Do I need to use structurally graded timber for deck boards?

From time to time, we get asked to provide our deck boards structurally graded to C16 or C24. This is something we can offer our clients on request, but is this really necessary for timber deck boards? In this blog piece, we explore how softwood boards are appearance graded at source and discuss how the structural grading rules cannot be applied to deck boards due to the application of the timber. 

Appearance grading

The majority of softwood deck boards in the UK is manufactured from European Redwood trees grown in the forests of Scandinavia, Russia and the Baltic States. This is because the cold climate and short growing season in these countries is ideal for producing a high quality, strong and close grain timber. With timber being a natural product there are variances between each board due to a number of different parameters which include live knots, dead knots, checks, wane, cross grain, compression, fungal attack, discolouration, and dimensional deviances. These different parameters will affect the quality and suitability of the timber.

Qualified and experienced graders working at the mills will grade the timber by checking and measuring the different parameters against the defined rules within the Swedish appearance grading standard for softwood - SS EN 1611-1. The timber is then sorted into groups (or grades) of broadly the same quality of timber which allows the end user and companies such as Gripsure to choose a grade suitable for the application in which the timber will be used. Grades I to IV (the higher grades) are sold together and tend to be used for internal joinery products where knot content and size of knots need to be minimised. The most common grade for deck boards is the Scandinavian V grade but the VI grade is sometimes offered as an economy option by some merchants and manufacturers.

Structural grading

The appearance grades which are common place within the decking industry are different to the structural strength grades and classes. Structurally graded timber is required for certain applications to ensure the timber is strong enough for the load being placed on it. Products which require structural grading include roofing rafters, floor joists, wall plates and stud work for partitions walls where it is crucial that the timber being used is suitable for its application.

The load bearing capabilities of a piece of timber is dependent on its dimensions, species and spans being used in the construction. Timber is graded against structural grading rules that limits strength reducing factors such as knot size, position of knots, rate of growth, sloping grain, fissures and fungal attack. The timber is passed through a machine or more commonly examined by trained graders who visually assess the timber against the structural grading rules which specify acceptable limits (EN 14081-1 which is supported by BS 4978 (softwoods) and BS 5756 (hardwoods)). This process produces two strength grades - GS (General Structural) or SS (Special Structural). These strength grades can then be used to sort timber into strength classes (most commonly C16 or C24) which contain timber from different species and strength grades as per BS EN 338.

Each piece of structurally graded timber is stamped which displays information about the method of grading, strength grade, strength class, the graders license number, certification body, company reference, species, standard reference and condition of timber (moisture content at time of grading). Structurally graded timber must be clearly marked to show that it complies with the correct standards and strength requirements as set out in the building codes and regulations.

Can structural grading rules be applied to timber used for deck boards?

The TDCA (Timber Decking & Cladding Association) who are an independent technical and advisory body state that there are no established structural grading rules for timber deck boards. Structurally graded timber against EN 14081-1 is designed for rectangular timbers used on edge. The structural grading rules cannot be applied to timbers which are used flatwise. Therefore, the strength classes of structurally graded timber (C16 and C24) are irrelevant when the timber is being used for timber decking as the rules cannot be applied to that application.

A simple analogy is that in the same way an internal floor joist is structurally graded (most commonly to C16) and the internal floor board used upon it is not structurally graded, an external decking joist is structurally graded (most commonly to C16) and the external deck board used upon it is not structurally graded. Both the internal floor joist and the external decking joist are used on their edge so the strength class can be applied to that application (as per EN 1408-1). The boards themselves are used flatwise so the structural grading rules cannot be applied.

It is understandable that we receive a handful of enquiries for structurally graded deck boards, as C16 and C24 graded timber is prevalent for many timber products. With the lack of easy to digest and clear information it is easy to believe that you need your deck boards structurally graded too.

What should I look out for?

If you are looking to use a treated softwood deck board, here at Gripsure we would recommend choosing a product which is;

- Scandinavian V grade European Redwood which has been appearance graded against SS EN 1611-1

- FSC or PEFC accredited timber to ensure that the timber is responsibly sourced and the forests are well managed and sustainable

- DeckMark Plus accredited manufacturer - Manufacturers who are annually audited and approved by the TDCA (Timber Decking & Cladding Association)

- Boards which are treated to the right treatment classification for the specific environment the boards will be used in to ensure longevity

- Boards which contain non-slip inserts with an initial PTV rating of 65 or above (extremely low slip potential) to mitigate the risk of personal injury in wet and slippery conditions

In addition to the deck boards we would recommend that the timber used for the sub structure is structurally graded as this will give the deck it's strength. The TDCA have calculated recommended joist centres in two span tables based on a domestic and commercial application which takes into consideration the size, profile and species of timber as well as the point and live loads placed onto the deck structure (as per BS EN 1991-1-1). We recommend following their recommended joist centre spans to ensure the deck isn't springy under foot which can be caused if the joist centres are too far apart. 

It is also important that the correct fixings and an experienced and competent installer is used to ensure the deck will pass the test of time. If any cuts are made to treated softwood boards, ensele must be used to seal the treatment to avoid premature failure.

I hope this blog piece clears up some of the grey areas around why the structural grading rules cannot be applied to deck boards. It is more important to focus on other factors such as treatment classification, appearance grade and slip potential when choosing a softwood deck board. If you would like to discuss any aspect of the blog further, feel free to get in touch with me.

Tom Anderson - tom@gripsure.co.uk