heartwood of naturally durable species (larch, oak, locust), thermally modified timber or composite — but each has its limitations and must still be built with ventilation.
Footing of posts
Posts for fencing, pergolas and raised terraces must stand above the frost line and dry, otherwise they are lifted by frost or rot at the base. Two things determine lifespan: the base does not stand in water, and the foundation reaches below the frost line.
- 01Dig below frost line: posts are free-standing and unheated, so typically about 1.2 m frost-free depth is calculated (the ~0.9 m applies only under heated buildings) — verify against geotechnics/municipal requirements for the specific location.
- 02Set the post in a post shoe/pile shoe on a concrete foundation so the timber is held free of soil and water — this significantly extends its life.
- 03Casting directly into concrete traps water at the post base and often rots faster; chosen only with NTR A timber and drainage under the post.
- 04Set the posts plumb and in line and brace until the concrete has cured.
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