Use Class 4 treated timber: specifying, buying and installing
Preservative treatment provides wood with added durability (against decay and insect attack). But, it’s a mistake to assume that all treated wood is the same. The level of preservative protection required depends on where it is to be used.
The technical standards for wood preservation are set out in BS 8417 and the Wood
Protection Association (WPA) Code of Practice. They require the loading and penetration
of preservative is tailored to the desired end use or ‘Use Class’ of a wood component.
The most common Use Classes are 2, 3(u) and 4.
Treatment specification must take into account: the timber species; the end
use; the desired service life (DSL); and the risk and consequences of failure.