7th Dec 2022 -

Untreated timber will be susceptible to rot and infestation when used externally, potentially ruining decking, fencing, and outer walls. When looking to use wood in areas of your property that are vulnerable to moisture, treated timber is the only option, as it successfully resists water and insect infestations. In this guide from Building Materials Nationwide, we’ll take a look through the benefits offered by treated timber, the ways it’s classified, and the potential drawbacks posed by it.

What is Treated Timber?

Treated timber is any timber that has been altered with preservatives, which is designed to protect its exterior and allow it to be used outdoors. While hardwood timbers can often be used outdoors untreated, softwood timber that has not been treated is likely to experience severe problems with rot should it be installed outdoors. An alternative product is fire-treated timber, which has a fire retardant applied to it.

Looking For Quality Treated Timber?


    • Ideal for general construction, fencing, garden and leisure timber applications
    • Pressure-treated
    Price from: £4.18 £5.02
  2. £3.11 £3.73

View All Treated Timber

What is Used to Treat Timber?

Treating timber is not merely a case of covering the outer surface of the beam. It is sometimes referred to as pressure-treated timber because the process of treatment involves forcing preservatives into the timber. To treat timber, it must first be kiln-dried to reduce moisture. It will then be loaded into a specialist machine called a vacuum cylinder; this will further reduce moisture within the wood before flooding the preservative into the chamber, using pressure to ensure a high level of penetration. The settings of the machine will be altered based on which variety of wood is being used.

There are a range of chemical preservatives that can be used. Water-based preservatives include alkaline copper quaternary, copper azole, or boron compounds. Solvent-based preservatives can include permethrins and azoles, which will be dissolved in white spirit. For fire retardant timber, halogenated organic compounds will be used.

A final option for certain uses is creosote. Creosote can be used to treat timber for industrial and professional uses, meaning things like railway sleepers and telegraph poles, along with agricultural fencing. Creosote-treated wood cannot be used within domestic settings, due to health risks. Also banned due to health risks is CCA (copper, chromium, arsenic) which was used to treat timber historically but was outlawed due to health risks. Timber treated with CCA that is currently in use can continue to be used.


What Are the Use Classes of Treated Timber?

Use classes are definitions of where treated timber should be used, designated based on the level of protection they can offer. When buying treated timber for specific uses, you should ensure that it meets the relevant use class. Here are the five categories available, with use class 4 offering the best protection (defined in BS EN 335-1):

  • Use Class 1: For internal dry usage only. No threat of moisture, but potentially some threat from insects.
  • Use Class 2: Internal use with occasional threat of moisture. This could be the framing of an external wall, or tile battens in a roof. There may also be threats of fungi which can damage untreated wood.
  • Use Class 3 (Coated): For external use above the ground when coated. This could be external cladding or window frames.
  • Use Class 3 (Uncoated): For external use above ground where there is no coating. An example of this would be fence rails.
  • Use Class 4: Direct contact with soil or freshwater. This could be everything from fence posts to docks and sea defences.

Can You Saw Treated Timber?

Treated timber can be sawn, though you should bear in mind that in cases where the timber is load-bearing, such as sawn carcassing timber, reducing its height or width will require the timber to be re-graded for strength before use, as the strength grade is determined based on previous dimensions. In any case, you will need to apply preservatives manually to the outer edges of any timber you have cut to ensure its continued performance. You should apply a suitable preservative to sawn-treated timber as recommended by the original manufacturer. It’s important to wear a full mask and protection when cutting treated timber.

What Are the Benefits of Treated Timber?

The majority of softwood timber (which is in itself the majority of UK timber) is naturally unsuited for outdoor use, as it is prone to decay when it encounters moisture. Treatment will allow timber to be used in outdoor areas, such as external walls, decking, and fencing. If looking to erect a wooden building, you will need to use treated timber for a variety of features within it.

The second benefit is protection from fungal and insect attacks. There are a variety of wood-boring insects, including woodworms, furniture beetles, deathwatch beetles and more. These can burrow into wood, laying eggs that will become larvae, and creating networks of holes in your timber. Not only is this unsightly, but it also reduces the strength of your wood, which is vital in load-bearing beams. Keeping your timber moisture-free and coated will resist insects that could otherwise burrow into it.  It will also help to resist fungal growths that can harm the strength of your timber. Treated carcassing timber can therefore provide benefits even when not exposed to moisture.

Lastly, fire-treated timber can be used within a building project if you are looking to combine timber construction with the highest possible levels of safety. Fire treatment will help to limit the spread of surface flames, alongside reducing smoke propagation. Both of these features will offer safety benefits within a building, allowing occupants a longer window to escape and lowering the chances of smoke inhalation.

Are There Any Drawbacks to Treated Timber?

The outlawed treated timber preservative CCA poses serious health risks, from inhalation of dust when cutting to leaching into water, or even just direct contact. They pose a carcinogenic risk and as such have been outlawed for use within the UK. Treated timber sold today will not contain CCA or its associated risks. You should still work to avoid inhaling any dust, however. It’s also important to not burn any treated wood, such as within a stove or a barbeque. Burning it will release chemicals that are unsafe to inhale. Another small drawback is that sometimes the treatment process can cause warping within wood whilst drying. As you will be buying wood that has already been treated you’ll be able to assess the level of warping before purchase.

Our Range of Treated Timber

Building Materials Nationwide stocks a comprehensive selection of treated timber beams for sale. These are perfect for all external uses within a building project, from decking to fencing to outer walls. Within our full range of timber and joinery, you’ll find treated timber beams, as well as treated timber decking. All of our products can be delivered across the UK. We also offer trade accounts to customers working within the industry, giving you access to our full product range, our lightning-fast delivery options, and a dedicated account manager. Sign up for a trade account through our website today.


    • Ideal for general construction, fencing, garden and leisure timber applications
    • Pressure-treated
    Price from: £11.73 £14.08

    • Ideal for general construction, fencing, garden and leisure timber applications
    • Pressure-treated
    Price from: £7.81 £9.37

    • Ideal for general construction, fencing, garden and leisure timber applications
    • Pressure-treated
    Price from: £7.83 £9.40

View All Treated Timber