HOW TO TREAT TIMBER

Natural durability and treatment considerations Timber is a variable material; different species have different properties. Some timbers, typically many hardwoods, are more naturally durable and more resistant to decay, fungal or insect attack. However, some softwoods contain natural chemicals that can also make them more resistant.

Because softwoods are inexpensive, readily available and renewable, they are popular for general construction work. Depending on where they are needed in a building, some will require additional protection by using a preservative treatment.

TYPES OF TREATMENT

  • Chemical wood preservatives
  • Vacuum, high pressure treatments
  • Double vacuum, low pressure treatments

  1. The chemicals used in wood preservatives contain biocides that are specifically targeted against the organisms they control and are less hazardous towards the wider environment, they also comply with current EU regulations. There are two main types of pre-treatment processes, both carried out by timber suppliers, merchants or joinery companies, in enclosed and strictly controlled industrial vessels.
  2. Vacuum, high pressure treatments are most suitable for the full range of timber end uses. They are particularly relevant for external applications, both in and out of ground contact – Use Classes 1 to 4 – providing a service life protection ranging from 15 to 60 years. They force the preservative deep into the cellular structure of the timber and generally result in a pale green colouration to the finished component. Additives are available that can give either a rich brown colouration, usually for rough sawn fencing and landscaping timbers, or an effective extra water repellency protection for decorative external timbers, such as decking and cladding timbers.
  3. Double vacuum, low pressure treatments can be used for building and joinery timbers in Use Classes 1, 2 and 3.1, delivering a 30-60 year service life protection. Treatment provides an effective ‘envelope’ protection around the timber and leaves the colour of the timber virtually unchanged. A colour indicator, as well as water-repellency, can be added to the treatment if required.

Use Classes and desired service life

The end use of treated timbers is classified into one of the four main categories shown in the table below. These Use Classes are based on the potential threat to the timber from decay or insect attack in its end-use application. For instance, internal building timbers in Use Classes 1 & 2 will be less vulnerable than timbers used externally in ground contact (Use Class 4). Therefore, they will require a lower level of protection than Use Class 4 timbers.

USE CLASS SUMMARY

1 Internal, dry (e.g. upper floor joists)

2 Internal, risk of wetting (e.g. tile battens)

3.1 Outdoors, coated, above ground (e.g. window frames*)

3.2 Outdoor, uncoated above ground (e.g. fence panels)

4 Direct soil or fresh water contact (e.g. fence posts)

*Exterior woodwork should be protected with an appropriate surface coating after preservative treatment, then regularly maintained

How naturally durable are timber species?

Many hardwood species are naturally durable and can be used outdoors untreated, but they are expensive and supplies of certified timber are limited. European oak has been re-classified as Durability Class 2-4; if you are using it outdoors, make sure it meets Durability Class 2-3.
Some softwoods are relatively durable, but most will need preservative pressure-treatment if used outdoors or in humid conditions. They are inexpensive, and certified supplies are plentiful.
Modified timbers, such as Accoya™, provide durability with sustainability, but are expensive and may need specialist stainless steel fixings.
Cutting or notching will expose untreated timber, which should be treated with a generous coat or two of brush-applied end-grain sealer or preservative.
Treated timber - fitness for purpose (Based on BS EN 335:1, which defines the treatment Use Class for different applications)
Use ClassService situationApplication examples
1Internal: permanently dryPitched roofs; floor boards; architrave; internal joinery; timbers in upper
floors not built into solid external walls.
2Internal: occasional wettingTiling battens; frame timbers in timber frame houses; timber in pitched roofs with high condensation risk; timbers in flat roofs; ground floor joists; sole plates (above dpc); timber joists in upper floors built into external walls.
3c (coated)External: out of ground, frequent wettingExternal joinery; roof soffits, fascias and bargeboards; cladding, valley gutter timbers; external load bearing timbers.
3u (uncoated)External: out of ground, frequent wettingFence rails and boards; gates; agricultural timbers not in soil contact;
landscaping and decking timbers not in contact with the ground.
4External: in permanent ground or water contactFence and deck post; gravel boards; agricultural timbers in soil; earthretaining walls; poles; sleepers, playground equipment; lock gates; jetties and boardwalk support.
5External: in permanent ground or water contactMarine piling, piers and jetties, dock gates, sea defences, ships hulls.

Treatment processes and preservative chemicals

The chemicals used in wood preservatives comply with current EU regulations. They contain specifically targeted biocides designed to present a minimum hazard to the wider environment. There are two main types of pre-treatment process, both carried out by timber suppliers, merchants or joinery companies in enclosed and strictly controlled industrial vessels.

Vacuum, high-pressure treatment

Suitable for the full range of end uses, but particularly for external applications. Both in and out of ground contact, it provides a 15 to 60-year service life. The preservative is forced deep into the cellular structure of the timber, typically giving a green tint. Additives can give either a rich brown colour, usually for fencing and landscaping timbers, or extra water repellency for decorative external timbers, such as decking and cladding timbers.

Double vacuum, low-pressure treatment

Used for building and joinery timbers in Use Classes 1, 2 and 3c to deliver a 30 to 60-year service life. Treatment provides an effective ‘envelope’ protection around the timber and leaves the colour virtually unchanged. A colour indicator, as well as waterrepellency, can be added to the treatment if required.