Knots are natural characteristics within the timber and add to the beauty of the material. However large knots and smaller knots which are present in clusters are weak points within the timber. By grading / sorting timber of the same species based on their knot content (as well as splits, shakes and other defects) you can create different classifications.
Generally, there are minimal knots in the tropical hardwoods that we offer due to the way the trees grow in the forest (a race to the forest canopy means energy needs to be concentrated on getting tall) and the higher grades that we offer.
Knots are also present in the temperate hardwoods that we offer, such as oak, with us offering two grades, the prime grade (fewer knots) and the character grade (knots permitted). These two grades are visual grades, so if you wanted less knots, you can choose the prime grade (at a premium cost) or if you enjoy the characteristics of knots within oak, the character grade is a good option at a more economical cost.
Knots are more prevalent in European Redwood (softwood) boards, even in higher grades used for internal joinery. The Timber Decking & Cladding Association (TDCA) states that there is no established UK structural grading rules for timber deck boards. The requirements for strength grading set out in BS EN 1408-1 are designed for rectangular timbers used on edge, not flatwise. Therefore, the strength grades are only applicable for timbers used on edge (I.e. timber joists) and not flatwise (I.e. deck boards).
The strength of the deck comes from the sub structure below and as long as the recommended joist spans are followed, and you use structurally graded timber joists to C16 or C24, you will avoid a springy deck.
On request, we can offer our deck boards structurally graded to either C16 or C24, however this is not necessary for deck boards with the majority of manufacturers not offering this as the structural grading rules cannot be applied to the deck board’s application.
Therefore, the most common standard for European Redwood boards is SS-EN 1611-1, which is the Swedish standard for appearance grading of softwoods. The boards are graded to this standard at the mills in Scandinavia before they are imported to the UK and look for knots, checks, wane, cross grain, compression, fungal attack, discolouration and dimension deviations.
All of Gripsure’s European Redwood deck boards are sourced sorted to the Scandinavian V grade which is most common grade used in the UK for deck boards. Scandinavian I-IV grades tend to be used for internal joinery products where knot content needs to be minimised. Within this grade, generally 90-95% of the boards we import meet the C16 strength grade. Gripsure also performs a final quality check to remove boards with excessive knot content.
With the absence of a structural grading rule for deck boards we would recommend choosing a manufacturer who carries the TDCA’s deckmark Plus quality accreditation such as Gripsure, a Scandinavian V grade European Redwood to SS-EN 1611-1, as well as following the recommended joist centres and ensuring the joists are structural graded to C16 or C24 for softwood joists.