What is Treated Timber?
Timber is ubiquitous in the building industry, used in timber frame construction, furniture and pallets. However, timber often needs to be treated before it’s suitable for use. This guide will talk you through the the different types of timber available, the treatment process, it’s benefits, and, of course, answer the question ‘what is treated timber?’
Contents
Intro
Wood, when found in its natural state, is not a durable material. Instead, it’s prone to shrinkage, rot, insect visitors, and, inevitably, degradation. The act of ‘treating’ timber sees to fix these issues, prolonging the longevity of wood, timber and their associated products, such as engineered wood. However, understanding how timber is treated, what preservation chemicals it’s treated with and how this will actually impact your project is all key in ensuring you decide upon the right timber for the job. So, without further ado, let’s try and understand the answer to: what is treated timber?
*** This guide has been crafted to help you to understand the uses and characteristics of treated timber. All information provided is compliant with BS 8417:2011+A1:2014 recommendations, which is the British Standard that provides guidance for the preservative treatment of wood to provide protection against biodeterioration in end-use situations. It gives recommendations for determining the need for treatment and specifying the type of treatment, tested to the requirements of BS En 599-1. ***
What is Treated Timber?
Although you can treat freshly-cut timber by simply using an external paint or protector (see our Choosing Exterior Paint, Varnishes and Oil Guide), properly treating timber is not merely a case of coating the outer surface of the wood, and frequently involves forcing preservatives through the timber.
What is Timber Treated with? There is no one kind of preservation used to treat timber and what kind you choose can depend on what you want to use your timber for. For example, if you need to fire-retardant timber, halogenated organic compounds will be used to preserve the timber. However, more common treatments include alkaline copper quaternary, copper azole or boron compounds. Solvent-based compounds are also available, such as permethrins and azoles, which will be dissolved in white spirit before use. Occasionally, creosote-treated timber is also an option for some industrial and professional settings, e.g. railway slippers, telegraph poles, and agricultural fencing. Treatment plants in the UK are normally designed and operated in accordance with the Wood Protection Association code of practice for the Safe Design and Operation of Timber Treatment Installations
How is Timber Treated? To treat timber, typically a softwood, such as Redwood, is kiln-dried to remove moisture. Next, it will be loaded into a vacuum cylinder, which is a specialist machine designed to reduce moisture further. Finally, the machine is able to flood preservation into the chamber, using enough pressure to ensure a high level of penetration of chemicals into the timber. Below is the step-by-step process of how timber is treated, with accompanying information provided by the Secretary of State’s Guidance for Local Air Pollution Control (LAPC) and Local Authority Pollution Prevention and Control (LAPPC):
What is Carcassing Timber?
Carcassing timber is usually made from softwoods, such as European Redwood or European Whitewood, and gets its name from being the timber that constructs the ‘carcass’ of a building. In essence, carcassing timber is the general name for timber that has been treated suitably for use in structural applications.
Applications of Carcassing Timber
Compared to joinery timber, carcassing timber has a much lower quality finish, possessing a sawn and knotty appearance. However, this doesn’t mean it can’t suffice in a large variety of applications, such as:
✅ Framing / Falsework - Falsework is a temporary structure that is used as a supportive feature during maintenance or production.
✅ Joists - Carcassing timber can suspend loft, roof and ground-floor joists and can even be used in the construction of flat roofs.
✅ Noggins - Carcassing timber can be utilised as noggins, which are wooden struts fixed within joist or stud frameworks, giving strength and stability to the structure.
✅ Rafters - You can create popular rafter designs (auxiliary, hip and valley) and even roof battens using Carcassing timber.
✅ Stud Partitions - Stud partition walls are used to break up a building into its separate rooms, but can also be utilised to apply insulation to solid walls.
✅ Wall Plates - Carcassing timber can be used to create wall plates, which are used in brick houses and typically refers to a length of timber bedded on top of mortar but below the joists of the ceiling.
What is Tanalised Timber?
Tanalised timber is named after its treatment agent, Tanalith E, which is used to prevent decay and insect damage. These days, most structural timber is treated because it lasts much longer than the untreated varieties and is mainly used in structural work in areas that are not visible and are prone to moisture and decay.
Is Tanalised Timber Safe?
Typically, Tanalith E will be a safe option for most jobs as it only contains organic biocides which are much safer than more traditional treatments, which often contain dangerous compounds such as arsenic or chromium. Fortunately, this old type of timber preservation - CCA (copper, chromium, arsenic) - has been outlawed in the UK due to the carcinogenic health risks that came with it.
Where is it OK to use? Indirect or incidental contact of food (e.g. desks, picnic tables, playgrounds) on treated timber will have no ill effect as the potential for the preservation chemicals to migrate onto the food is little to none. Treated timber is also suitable for garden use and even for growing food, e.g. raised flower beds, landscaping, mushroom trays, tomato stakes, and greenhouses
Where should it be avoided? Despite it being a generally safe option, treated timber should not be used in direct contact with human food or drinking water, either during preparation or storage. In addition to this, it’s important to note that you shouldn’t burn any treated wood (e.g. within a stove or BBQ) as this will release dangerous chemicals that are unsafe to inhale. When cutting or handling any kind of treated wood, you should also take measures to avoid inhaling dust by wearing a dust mask.
What is Pressure Treated Timber?
‘Pressure-treated’ is just a synonym for tanalised and the two phrases can be used interchangeably. The namesake of pressure-treating refers to forcing preservative chemicals into the wood with a vacuum. The final result is a durable and stable product, resistant to rot and decay.
Applications of Pressure Treated Timber
Thanks to the treatment it receives, tanalised or ‘pressure-treated’ timber is ideal for:
✅ Joists for roofs, floors and decking
✅ Stud partition walls
For further information regarding the legal standards surrounding the treatment of timber, please see the Timber Preservation Page provided by the NHBC (National House Building Council).
If you’d like more information on what applications treated timber is apt to tackle or what different kinds of timber are available, take a look through our other timber guide pages: