Rock Elm

Ulmus thomasii · Hardwood · North America

Janka 1,320 lbf≈39 lb/ft³ dried (est.)

Properties

Janka hardness	1,320 lbf (5,871 N)
Average dried weight (est.)	39 lb/ft³ (630 kg/m³)
Specific gravity	0.63
Modulus of rupture	14,800 psi (102.0 MPa)
Modulus of elasticity	1.54 million psi (10.62 GPa)
Crushing strength	7,050 psi (48.6 MPa)
Shrinkage — radial	4.8%
Shrinkage — tangential	8.1%
Shrinkage — volumetric	14.9%
Shrinkage T/R ratio	1.7

Values at 12% moisture content, clear-wood averages. Source: USDA Forest Products Laboratory, Wood Handbook FPL-GTR-282 (2021), Table 5–3b p. 5–9; shrinkage Table 4–3 p. 4–8.

Rock elm is the strong end of the elm group (specific gravity 0.63, Table 5-3b Janka hardness 1,320 lbf), a hard elm chosen where bending strength and toughness matter. Today it goes mostly into furniture and decorative panels.

Appearance & grain

The sapwood is nearly white. The heartwood is light brown, often tinged with red. Rock elm belongs to the hard elm class, which runs somewhat heavier than the soft elms.

Workability

The Handbook credits the elms with excellent bending qualities, and the hard elms — rock elm among them — are preferred where greater strength is required. That toughness is why elm historically served in vehicle parts and farm implements.