Timber defects — cracks from exterior surface shrinkage Cracks caused by shrinkage of the outside surface of wood exposed to the atmosphere are called:

Introduction / Context:
Correct identification of timber defects is important in carpentry and structural applications. Surface cracks due to weathering and drying influence aesthetics, durability, and paint or preservative performance.

Given Data / Assumptions:

• Lumber is exposed to atmospheric conditions leading to moisture loss.
• Cracks originate at the exterior face due to rapid surface shrinkage.
• Terminology follows standard woodworking practice.

Concept / Approach:
When the surface dries faster than the core, tensile stresses develop at the face, forming narrow, shallow cracks known as wind cracks. Shakes are different: radial/heart shakes often originate in the log during growth or seasoning and extend along the heart/annual rings, not merely at the outer face.

Step-by-Step Solution:
Link cause (surface shrinkage from weathering) to defect name (wind cracks).Differentiate from radial and heart shakes that relate to internal separations within the log structure.Exclude fibre distortion defects (twist) which are due to growth spiral, not surface shrinkage.

Verification / Alternative check:
Visual inspection: wind cracks are surface-limited, narrow, and often longitudinal; shakes are broader separations extending deeper along rings.

Why Other Options Are Wrong:
Radial/heart shakes are internal separations along rays or near the pith. Twisted fibres describe grain distortion, not cracking.

Common Pitfalls:
Confusing weather checks (wind cracks) with end splits from rapid kiln drying; the location and pattern provide the clue.

Final Answer:
Wind cracks