When buying or specifying wood that will be used out-door or is expected to be exposed to moist or biological attack or other tough environment it is necessary to asses if a preservative treatment is required according to building and construction codes or could be used to increase service life and reduce the total economical and environmental cost. The level of protection required depends on a number of aspects:
- The timber species – its durability and ability to accept treatment
- Where the timber component is to be used – eg, fence post, cladding etc,
- Exposure scenario – geographical, weather, sun, use, wear and tear
- For 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 the treatment of wood are set out in NTR wood protection classes and is also defined in the European standard EN 351-1 and EN 335. These standards consider all of these elements and gives guidance on the loading and penetration of timber preservative, to ensure treated timber is fit for its desired end use.
Guidance and building codes often refer to Use Clas 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 being NTR A, NTR AB, NTR B and NTR GRAN.
HOW TO SPECIFY TREATED TIMBERS
Some softwoods species are more naturally durable than others and in some situations a preservative treatment may not be required at all. However, to ensure an extended and trouble free service life for general construction and landscaping timbers, controlled, industrially applied treatments are the best choice.
These treatments are available through a wide network of timber companies, sawmills, joinery manufacturers and specialist treatment organizations through Europe. Ready treated supplies of preservative treated timbers for many end markets are also readily available on the European markets under a wide range of product brands.
The main consideration when specifying preservative treatments for timber is its intended end use. European standard EN335 defines several Use Classes for treated timbers and these are shown below.
Use class summary
- Use Class 1 Interior timbers – no risk of wetting – for example, upper floor joists
- Use Class 2 Interior timbers – risk of wetting – for example, tile battens
- Use Class 3 Coated Exterior timbers – used above ground contact and with an appropriate and maintained coating – for example, cladding
- Use Class 3 Uncoated Exterior timbers – used above ground contact without a coating – for example, fence rails
- Use Class 4 Exterior timbers – used in permanent ground or fresh water contact – for example, fence posts
- Use Class 5 Timbers used in sea water contact
See detailed description of NTR- End Use
Low Pressure Treatments
Use Classes 1-3 coated timbers can be effectively protected using low pressure/double vacuum treatment process and in some markets controlled immersion technologies. These treatments use water based micro emulsion type preservative products that leave the appearance almost unchanged. However colour dyes or pigments can be added at the point of treatment to aid treatment identification. If these treated timbers are used externally then they will require a suitable and well maintained surface coating protection to maintain the preservative protection.
General building timbers, claddings, truss rafters, carcassing and timber frame material, joinery components.
High Pressure Treatments
High pressure preservative treatments can generally be used for all Use Classes from 1-5. These treatments utilise copper organic based products that generally leave the treated timbers with a pale green colouration that slowly weathers to a silver grey over time. Again colourants can be added at the time of treatments to produce different treated timber options for different markets. Water repellents can also be included to add extra weathering protection to decorative external timbers. Creosote based treatments are also still an option for some heavy duty timber markets.
General building timbers, fencing, decking and landscaping timbers, playground timbers, sleepers, transmission poles, engineered timbers.
Controlled dipping treatments are generally utilized where anti-sapstain protection is required for freshly harvested and sawn timbers. In some markets dipping treatments can also be used for Use Class 1, 2 and 3 coated construction timbers.
Freshly harvested timbers, pallet and packaging timbers, general construction timbers.