Firstly it is important to understand that the wood will not rot if it stays dry (less than 20% moisture content). When used outside this is not always possible so treatments can be introduced to certain timber species. Treatment is not to be confused with decorative or protective coatings – these are often recommended in addition to any chosen treatments. Your desired overall affect/appearance will determine what you do to your timber.
When considering using timber for external applications it is important to understand that timber is a variable material; different species have different properties. Some timbers, typically many hardwoods, are naturally durable and resistant to decay. And due to their density, treatments are not affective (or necessary). Our materials section indicates the durability of various species to help with this choice.
Our durability league table will also help to determine which woods are good for which applications. In addition to this, if you select one of our products, we identify which timbers are recommended for the chosen item.
Softwoods can be an effective alternative to choosing a durable hardwood, which are often expensive. When choosing a softwood for an external application or for when resistance to decay is a priorty – treatment is required. To significantly extend the service life of non-durable timbers, there are a number of treatment options available.
The type of treatment, and how it’s applied, will depend on several factors:
a) The natural durability of the species of timber you are using.
b) Its resistance to penetration by preservatives (its ‘permeability’) – eg. Douglas Fir is a softwood but considerabiley dense and cannot affectively be treated by the methods we will describe below.
c) The end use of the timber
d) The service life required
e) The ease of any future maintenance – treatments can affect the application of surface finishes and coatings.
We describe three commonly used treatment methods but there are many more available;
1) Tanalisation –
TANALITH® E is the latest generation wood preservative. Already established in markets throughout the world, TANALITH® E treated timber provides a proven alternative to traditional CCA treated timber for the timber trade and for the general public.
TANALITH® E preservative has a unique copper and organic biocide (triazole) formulation allowing TANALITH® E treated timber to be used where special environmental concerns or restrictions exist, as well as the everyday construction, landscaping and engineering applications for which treated timber is renowned, e.g. children’s play areas.
1. The Tanalising process involves placing the timber within the treatment cylinder and creating an initial vacuum within the timber cells.
2. The cylinder is then flooded under vacuum with the preservative treatment
3. Hydraulic pressure is then applied forcing the preservative deep into the timber cells
4. After a pre-determined period of pressure depending on the species of timber being treated and its eventual use, the treatment solution is pumped back into storage and a final vacuum extracts any excess treatment solution from the timber
5. Low pressure inside the timber draws in surface solution when vented to the atmosphere and the treated timber is left for a specified period for fixation of preservative to occur
These types of vacuum, high pressure treatments are particularly relevant for Use Classes 1 to 4 (see classes tables below) – providing a service life protection ranging from 15 to 60 years. They force the preservative deep into the cellular structure of the timber and generally result 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.
2. Double vacuum, low pressure treatments can be used for building and joinery timbers in Use Classes 1, 2 and 3.1, 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. Protim Osmose is one of many manufacturers that provide the treatments for these double vacuum plants. These are located across the UK. Click here for general information on Protim Osmose and here for more on Protim Osmose 418.
3. Brush applied or dipped treatments – There are many manufacturers that offer treatments of this type. These are superficial treatments and they are less effective than the two treatment options highlighted above. The advantages are that are easy to apply and cost effective. You should consult the coating manufacturer if you are considering this type of treatment.
Timber use classes
Please note 3.1 - Exterior woodwork should be protected with an appropriate surface coating after preservative treatment, then regularly maintained.
Use Class 4
Timbers destined for Use Class 4 situations will be permanently exposed to wetting in either groundor fresh water contact. For optimum durability it is important to ensure the correct specification. Make sure you ask for timber treated to Use Class 4.
Posted by Paul Hayman on Monday 26 June 2017 at 10:46