What Shakes Are
A shake is a separation along the grain of timber — a crack or split that follows the wood fibres rather than crossing them. Unlike checking, which develops during drying as a surface defect, shakes typically originate in the standing tree or during felling and exist before any processing begins. They may not be visible on the surface of sawn boards, becoming apparent only when the board is cut or when shear force is applied in service.
Shakes are particularly serious in structural timber because they occur parallel to the board surface and directly reduce shear capacity. A beam carries shear stress through horizontal planes parallel to its axis — a shake along exactly this plane eliminates the timber's ability to transfer shear across that plane.
Types of Shakes
Heart shakes radiate from the centre (pith) outward, following the rays. They are caused by internal stresses in old trees where the heartwood has shrunk relative to the sapwood. In sawn boards they appear as cracks running from the middle toward the edge.
Ring shakes (or cup shakes) follow growth ring boundaries, separating one annual ring from the adjacent one. They are caused by frost damage, wind stress, or severe growth stress in the standing tree. In sawn boards they appear as arcs following a growth ring. They are the most dangerous type because they can be entirely internal — no surface expression — and only become visible when the board is cross-cut or fails in service.
Wind shakes are caused by excessive bending stress on the standing tree during storms. They follow the grain direction across the cross-section and produce a fibrous, torn appearance distinctly different from a clean drying check.
- Heart shake: from pith outward — visible on face and end grain
- Ring shake: along growth ring — may be entirely internal and invisible on faces
- Wind shake: across grain from mechanical stress — fibrous, torn appearance
- All shakes reduce shear capacity — structural impact depends on location and extent
Why Ring Shake Is the Most Dangerous Defect
Ring shake is particularly dangerous because it cannot always be detected by visual inspection of board surfaces. A board with a severe internal ring shake may look clean on all four faces and both ends. The defect only reveals itself when the board is cross-cut — exposing the ring separation on the end grain — or when the member fails in shear.
This failure mode is most concerning in roof rafters and floor joists, where horizontal shear stress is highest near the supports. A shake running along the beam at mid-depth can cause sudden, catastrophic failure under a peak load event with almost no prior warning.
How to Check for Shakes at Delivery
Inspect the end grain of every piece in a structural delivery. The end grain cross-section reveals heart shakes as radial cracks from the centre and ring shakes as separations along ring boundaries — both are clearly visible even when invisible on the face. Any timber with a ring shake visible on the end should be rejected for structural use regardless of face grade.