Is CLS timber OK for decking? Or is there a better option?

In this article we will be looking at whether it is OK to use CLS timber for creating a decking frame.

Here in the UK, when someone mention CLS timber we usually think of internal studwork and framing, and this is its most popular use. Most timber merchants and builders’ merchants will stock CLS in a variety of dimensions and lengths.

CLS timber can be used for decking. However, you must ensure that the timber is treated. You should be looking to purchase tanalised (pressure treated) timber. This is timber that has been treated under pressure to force chemicals into the wood.

This process makes the timber more resistant to outdoor conditions and will prevent rot and mould from setting in.

is cls timber ok for decking
50m2 deck I built in my garden using tanalised C24 timbers

What is CLS timber

In the construction industry, and specifically joinery, if somebody says CLS, they are usually referring to timbers used for studwork. This is generally 3 x 2 timber and most commonly comes in lengths of between 2.4m and 4.8m.

However, the term CLS is a broader term, which determines how the timber is graded. CLS is an acronym for Canadian lumber standard. This type of grading looks at the quality of the timber to determine its grade. Things that are considered include:

  • How straight the timber is
  • Tightness and direction of grain
  • Size of knots
  • Strength and durability

CLS timber is kiln dried softwoods, such as pine, spruce, and fir. It is planed on all surfaces and has rounded corners. The most common grades are C16 and C24, with your average stud work timber being C16 grade.

C24 is a higher quality timber and usually contains smaller knots and a tighter grain, this makes them stronger and more durable.

Using a higher-grade timber can offer benefits. For example, due to its higher quality, it will be able to span larger distances and the joists will be able to support heavier loads.

Do you have to use treated CLS for decking?

When you are building your decking frame, the most important thing to ensure, is that you use treated timbers. When the timber is treated, it is protected from the elements and will last much longer than untreated timber.

Untreated timber is not intended for outdoor use; therefore, it will suffer from things like rot, mould, and insect infestation much quicker.

When timber is treated, a chemical called Tanalith E is forced into the wood under pressure. This process saturates the wood in this chemical, which preserves the timber and protects it against harsh outdoor conditions. Also, due to the nature of the chemicals, it will repel insect attack, as the chemicals are poisonous to wood boring insects.

In theory the timber does not need to be graded, and you could use rough sawn tanalised timber. However, you should be aware that the quality is lower in this type of timber.

As rough sawn timber is not strength graded, it is generally lower quality and less durable. With that said, it could be suitable on smaller decking areas, that will not be spanning larger distances and supporting heavier loads.

If you do opt for ungraded timber, then you are more likely to see defects in the timber, such as:

  • Large knots
  • Wider grain
  • Less uniform shape
  • More twisting and warping

If you order this type of timber delivered, then don’t be surprised if the quality is quite mixed. If you do opt for non-graded timbers, you are better visiting the merchant and picking the timbers yourself. By doing this you can pick the best ones and avoid any banana shaped pieces of wood.

The main advantage of graded and treated timber

The main advantage with graded timber, is the quality and consistency you will get. If you order this type of timber delivered, it is far more likely to be of consistent quality.

Also due to the grading process, you know this timber will have less knots, less defects, and a tighter grain. This all means that the timber will be stronger and more durable. It will be slightly more expensive than ungraded timber, but it will create a much sturdier frame for your decking.


CLS is certainly OK to use for decking. However, you must ensure that you use a treated timber. Standard CLS studwork timber will not stand the test of time. This type of timber is untreated and not suitable for outdoor use.

By choosing a high grade treated timber, you will give your decking a high-quality, sturdy base, that will last a long time. You will also ensure it remains free from rot, mould, and insect damage.

Opting for a graded timber, as opposed to a lower quality tanalised timber, will also improve the quality of your finished decking frame. Higher graded timber will offer additional strength and durability, with less defects and a more consistent quality product.