Some wood species are more naturally durable than others. Durability of hart wood is specified in the European Standard EN 350. Sapwood has no durability in any species and has to be removed. The variance in physical properties will require detailed knowledge regarding the specific wood product, how it is sawed and handled. No Nordic wood species is durable to used in Use Class 3 or more.
When buying or specifying wood that will be used outdoors or will be exposed to tough environments such as moisture or biological attack, there should be as an assessment whether a preservative treatment is required according to building and construction codes, or whether the treatment could increase service life and reduce economic and environmental costs. The level of protection required depends on a number of aspects:
- The timber species – its durability and ability to accept treatment
- How and where the timber component is to be used – fence post, cladding, etc
- Exposure scenario – geographical, weather, sun, use, wear and tear
- How long the component needs to perform before it is either replaced or taken down – usually 15, 30 or 60 years.
- Safety, and maintenance – access to the component or project site in addition to cost restrictions and service intervals.
The minimum standards for wood treatment are set out in NTR wood protection classes and the European standard EN 351-1 and EN 335. These standards consider all of the above aspects and give guidance on the loading and penetration of timber preservative, to ensure treated timber is fit for its intended use.
Guidance and building codes often refer to Use Class or NTR wood protection classes. One typical example is the specific requirements of load bearing structures and service life.
Durability of wood and wood-based products is grouped into NTR-wood protection Classes. The most commonly used classes for preservative treatment are NTR A, NTR AB, NTR B and NTR GRAN.
How to specify treated timbers
The main consideration when specifying preservative treatments for timber is its intended use. The NTR wood durability system and European standard EN335 set out the following Use Classes for treated timbers:
1. What Use class / NTR Wood Protection Class is required?
Establish the specific use class that is required in the current project. There could be more than one class in the project, depending on construction and exposure.
NTR B / Use Class 3.1 Coated Exterior timbers – used above ground contact and with an appropriate and maintained coating – for example: cladding
NTR AB / Use Class 3.2 Uncoated Exterior timbers – used above ground contact without a coating – for example: external cladding, decking, garden timbers
NTR A / Use Class 4 Exterior timbers – Wood in contact with the ground or fresh water or severely exposed to the weather; or if wood component is inaccessible or where the consequences of failure will be particularly serious.
NTR M / Use Class 5 Timbers used in seawater contact
See detailed description of NTR- End Use
2. Find timber that meets the specified use class
2a. Type of wood: a specific wood species or preservation?
Some wood species are naturally more durable than others. Durability of heartwood is specified in the European Standard EN 350. Sapwood has no durability in any species and has to be removed. The variance in physical properties will require detailed knowledge regarding the specific wood product, regarding how it is sawed and handled. No Nordic wood species is durable to use in Use Class 3 or more.
With wood preservative or modification all use classes can be met.
2b. Documentation for specified use class
With the right documentation a timber product is fit for use in the specified use class. However, the scientific definition of a product for a given use class is a complicated process. There are a number of things to include in a proper documentation regarding wood durability:
- Documentation proving sapwood penetration (according to EN 351-1)
- Proof that the wood treatment chemical in use is effective. This includes laboratory test results and field test results.
- Production control, CE Mark and certification from an ongoing quality scheme, including third-party industrial control.
All this is included in the NTR wood durability quality system and the NTR label is used to mark wood products that are produced according to NTR standards.
3. Compare alternative materials
Time, money and sustainability are the main concerns when selecting building material.
Start with defining the expected service life and index the cost to this time. It’s also important to specify the costs of the entire product lifecycle, such as material, construction, transport, maintenance and disposal.