Question 1

What are timber treatment use classes?

Accepted Answer

Timber treatment use classes (UC1 to UC5) defined by BS 8417 describe the service conditions a timber member will face, which determines the level and type of preservative treatment required. UC1: timber in dry interior conditions - no treatment required (internal joists, studs, rafters in properly ventilated buildings). UC2: timber in covered exterior locations protected from direct wetting - light treatment required (covered outdoor structures, car ports, concealed cladding battens). UC3: timber exposed to the weather but not in ground contact - moderate treatment required; split into UC3a (above dip line, less exposed) and UC3b (more exposed, including end grain). UC4: timber in ground contact or fresh water - heavy treatment required (fence posts, deck joists on or near the ground, pergola posts, garden sleepers). UC5: timber in salt water or salt water splash - very heavy treatment required (marine piling, jetty timbers). For most domestic construction, UC1 to UC4 are the relevant classes.

Question 2

What is pressure impregnation?

Accepted Answer

Pressure impregnation (also called pressure treatment or vacuum pressure impregnation) is the most effective method of applying preservative to timber. The timber is placed in a sealed cylindrical vessel (an autoclave); the vessel is first evacuated to remove air from the timber's cell structure (initial vacuum), then flooded with preservative solution and pressurised to force the preservative deep into the timber (pressure phase), then the pressure is released and a final vacuum removes excess preservative from the timber surface. The result is preservative penetrating deep into the sapwood of the timber, not just coating the surface. Pressure-impregnated timber is far more durable than surface-treated timber and is the standard treatment method for structural carcassing, fence posts, decking, and any timber in at-risk locations. It is recognisable by a slight green or brown colouration from the preservative chemistry. The trade name 'Tanalith' (from Lonza, one of the leading preservative manufacturers) has become a generic term for pressure-impregnated timber in the UK, leading to pressure-treated timber sometimes being called 'tanalised' regardless of the actual preservative used.

Question 3

What types of timber preservative are used?

Accepted Answer

The main timber preservative types used in the UK are: copper-based organic preservatives (such as copper azole - CA, and micronised copper - MCQ), which are the most widely used for structural carcassing and decking and give the characteristic green tint; boron compounds (sodium octaborate), used mainly for treatment of green (wet) timber during manufacture of structural engineered timber products, and for remedial treatment of existing dry rot or woodworm infestations (boron diffuses through wet timber but is not suitable for surface application to dry timber as it will not penetrate); organic solvent preservatives (now less common due to environmental concerns, formerly including pentachlorophenol and lindane which are now banned); and water-based emulsions and micro-emulsions (newer chemistry with lower environmental impact). CCA (chromated copper arsenate), formerly the most common treatment, has been withdrawn for most end uses due to the arsenic and hexavalent chromium content - it is still found in older pressure-treated timber but should not be used in new work. For existing dry rot or woodworm problems, specialist remedial treatments applied by injection, gel, or paste are available.

Question 4

When do Building Regulations require treated timber?

Accepted Answer

Building Regulations Approved Document A (Structure) and the associated technical standards require preservative treatment for timber in locations at risk of biological attack. Specific requirements include: timber built into external masonry walls (where it may be exposed to moisture migrating through the wall) must be treated to at least UC2; timber in flat or pitched roofs where ventilation may be inadequate must be treated; timber in ground floor construction where a ventilated void exists beneath must be treated; timber in contact with or close to concrete or mortar (which can be alkaline and remain damp) should be treated; timber in conservatories, garages, and other semi-exposed structures should be treated. NHBC Standards (for new-build housing) are more prescriptive and require treatment for specific timber elements by location. In practice, specifying pre-treated (pressure-impregnated) carcassing for all structural timber regardless of location is good practice that adds minimal cost and eliminates the need to make location-specific judgements.

Timber Treatment

Just so you know...