When purchasing pressure treated timber, customers will notice a Use Class designation, usually UC3 or UC4 for exterior uses. Use Classes indicate timber protection and applicable conditions. This information can usually be found on product specifications or labels. Many customers disregard this detail, assuming the number impacts the value of the product.

This is false. Many customers do not notice Use Class UC2, resulting in a significant loss of value.

Class identifiers are listed in the BS EN 335 European standard for timber durability and preservative. Classes run from UC1 to UC5, and each describes a different exposure scenario. Most homeowners are primarily focused on the middle of the scale.

UC1 includes timber that will be indoors and in dry conditions, and with no chance of being wetted, such as internal joinery, furniture, and flooring of heated rooms. UC2 includes timber that will be indoors and with some chance of wetting, but again, not for extended periods of time and not with ground contact. Both UC1 and UC2 are for interior applications. Neither of them are suitable for timber that will be used in any exterior applications that will be exposed to the weather.

The first level for exterior applications – UC3

UC3 is the lowest level of classification for timber that will be used outdoors and hence will be exposed to the weather, but will not be in contact with the ground or fresh water. Within UC3, two subclasses are of particular interest: UC3a describes timber that will not be subjected to prolonged wetting, which in this classification is typically referred to as painted or coated timber; and UC3b describes timber that will be subjected to prolonged wetting, including uncoated timber and positioned horizontally where water can pool.

For most species and applications, the UK has exterior cladding that falls into the UC3b classification. Similarly, UC3b is the classification assigned to softwood decking boards, fence rail, and components of pergolas. The specifications for treatment of timber in UC3b is more onerous than those in UC3a, with greater amount of preservative required to be retained, as well as greater depth of penetration, being demanded. If a timber product is described as being of classification UC3 but has no subclass, it is fair to ask what subclass it is.

Ground contact: UC4

Ground contact UC4 refers to timber that makes contact with the ground or with freshwater. Examples would be posts, soil driven fence posts, sleepers, and the construction components of garden buildings that are in or on the ground. The biological hazard is far greater with ground contact — soil holds micro-organisms, moisture and the conditions needed for decay and insect colonization. Therefore, treatments for UC4 require higher preservative loads and, for some timber species, longer treatment cycles.

An example of this would be a fence post treated to UC3b that has been driven into the ground. It will decay faster than the fence boards that it supports. This is referred to as a treatment mismatch in domestic timber installations and typically comes to light in the first three to five years when the post above the ground still appears structurally sound, while the rest of the structure looks sound.

Marine exposure, tidal and saltwater environments, are also included in UC5. This does not pertain to most domestic projects.

What is involved in the treatment process

An advanced timber treatment method called pressure treatment is by far the best method to treat timber. This process uses both vacuum and pressure to push the timber preservatives deep within the cells of the timber and not just the surface. The degree of depth of the timber treatment is one of the criteria’s used, along with the retention level, to determine what ‘use’ class the timber is designated. This information is usually listed on the treatment certification or data sheet for the timber.

Although there are different types of preservatives used on treated timber, for the UK, the most common type of preservative used on treated timber is copper, and more specifically, microencapsulated copper. This type of treated timber should not be used in any situation to grow food, and when the treated timber is to be disposed of by fire, it should not be burned as it will produce a hazardous waste. There are different types of microencapsulated copper preservatives, with the obsolete ones, such as CCA (copper, chrome and arsenic), being less common than the more modern ones called ‘copper azole’ and ‘copper boron’.

What to do before buying

Get the treatment data sheet and ask for the use class. Do not accept the sales information stating the timber is treated as anything substantial. For all timber used for building outdoor structures such as a fence, deck, pergola etc, be sure the timber is treated for UC4. For also outdoor timber used for building structures that do not come into contact with the ground, UC3b is the minimum that should be required.