Different Use Classes

There are 4 use classes of treated timber, starting from 1 being the least resilient to 4 being the longest lasting and best protection. The stages are featured in the table below which shows what you can expect from each use class.

Use Class Table

The Kiln Drying Process Prior To Preservative Treatment

The preservative in UC4 treated timber is the same, but the process prior to treatment is changed to enable more of the treatment to penetrate deeper into the timber; the timber will be dried in a kiln to reduce the moisture content down to around 28% and in some cases, less. The extent of moisture reduction can be identified through a number of moisture measurement methods.

Moisture content is expressed as a percentage of the weight of the moisture timber contains, compared to it’s completely oven dry weight (all possible moisture is lost). The moisture present in wood can weigh more than the dry weight of the wood; therefore it is possible to have a moisture content over 100%, which is termed as free water, when it is freshly cut. The kiln used, in basic terms, is a large metal shed with fans and heaters.

The drying process is computer controlled, by a proven tested method to ensure the removal of free water from the timber. When completed in this manner it preserves the structural integrity of the timber. Due to the slow process of drying the timber there is no splitting or warping caused. To give an idea of the speed, it takes 16 hours to get the kiln to the required temperature to start drying and can take 5/6 days in the kiln for the timber to reach the desired moisture content. The process can be aided by air drying the post first to shorten the time needed in the kiln.

The Final Resulting Treated Timber

Upon completion of this process the wood will be left for 48 hours to dry before delivery. It is important to remember that the preservative treatment is a seal, a barrier to the moisture getting into the post. Therefore if any cross cut, notch or mortice is made in to the post the treatment may fail; as the barrier has been broken.

Therefore we must remind you of the following for any ground contact timbers:


  • A cross cut end of a treated post must never be embedded in the ground.
  • Any cuts or notches into the treated post should be re-coated with two liberal brush coats of endcoat
  • Treated wood must never be rip sawn along its length.
  • If concreting the post in the ground, there must be sufficient drainage; so do not create a boot for the water to collect in.