Timber chooser

Treated: which use class?

Treated wood isn't all the same. One piece can look very like another while carrying a very different level of protection, because the standards tailor the preservative to the end use¹. The Wood Protection Association groups uses into use classes, the main three being 2, 3 and 4¹.

• Treatment also has a service life, usually 15, 30 or 60 years: how long the part must last before it's replaced². For decks, 15 years is the trade body's minimum for quality installations⁴.
• If in doubt, move up a class, for example from Use Class 3 to Use Class 4².
• The minimum standards for treating wood are set out in British Standard BS 8417². The use classes themselves come from BS EN 335-1⁵.
• Wood treated for outdoor use shouldn't be sold for indoor jobs either: over-treatment is no better practice than under-treatment¹.

At the counter

The Wood Protection Association's checklist for buying treated wood¹:

1. Work out the use class you need before you order.
2. Tell the supplier in writing that the wood must be treated to that use class to comply with BS 8417.
3. Ask them to confirm it meets that use class on the delivery note and invoice, or a treatment certificate.
4. When buying from stock, check which use class the wood was treated to.
5. Never use wood treated for an indoor job outside.

The association also says to ask for product documentation, including proof of treatment³.

Cut ends

• When you cross-cut, notch or drill treated wood, always put end grain preservative on the fresh wood. Never put cut ends in the ground, even if you've coated them¹.
• Timber Development UK says two liberal brush coats on each cut end, and that a treated post should go in uncut end down⁷.

C16 and C24

• The number is the bending strength of the class: C16 for 16 N/mm² and C24 for 24 N/mm². They're two of the twelve strength classes in BS EN 338⁸.
• Every piece of structural timber carries a strength-grading mark showing its class and source. Timber that isn't marked piece by piece shouldn't be bought for structural use⁸.
• A strength class says nothing about durability⁸: treatment is a separate choice.
• Most C16 and C24 sold in the UK is kiln-dried and surfaced⁸.
• Decks: C16 softwood joists are recommended for decks below 600 mm high and are the minimum above that⁹.
• Which class a joist or rafter needs, and at what size and span, is for the designer or Building Control.

Sizes as sold

CLS

• CLS is the usual timber for studding and internal partitions. It's supplied regularised (surfaced) with eased edges, usually kiln-dried whitewood (spruce), strength-graded to C16¹⁰.
• The most common finished sizes are 38 x 63, 38 x 89 and 38 x 140 mm, in lengths from 2.4 m to 4.8 m, with 5.4 m and 6.0 m to order¹⁰.
• Ungraded CLS can only be used where it doesn't carry a load¹⁰.

Sawn or planed

Structural timber comes in two tolerance classes under BS EN 336¹¹:

Class 2 is Class 1 timber planed down, which is why its standard sizes are smaller, with a smoother surface and often eased edges¹¹. So a planed piece is a little smaller than a sawn piece of the same name: check the actual size on the label or ask.

Deck boards

Supplied from 1.8 m to 6.0 m long, 19 to 45 mm thick and 70 to 145 mm wide¹². Work out how many with the decking calculator.