What is the KAR (Knot Area Ratio) and why does it matter?

KAR is the ratio of the cross-sectional area of knots to the total cross-sectional area of the timber piece at the critical section. It is the single most important parameter in visual strength grading because knots are the primary strength-reducing defect in softwood.

KAR is the ratio of the cross-sectional area of knots to the total cross-sectional area of the timber piece at the critical section. It is the single most important parameter in visual strength grading because knots are the primary strength-reducing defect in softwood. For C24 grade (SS to BS 4978), the maximum KAR is typically 0.33 (one-third of the cross-section). For C16 (GS), the limit is 0.50. KAR is measured by projecting all knots within a 150 mm length onto the cross-section and calculating the total knot area as a proportion of the section area.

Why this matters

This question relates to European Timber Grades Explained: Nordic, German & British Systems, which covers the broader topic in detail. Understanding this specific aspect helps architects, builders, and timber dealers make informed specification decisions.

For full context and additional technical detail, see our comprehensive guide:

→ European Timber Grades Explained: Nordic, German & British Systems

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About this answer

This answer is part of the PuurModulair Knowledge Base — comprehensive timber specification guidance for architects, builders, and dealers across the UK, EU, and Scandinavia.