UC4 Timber Preserving Treatment

The History Behind UC4 Treated Timber

Since the change in treatment some believe that this affected the life span of treated timber, especially when in ground contact, due to what appeared to be many failings of posts. But, according to the Wood Protection Association, there has been no change in the number of samples that have been returned for premature failure analysis. Since 2004 analysis of posts that have failed after just a few years service have included examples of CCA and Copper Chrome preservatives as well as the so called “new generation” Copper-organic formulations; reinforcing the view that it is not the efficacy of the “new generation” preservative products that is in question.

No wood preservative can be expected to deliver a BS8417 service life if the treatment is incorrect, or, if wood with too high a moisture content is sent for treatment. Similarly if the products integrity is compromised with bad end user practice.

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 Timber Preserving Process

Stage 1 - The timber is loaded into the treatment vessel.

Stage 2 - Initial vacuum applied and the timber cells are evacuated of air. Vacuum held.

UC4 Stage 1 UC4 Stage 2

Stage 3 - Cylinder flooded under vacuum with preservative.

Stage 4 - Hydraulic pressure applied, forcing the preservative deep into the timber.

UC4 Stage 3 UC4 Stage 4

Stage 5 - Final vacuum extracts excess preservative solution, which is pumped back to storage.

Stage 6 - The low pressure inside timber draws in surface solution when vented to atmosphere. Treated timber is left to dry.

UC4 Stage 5 UC4 Stage 6

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.

  • The same endcoat will be used in our workshops as available for AVS customers to buy; always remember if cutting or notching yourself, endcoat should be used to re-treat any cuts.


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