The thickness edge of a board or beam is the narrow, thicker side — that is, the side whose height (thickness) is greater than the width of the other face. On a typical 25 × 150 mm board, the thickness edge is the 25 mm wide side, while the 150 mm face is called the wide face or flat side.
Why does the thickness edge matter?
In woodworking, the orientation of timber is critical for stability, strength and the finished look:
- Dimensional control — when planing or routing, the thickness edge serves as a reference face; that is also why a planing machine that works parallel to it is called a *thicknesser*
- Movement of wood — shrinkage and swelling are mostly in the width direction; the thickness edge stays more dimensionally stable
- Structural strength — a beam is always weakest about its thickness; that is why joists and rafters are placed *on edge*, with the height resisting the load
Application
The thickness edge and wide face play a role in:
- Floor joists — installed with the height vertical (flat face up and down), maximising bending strength
- Roof rafters and purlins — the same; the narrow thickness edge faces you in a roof structure
- Skirting boards and panelling — the wide face is visible, the thickness edge disappears against the wall
- Planing work — when flattening a board, you first plane the wide face, then the perpendicular thickness edge
Planing sequence
When hand-planing, the standard order is:
- Face side — plane one wide face perfectly flat
- Edge (thickness edge) — plane the adjoining narrow face square to the face side (check with a try square)
- Second wide face — parallel to the face side, down to required thickness
- Second edge — parallel to the first edge, down to required width
On machine equipment, steps 1 and 2 are done on a jointer, steps 3 and 4 on a thickn
esser.
Terminology variants
- Edge or narrow face — synonyms for thickness edge
- End grain — *not* the same: that is the end of the beam (where annual rings are visible)
- Face or wide face — synonym for the broad flat side
Related terms
- End grain
- Planing
- Thicknesser
- Timber
- Woodworking
Want to know more about construction terms? Visit our knowledge base on fredsdiyplans.com.