Making preservative-treated timber last longer

The Wood Protection Association explains why cross-cut treated wood products must have their cut ends treated to maintain their integrity and durability. 

Preservative treated, softwood construction components such as Pine or Spruce have been through a factory controlled, industrial treatment process. This results in the timber having an effective barrier of protection against decay and insect attack. It provides added durability, extending the service life of the timber and allowing plentiful, fast grown species to be used in areas that wouldn’t ordinarily be practical.

The penetration and retention of wood preservatives are defined in British Standards and are aligned to their end use through the Use Class system.

Re-working treated timber

Treated products are designed to be installed without modification. For this reason, re-working pre-treated timber during installation should be avoided if possible. Treated wood must NEVER be rip sawn along its length.

However, certain components do need to be cross-cut to size – the fact that timber is easily worked is one of its advantages. But by cross-cutting, or even drilling, boring or notching the treated timber, you are likely exposing the timber’s untreated core and breaking the ‘envelope’ of preservative protection. However, there is an easy solution.

A cross-cut preservative treated post showing extent of formulation penetration.

Cut-end brush-on treatment

You can maintain the integrity of the pre-existing treatment by giving each cut end two liberal brush coats of a suitable end grain wood preservative or protector. This also applies to areas which have been bored, drilled or notched – for example to accept fixings.

Your timber supplier should be able to recommend a suitable product to use, which may well originate from the manufacturer of the preservative used in the treatment.


Post installation

It’s important to remember that preservative penetration achieved by brush application is less effective than that achieved by the industrial treatment process. So even if an end grain preservative has been applied to a cross-cut end of a treated post, this end must NEVER be embedded in the ground.

Instead, put the un-cut end in the ground. Mechanical incising of the surface is now used widely to help achieve desired preservative penetration levels. Some fence posts only have incising along part of their length and, in such circumstances, the un-cut, incised end of the post must be placed in the ground.

If a post does needs shortening, then cut the top (at an angle to shed water), apply two coats of brush-on treatment and fit the cut top with a post cap to reduce water penetration into the end grain.

If you need to source a cut-end product, you can contact the Wood Protection Association online at