Grain deviation, also called slope of grain, is the angle by which the wood fibres deviate from the long axis of a board, beam or batten. In ideal timber, the fibres run exactly parallel to the length; with grain deviation, they run at an angle or in a spiral.
How does grain deviation arise?
Grain deviation occurs because:
- The tree grew with a twist — some species have a natural spiral growth
- The log is sawn off-axis — crooked logs or wrong saw setting
- Knots and local growth disturbances cause local deviation in the fibre direction
- Reaction wood — formed when the tree grew leaning due to wind or slope
Effect on properties
Grain deviation strongly affects the properties of wood:
- Reduced strength — especially in tension; strength drops sharply when the fibres run at an angle
- Reduced stiffness — less rigid under load
- Movement — timber with grain deviation tends to warp more easily
- Workability — harder to plane cleanly
- Splitting — tendency to split along the grain
Indication in practice
Grain deviation is often expressed as a ratio, e.g. 1:10 (over 10 cm of length, 1 cm of fibre displacement across the board). Structural grading standards reflect this:
- Strength class C — a maximum slope of grain per class (e.g. 1:10 for C18)
- Higher quality — demands smaller deviation (e.g. 1:14 for C24)
Application and quality control
When selecting structural timber, grain deviation is always assessed:
- Purlins and rafters — low slope of grain required
- Floor joists — straight grain preferred
- Joinery, windows and doors — low deviation for dimensional stability
- Formwork timber — more tolerant, since requirements are lower
Visual inspection
An experienced carpenter or grader spots grain deviation quickly: the lines between annual rings on the board run visibly at an angle or in a slight curl.
Related terms
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