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 grade rules for the use of structural timber are set out within BS5268-2:2002 but how do you know which grade to use?
Deck boards are a key structural part of the deck and they should always be graded. The most common timber grades for softwood deck boards are C16 and C24.
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.
Boards graded to C24 is a more demanding grade. Boards will have fewer knots and the knots in the board will be smaller than in a board graded to C16. C24 graded decking boards are typically used where higher loads are required or wider spans are required.
The main advantages of using C24 graded deck boards are firstly that in the same section as a board graded to C16 a wider rafter centre can be used or, a smaller deck board section can be used on the same centres.
The 3 common uses for C24 that we see 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
- to optimise the aesthetic appearance of the project
One of the main considerations is of the timber deck is deflection. The most crucial load is the point load and the limited factor in setting spans is deflection. A deck board that has too much deflection can be a trip hazard. Marley Eternit Deck Board spans assume a maximum 3mm deflection which we view is the best or optimum specification.