Timber - Treatments & Use Classes
Timber Treatments
Timber can either be untreated or treated.
Untreated timber will not rot if it remains dry with less than 20% moisture content. When considering using timber for external applications it is important to understand that different species have different properties.
Some timbers, typically many hardwoods, are naturally durable and resistant to decay and, as such, treatments are not necessary. Softwoods can be an effective alternative to choosing an often more expensive durable hardwood. However, treatment is required when choosing a softwood for an external application or for when resistance to decay is a priority.
There are a number of treatment options available to significantly extend the service life of non-durable timbers.
The type of treatment and how it is applied will depend on factors such as :-
-
the natural durability of the species to be used;
-
its resistance to penetration by preservatives i.e. its permeability;
-
the end use of the timber;
-
the service life required;
-
the ease of any future maintenance.
Three of the most commonly used treatments are tanalisation, double vacuum low pressure treatments, and brush applied or dipped treatments.
Tanalisation - is a high pressure vacuum impregnation of the timber, providing a service life protection ranging from 15 to 60 years.
The preservative generally results in a pale green colouration to the finished component. Additives are available that can give either a rich brown colouration, usually for rough sawn fencing and landscaping timbers, or an effective extra water repellency protection for decorative external timbers such as decking and cladding timbers.
Double vacuum low pressure - impregnation treatments can be used for building and joinery timbers delivering a 30-60 year service life protection. Treatment provides an effective ‘envelope’ protection around the timber and leaves the colour of the timber virtually unchanged.
A colour indicator, as well as water-repellency, can be added to the treatment if required.
Brush applied or dipped treatments - are superficial treatments and are less effective than those previously mentioned. Their advantages are that they are easy to apply and they are cost effective. These preservatives range from water based, spirit based, paint, tar, liquid asphalt, oils, and creocote.
Timber Use Classes
The eventual end use of preservative treated timber is classified into one of five main categories. These Use Classes are based on the potential threat to the timber from decay or insect attack in its eventual application.
For instance, internal building timbers in Classes 1 and 2 will be under less threat than timbers used externally in ground contact i.e. Use Class 4. Use Class 4 timbers will require a higher degree of protection.
The Use Classes are as follows :-
-
Use Class 1 – Internal, dry (e.g. upper floor joists);
-
Use Class 2 – Internal, risk of wetting (e.g. tile battens);
-
Use Class 3 – Coated (3.a) – Outdoors, coated, above ground (e.g. soffits/cladding), Uncoated (3.b) – Outdoors, uncoated, above ground (e.g. cladding/fence rails);
-
Use Class 4 – Direct soil or fresh water contact (e.g. fence posts).
The diagram below illustrates the Use Classes.
.jpg)
-
Use Class 1 – Domestic pitched roofs all structural timbers, first floor joists;
-
Use Class 2 – Condensation risk pitched roofs all structural timbers, tiling/slating battens, flat roof timbers, external frame – inner leaf – except sole plates, frame sheathing – plywoods, groundfloor joists;
-
Use Class 3 – External cladding, battens for fixing external cladding, sole plates above dpc, timber garden products;
-
Use Class 3a – Barge boards, fascia boards, external joinery, surrounds for non-wood windows, external doors, external door frames;.
-
Use Class 3b – Decking;
-
Use Class 4 – Sole plates below dpc, sole plates above dpc, fence posts, swimming pool surrounds external.
