When pressure preservative treated, timber can provide decades of reliable service exposed to the harshest of climates. Southern Yellow Pine is the preferred species when pressure treatment with wood preservatives is required. The unique cellular structure of Southern Yellow Pine permits deep and uniform penetration of preservatives, rendering the wood useless as a food source for fungi, termites and micro-organisms.

Common outdoors applications for preservative treated timber include:

  • Decks
  • Boardwalks
  • Bridges
  • Marinas & Piers
  • Cladding
  • Playgrounds

Structural timber products are generally bought, sold and specified for end use according to species combinations with design values assigned for each species. Adjustment factors can be applied to structural timber to calculate the performance of different sized members in end use. These are published by various governmental agencies in destination countries. American softwoods comply with BS 5268 Pt 2: 1996 and EN 1912 “Structural Timber – Strength Classes.”


Use Class Guide

For outdoor use, it is important to specify the correct timber Use Class and appropriate preservative retention level in order to prevent rot and decay. The level of preservative needed is determined by the end use of the wood. For example, timber used for swimming pool surrounds will require a higher level of preservative treatment than ground floor joists.

Use Class Guide


Use Class Categories

To make it easier to select timber with the correct level of treatment, timber is categorised into four Use Classes dependent upon its end application.

Use Class 1:

Situation in which the wood or wood-based product is under cover, fully protected from the weather and not exposed to wetting.

Use Class 2:

Situation in which the wood or wood-based product is under cover and fully protected from the weather, but where high environmental humidity can lead to occasional but not to persistent wetting.

Use Class 3:

Situation in which the wood or wood-based product is not covered and not in contact with the ground. It is either continually exposed to the weather or is protected from the weather but subject to frequent wetting.

Use Class 4:

Situation in which the wood or wood-based product is in contact with the ground or fresh water and thus is permanently exposed to wetting.