Use classes and pressure impregnation of wood
From dry indoor climate to ground contact — how wood is protected against moisture and rot
Wood is strong and durable as long as it is kept dry. Problems arise when moisture content is high enough for rot fungi to grow for extended periods. Therefore a key part of material science is assessing how wet the wood will get in the specific construction — and choosing a protection that fits the load, neither more nor less.
§The three application classes
To describe the wood's moisture effect, three classes of use are used. They control both how durable the wood must be and what calculation values you are allowed to use in a load-bearing structure, because moist wood is weaker than dry.
| Use class | Environment | Example |
|---|---|---|
| Class 1 | Dry, heated indoor climate | Internal walls floors ceilings |
| Class 2 | Covered but damp/unheated | Ventilated roof structure, carport |
| Class 3 | Exposed to weather or soil contact | Terrace, fence, external cladding |
§When should the wood be impregnated?
In use class 1 and 2 ordinary dry construction timber does well as long as it is kept ventilated and can dry out again after moisture exposure. It is in use class 3 — where timber is exposed to rain prolonged moisture or direct ground contact — that you normally should use pressure-treated timber or a naturally durable timber species. If you choose untreated spruce for a post in the ground it can be rotten in a few years.
§NTR classes
Pressure-treated wood is marked according to the Nordic NTR system, which classifies wood according to the severity of moisture load the treatment is designed for. The heavier the load, the more preservative is pressed into the wood.
- 01NTR class A – for wood in and near soil, in fresh water and for structures with safety risk; thoroughly impregnated
- 02NTR class AB – for cladding, facade and fences without ground contact; thoroughly impregnated but with less uptake than class A
- 03NTR Spruce – spruce has a closed pore structure and is impregnated only at the surface; used as priming before subsequent surface treatment
§Constructive solutions instead of chemicals
The best protection is often constructive. By keeping wood away from the ground, ensuring drainage, roof overhangs and ventilation, in many cases you can use a lower use class and avoid impregnation. A post on a galvanized post shoe rather than cast directly into the ground lasts much longer — and a good carpenter always thinks about how water gets away from the wood.
Matching wood species, impregnation and construction to the correct application class is both good economics, good craftsmanship and good environment. It is an assessment that the carpenter, floor layer and thatcher must each be able to make in their own field.