Two numbers describe how a wood species moves across the grain: its tangential shrinkage and its T/R ratio, which is tangential shrinkage divided by radial shrinkage [1]. Tangential shrinkage sets how far a flat-sawn panel moves. The T/R ratio sets how unevenly the wood moves, which drives cupping and decides how much quartersawing helps [1].
The two numbers
Tangential movement runs along the growth rings and dominates the width of flat-sawn lumber. Radial movement runs across the growth rings [1].
A high T/R ratio means a species moves much more in one cross-grain direction than in the other. That difference makes wide flat-sawn boards in the species cup more [1].
A low T/R ratio means a species moves more evenly in both cross-grain directions. A low-ratio species cups less and is more forgiving in panel glue-ups that mix grain orientations [1].
Movement sets float clearances, breadboard slot lengths, door gaps, drawer sizing and whether tabletop attachment hardware has enough travel [1][2].
Four species compared
| Species | Tangential shrinkage | T/R ratio | 24" flat-sawn panel, 4% MC swing (example) |
|---|---|---|---|
| Black cherry | 7.1% | 1.92 | about 1/4" |
| Black walnut | 7.8% | 1.42 | about 1/4" |
| White oak | 10.5% | below hard maple's 2.06 | about 3/8" |
| Hard maple | 9.9% | 2.06 | about 11/32" |
Tangential shrinkage and T/R ratios in the table are green to oven-dry values [1].
The panel movements in the table are a worked example: a 24-inch flat-sawn panel through a 4 percentage point seasonal MC swing, calculated as width × MC swing × the species' tangential dimensional change coefficient per 1% MC [1].
The table shows the pattern between species. Sizing an actual joint calls for a float clearance table by species, panel width and the MC swing of the room the piece will live in [1][2].
Walnut: the most even mover
Black walnut has the lowest T/R ratio of black cherry, black walnut, white oak and hard maple: 1.42 [1].
Walnut moves more evenly across its two cross-grain directions than cherry, white oak or hard maple, which makes it the most forgiving of the four in wide flat-sawn panels and mixed-grain glue-ups [1].
A 24-inch flat-sawn walnut top moves about 1/4" through a 4 percentage point MC swing [1]. At that swing, standard Z-clips have enough travel for most species at most tabletop widths [2].
Because walnut's T/R ratio is low, quartersawn walnut gains less stability over flat-sawn walnut than a high-ratio species gains from quartersawing [1].
Hard maple and white oak
White oak's tangential shrinkage is 10.5% and hard maple's is 9.9%, so a flat-sawn white oak panel moves slightly more across its width than a flat-sawn hard maple panel of the same width [1].
Hard maple's T/R ratio, 2.06, is higher than white oak's [1]. Hard maple moves less in total than white oak but less evenly, so flat-sawn hard maple cups more readily and gains more from quartersawing [1].
Hard maple is often treated as the more stable species because of its density and fine grain. Its movement numbers do not support that reputation [3].
A wide hard maple panel built the way a walnut panel is built, with walnut's clearances, is undersized for hard maple's movement [1].
The delivery environment
Seasonal movement scales in proportion to the MC swing. A piece built in a shop with a 4 percentage point swing and delivered to a house with a 6 point swing moves about one and a half times as far at the client's house as it did in the shop [1][4].
Six percentage points is a safe default MC swing for unmanaged residential interiors [4].
Attachment hardware, slots and clearances should be sized for the swing at the client's site, not the swing in the shop [4].
Choosing a species is choosing a number
Walnut and cherry move least across a flat-sawn panel. White oak and hard maple move most. Hard maple moves least evenly of the four [1].
Knowing a species means knowing both how it behaves under tools and finish and how far and how evenly it moves [1].
Sources
- USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (FPL-GTR-190), Chapter 4, Table 4-3 (Shrinkage Values); Skaar, C. (1988), Wood-Water Relations, Springer-Verlag (dimensional change coefficients); Hoadley, R.B. (2000), Understanding Wood, 2nd ed., Taunton Press, Chapter 4.
- Hewn heuristic data, HEU-R1249 (tabletop attachment travel) and HEU-R1235 (wide-panel float by MC swing, from USDA Wood Handbook coefficients), Reed Additions, provisional and unreviewed.
- Hewn heuristic data, HEU-R1234 (hard maple stability reputation), Reed Additions, provisional and unreviewed.
- USDA FPL GTR-282, Chapter 4 (Moisture Relations) (as listed against Hewn heuristic R736, MC swing by environment, in Hewn's notes).