The wooden stave church of Borgund, in Norway, has been standing for about eight centuries. The wood is pine, not among the most durable, but it has been kept dry by wide overhangs, a stone base, shingles to be renewed and tar. The same wood left wet decays in a few years. The durability of wood is a question of water before it is a question of species: below about 20 per cent moisture content decay fungi do not develop, and most damage in buildings starts from a detail that allowed water to arrive and stay.
The use classes of UNI EN 335
UNI EN 335 classifies the environment in which the wood works into five use classes. The class establishes which organisms are present and therefore what durability or treatment is needed. It should not be confused with the service class of Eurocode 5, which concerns mechanical strength and equilibrium moisture content.
| Class | Situation | Example |
|---|---|---|
| 1 | indoors, always dry | floor joists in a heated room |
| 2 | indoors or covered, occasional wetting | ventilated attic, structure under a canopy |
| 3.1 | outdoors above ground, protected, short wetting | covered pergola, cladding with a wide overhang |
| 3.2 | outdoors above ground, exposed, prolonged wetting | decking, unsheltered cladding and beams |
| 4 | contact with ground or fresh water | posts, fences, walkways in wet areas |
| 5 | contact with salt water | jetties, piles in the sea |
Natural durability
EN 350 classifies the resistance of heartwood to fungi in five classes, from 1 (very durable) to 5 (not durable). It applies to heartwood, the darker central part of the trunk. Sapwood, the lighter outer band, is in class 5 for all species. In practice, a species in class 3 or 4 can stay in use class 3.1 if well detailed, but not in class 4 without treatment.
| Species | EN 350 class | Outdoor use without treatment |
|---|---|---|
| Teak | 1 | suitable even for exposed outdoor use |
| Black locust | 1-2 | suitable for posts and furniture |
| Sweet chestnut | 2 | suitable above ground, fair in the ground |
| Oak | 2 | suitable above ground, fair in the ground |
| Larch | 3-4 | above ground, with good details |
| Douglas fir | 3-4 | above ground, with good details |
| Scots pine | 3-4 | only if treated |
| Norway spruce and silver fir | 4 | only if sheltered or treated |
| Beech | 5 | indoors only |
Fungi and insects
Decay fungi are divided into brown rot, which consumes cellulose and leaves the wood brittle and cracked into cubes, white rot, which also attacks lignin, and soft rot, typical of very wet wood in contact with the ground. The most feared case in buildings is dry rot (Serpula lacrymans), the house fungus, because it carries water through its mycelial strands and can attack apparently dry wood far from the source of moisture.
The most common insects are the house longhorn beetle (Hylotrupes bajulus), which attacks the sapwood of conifers, the common furniture beetle (Anobium punctatum), found in old, poorly ventilated members, and subterranean termites (Reticulitermes), also present in Italy and requiring specific interventions. Dry, well-ventilated and periodically inspected wood is the most effective way to keep them away. Exit holes with fresh frass show an attack in progress.
Treatments and modified wood
Preservatives are applied differently according to the use class. By brush or spray in use classes 1-2, with boron salts for interiors. For classes 3 and 4 a pressure treatment in an autoclave is needed, which carries the product deep into the wood (penetration is classified with EN 351). Autoclave treatments, however, complicate end of life, because wood treated with some preservatives cannot be recycled as clean wood.
Modified wood changes the structure of the material to increase durability without biocides. Thermally modified wood (180-230 °C) becomes more stable and less prone to attack, but loses bending and impact strength, so it is not used for load-bearing members. Acetylated wood (such as acetylated radiata pine, Accoya) and furfurylated wood (such as Kebony) improve durability and dimensional stability, with durability classes declared by the producer up to class 1. They cost more than autoclave treatments, but are used without biocides for cladding, windows and outdoor decking.
The construction detail as protection
In the German-language technical tradition we speak of constructive wood protection, and the rule is that it always comes before chemical protection. The measures are simple and must be decided at design stage. Eaves overhangs keep rain away from facades. Structures are lifted off the ground by at least 15-20 cm. The ends of beams, which absorb water many times faster than the side faces, must be protected or ventilated. Bearings on damp masonry are made with an impermeable layer and an air gap. In external cladding a ventilated air cavity is left behind the boards. Metal plates are kept off the wood so that condensation forming on the metal does not stagnate.