C16 Versus C24 for Structural Decking

All timber decks, whether domestic or commercial, are designed to carry specific loads; these include both uniformly distributed and point loads. The grading rules for structural timber are set out within BS 4978, but how do you know which grade (C16 or C24) to use?

Deck boards are a key structural part of the deck, and they should always be graded - this helps to ensure strength and durability and could also lead to cost savings on the sub-structure. 

The most common timber grades for softwood deck boards are C16 and C24. The difference between these two options is based on the performance properties of the timber, and the right choice will depend on the project.

What is C16 timber?

Boards graded to C16 are generally recommended for most applications and, if the correct spans are used, this is usually the most cost-effective option that will still meet the performance requirements.

What is C24 timber?

C24 is a more demanding grade. Boards will have fewer knots and the knots in the board will be smaller than in boards graded to C16. C24-graded decking boards are typically used where higher loads or wider spans are required.

The main advantages of using C24-graded deck boards are that a wider rafter centre can be used in the same section as a board graded to C16, and a smaller deck board can be used on the same centres. 

Choosing between C16 and C24

While C16 is recommended for most projects, there are situations where C24 deck boards are the better choice. The three most common uses for C24 deck boards are:

• Balconies – when trying to keep the deck sections thin to match up with door levels.
• Bridges – to optimise the use of large beams or to use fewer beams in the structure.
• Aesthetics – Fewer knots in the wood can be preferable for projects where appearance is important.

One of the main considerations for the timber deck is deflection. The most crucial load is the point load, and the limiting factor in setting spans is deflection. A deck board that has too much deflection can be a trip hazard and may even give the deck a “bouncy” feel. Marley deck board spans assume a maximum 3mm deflection, which we view as the best or optimum specification.

All Marley timber decking is supplied to a minimum grade of C16, and we also offer boards graded to C24. The grading is carried out in house by our fully trained team.