Flat Sawn Vs Quarter Sawn And Movement

Two boards cut from the same log, at the same width, can move to two noticeably different degrees under the identical humidity swing — and the entire difference comes down to the angle the growth rings make with the wide face, nothing else.

What the two cuts actually are

Flatsawn (also called plain-sawn) lumber is cut so the growth rings run roughly parallel to the wide face — the most common and most economical way to saw a log, because it yields the widest boards with the least waste. The face you see is dominated by the cathedral-arch grain pattern most people picture when they think "wood grain."

Quartersawn lumber is cut so the growth rings run roughly perpendicular to the wide face — achieved by first quartering the log, then sawing each quarter radially outward from the centre. It yields narrower boards, produces more waste per log, and costs more per board foot as a direct result of that lower yield — but it exposes the medullary rays as a flake or fleck figure in species (like white oak) where those rays are prominent, and it changes how the board moves.

Why the ring orientation changes the movement number

Wood shrinks roughly twice as much tangentially (along the direction of the growth rings) as it does radially (across them, toward the pith) — the exact ratio varies by species but is almost always in this direction. A flatsawn board's wide face is oriented tangentially, so as the board dries or picks up moisture, that face moves at close to the tangential rate. A quartersawn board's wide face is oriented radially, so it moves at close to the smaller radial rate instead. Nothing about the wood itself changed between the two cuts — same tree, same species, same density — only the angle the saw took through the log, and that angle alone is what determines which of the two shrinkage numbers governs the board's real-world behaviour.

A concrete before-and-after

Take white oak, a species with a wider-than-average gap between its two shrinkage figures (tangential 10.5%, radial 5.6%). A 12"-wide flatsawn panel moving through a typical 6%-to-12% moisture-content seasonal swing shifts about 0.252" across its width. Reorient the identical species and width to quartersawn instead, and the same swing moves it only about 0.134" — nearly half. That's not a marginal improvement; it's the difference between a movement allowance that comfortably fits inside a standard frame groove and one that pushes the limits of what a joint can accommodate without splitting.

Cupping is the other half of the story

Flatsawn boards are also the ones that cup as humidity swings, because the tangential shrinkage is concentrated on one face relative to the growth-ring curvature running through the board's thickness — the face further from the pith (typically the "bark side") shrinks more in proportion to its position than the face closer to the pith, pulling the board into a curve across its width. Quartersawn boards, with their growth rings running through the thickness rather than across the width, resist this distortion far better and stay flatter through the same humidity cycles — a second, independent reason wide quartersawn stock is specified for tabletops and other surfaces where staying flat matters as much as staying the right size.

When the extra cost is worth it

Quartersawn stock costs more and is harder to source in wide widths, so the honest answer is: spend it where the movement or the flatness genuinely matters — wide panels, tabletops, and anything glued rigidly into a frame with little tolerance for shrinkage — and save flatsawn for parts where the wider, cheaper boards' greater movement is a design non-issue, like show-hidden carcase parts or narrow face-frame stock. Run both orientations for a specific species and width through the wood movement calculator before deciding which one a given part actually needs.