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 eventual 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 (or Use Class 3.1)
Exterior timbers – used above ground contact and with an appropriate and maintained coating – for example, cladding
Use Class 3 Uncoated (or Use Class 3.2)
Exterior timbers – used above ground contact without a coating – for example, fence rails
Use Class 4 (and Use Class 4SP)
Exterior timbers – used in permanent ground or fresh water contact – for example, fence posts
Use Class 5
Timbers used in sea water contact
Generally the Use Class increases where there is a greater risk of attack from decay organisms. For instance, timbers in Use Class 1, which are under cover within a building with no chance of wetting, are much less likely to be attacked than Use Class 4 timbers that are used externally and are permanently in ground contact.
Within Use Class 4 some markets identify a special treatment level where extended service lives than the standard (usually 15 years) are required, usually up to 30 years. Preservative treatments are not allowed for Use class 5 sea water applications. These are usually managed through the use of specialist hardwoods or other construction materials.
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, engineered timbers, poles and sleepers.
High Pressure Treatments
High pressure preservative treatments can generally be used for all Use Classes from 1-4. 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.