Decking Board selection
Common sizes for seasoned boards include 90,120 and 140mm widths, and 35 or 45mm thicknesses. Grades included F17 and F27.Unseasoned board sizes include 75, 100, 125 & 150 widths, and 38 or 50mm thicknesses. Grades include F14 and F17, with some F11 available on a limited basis. Details of spans achievable with these grades and sizes are given in Table 2.

Decking profiles may be dressed, rough sawn or reeded. Edges may be square, rounded or arrised - as shown in Figure 2. Selection issues include:

• Rough sawn or reeded boards offer varying degrees of slip resistance. Reeded boards only provides increased slip resistance when travelling perpendicular to the grooves. Reeded boards will also tend to collect and hold dirt in the grooves.
  
• Arrised and rounded edge boards reduce splintering and improve the performance of coating systems as there is reduced risk of peel.
  
• The degree of cupping in decking can be minimised if the section ratio does not exceed 1:3 depth to width (approximately).
  
• The effects of weathering are not as obvious on rough sawn decking as they are on dressed material.
  
• To increase service life, boards should be laid with the concave growth rings facing downwards, as shown in Figure 2. This reduces or limits checking and hence consequential water retention.
  The closer the joist spacing the better the restraint for decking boards, thus reducing distortion.
  
• Unseasoned boards may shrink and should be laid with the in-service life moisture content in mind.

Figure 2: Common decking profiles