Rock Elm
Ulmus thomasii · hardwood · Eastern North America · moderate movement
- Janka hardness 1,320 lbfred oak ≈ 1,290 lbf
- Density ≈ 44 lb/ft³specific gravity 0.63 at 12% MC
- Shrinkage — tangential 8.1%flatsawn width, green → ovendry
- Shrinkage — radial 4.8%quartersawn width
- T/R ratio 1.7 : 1higher = more cupping in flatsawn stock
Shop notes The heaviest elm, once boatbuilding and hockey-stick stock. Hard on edges; interlocked grain resists splitting hardware.
Seasonal movement at a glance
Width change for rock elm across a typical indoor year — 6% moisture content in the heated winter to 12% in a humid summer:
| Board width | Flatsawn | Quartersawn |
|---|---|---|
| 6″ | 3/32″ (0.104″) | 1/16″ (0.062″) |
| 12″ | 7/32″ (0.208″) | 1/8″ (0.123″) |
| 24″ | 13/32″ (0.417″) | 1/4″ (0.247″) |
| 36″ | 5/8″ (0.625″) | 3/8″ (0.370″) |
Run your own numbers
Different width, different climate swing — the calculator below is preset to rock elm:
Typical swings:
Rock Elm questions
How much does rock elm move with the seasons?
A 12″-wide flatsawn rock elm board moves about 7/32″ across a typical indoor swing from 6% to 12% moisture content; quartersawn, about 1/8″. Its tangential shrinkage is 8.1% and radial 4.8% (green to ovendry) — a moderate-movement wood.
How hard is rock elm?
Rock Elm rates about 1,320 lbf on the Janka scale — in red oak’s hardness neighborhood (red oak is roughly 1,290 lbf). Density runs about 44 lb/ft³ at indoor moisture levels.
Is rock elm good for wide panels and tabletops?
Yes, with normal precautions: its 1.7:1 tangential-to-radial ratio is moderate. Still fasten solid tops so they can move across their width.
Similar species
White Ash · American Beech · Northern Red Oak · White Oak · Post Oak · Bur Oak
Shrinkage, specific gravity, and hardness from the Wood Handbook, FPL-GTR-282 (2021 edition), USDA Forest Products Laboratory (public domain). Values are species averages — individual boards vary with growth conditions and how they were dried.