Edinburgh Napier University

  The UK has a British Standard for external timber cladding, BS8605-1, in which support battens are defined as timber components of a section, grade, and durability, suitable for supporting external timber cladding. The term includes both battens (which are used horizontally or diagonally) and counter battens (which are only used vertically). Clause 17 states that where the density of timber for support battens is required, this should be determined in accordance with EN384. This is because design of support battens might involve structural calculation, for which a characteristic density value (5th percentile, with appropriate adjustments) is necessary. This characteristic density is conservative on the side of underestimating real density, since this density value is used as an estimator for fixing performance and charring rate (fire resistance). Minimum mean density, which is calculated from characteristic density (a factor of 1.2 in EN384), is used as a threshold for reaction to fire classification without need for further testing.
This paper examines the effectiveness of grading by ring width for characteristic density, using UK and Irish grown British spruce as the case study. British spruce has relatively wide growth rings compared to other common timbers. There is a separate question as to whether ring width affects fastener performance and charring rate separately from density, but for this paper it is assumed that the equations given in EN384 and EN1995-1-1 are correct for any ring width.