Radial vs tangential shrinkage, in numbers
By Lucas Anderson · Published · Data: Wood Handbook 2010
Wood shrinks more along its growth rings (tangential) than across them (radial). In the Wood Handbook data this holds for all 155 woods: the median tangential shrinkage is 7.8% against 4.4% radial. The ratio of the two, the T/R ratio, shows how unevenly a board will move as it dries, and so how likely it is to cup.
Two directions, two numbers
A log’s growth rings run around the tree. Tangential shrinkage is measured along those rings, radial shrinkage across them, from the pith outwards. The handbook gives both as a percentage of the green dimension lost when the wood is dried to oven-dry. In the 155 woods on this site, the median tangential figure is 7.8% and the median radial figure 4.4%. Every one of the 155 woods shrinks more tangentially than radially; there is no exception in the tables.
A third number, volumetric shrinkage, is the loss of volume. For most woods it lands close to the two directional figures added together: the median gap between volumetric shrinkage and tangential plus radial is 0.5 percentage points. Volumetric shrinkage is a good single figure for “how much does this wood move,” but it hides the direction, and the direction is what decides whether a board stays flat.
What it means for a board
The cut of a board decides which of the two numbers acts on its width. In a flatsawn board the rings run across the wide face, so the width follows tangential shrinkage. In a quartersawn board the rings run through the thickness, so the width follows the radial figure. Take white oak: 10.5% tangential and 5.6% radial. A 6-inch board cut green and dried to oven-dry would lose about 0.63 in. of width if flatsawn and 0.34 in. if quartersawn. At the medians for all 155 woods, the same board loses 0.47 in. and 0.26 in.
Oven-dry is the extreme case. Wood in use rarely gets that dry, so real boards move less than these figures, but the tangential direction still moves more than the radial one. That uneven pull is what makes a flatsawn board cup.
The T/R ratio
Dividing tangential by radial shrinkage gives the T/R ratio. A ratio of 1 would mean the wood shrinks the same in both directions. The median across the 155 woods is 1.81, so a typical wood moves 1.8 times as much along the rings as across them. Only 24 woods have a ratio below 1.5, while 46 reach 2.0 or more.
The ratio and the amount of shrinkage are separate questions, and the data show how far apart they can be. Greenheart has the lowest ratio of all, 1.09, yet it shrinks 9.6% tangentially, more than the median wood: it moves a lot, but evenly. Eastern white pine is the opposite. Its tangential shrinkage of 6.1% is below the median, but its radial figure is only 2.1%, giving the highest ratio in the tables, 2.90. Macawood manages both: 3.5% tangential, the lowest of the 155, and a ratio of 1.30.
| Lowest T/R | Tangential | Radial | T/R |
|---|---|---|---|
| Greenheart | 9.6% | 8.8% | 1.09 |
| Ipe | 8.0% | 6.6% | 1.21 |
| Southern magnolia | 6.6% | 5.4% | 1.22 |
| Kempas | 7.4% | 6.0% | 1.23 |
| Caribbean pine | 7.8% | 6.3% | 1.24 |
| Macawood | 3.5% | 2.7% | 1.30 |
| Yellow birch | 9.5% | 7.3% | 1.30 |
| Azobe | 11.0% | 8.4% | 1.31 |
| Highest T/R | Tangential | Radial | T/R |
|---|---|---|---|
| Eastern white pine | 6.1% | 2.1% | 2.90 |
| Subalpine fir | 7.4% | 2.6% | 2.85 |
| Black oak | 11.1% | 4.4% | 2.52 |
| Laurel oak | 9.9% | 4.0% | 2.48 |
| Scarlet oak | 10.8% | 4.4% | 2.45 |
| Andiroba | 7.6% | 3.1% | 2.45 |
| Southern red oak | 11.3% | 4.7% | 2.40 |
| Silver maple | 7.2% | 3.0% | 2.40 |
Oaks stand out on the high side: four of the eight highest ratios belong to them, and the median for the 15 oaks in the handbook is 2.15, above the overall 1.81. On an oak species page, the gap between the flatsawn and the quartersawn board drawing is correspondingly wide.
Groups and volume extremes
The three groups on this site shrink by different amounts but with much the same balance. Median tangential shrinkage is 8.9% for U.S. hardwoods, 7.1% for U.S. softwoods and 7.9% for imports, while the median T/R ratios are 1.88, 1.86 and 1.74. In other words, a typical softwood moves less than a typical hardwood, but no group is clearly better at moving evenly.
By volume, the steadiest wood in the tables is macawood at 6.5%, followed by redwood (old-growth) and western redcedar at 6.8%. At the other end karri loses 20.2% of its volume, about three times as much, with shellbark hickory (19.2%) and laurel oak (19.0%) close behind. Hardness is no guide here: the hard shellbark hickory is second from the top, while the much softer American basswood, at 15.8%, still shrinks more than 126 of the 155 woods.
Using the numbers
For a wide panel or a tabletop, look at tangential shrinkage first if the boards will be flatsawn, since that sets how much the width changes. Look at the T/R ratio for the tendency to cup. The shrinkage ranking lists all 155 woods by ratio, each species page draws the flatsawn and quartersawn board to scale, and the finder filters woods by a maximum face shrinkage and a maximum T/R ratio at the same time.
Sources and method
USDA Forest Service, Forest Products Laboratory. Wood Handbook: Wood as an Engineering Material (2010, FPL-GTR-190), Tables 4-3 and 4-4 (shrinkage) and 5-3b and 5-5b (strength and hardness). Public domain. Medians, counts, ranks, correlations and percentages are calculated by TimberLedger from those tables across the 155 woods on this site. Data as of: Wood Handbook 2010 edition (published test averages of small clear specimens). How we use the data.