Wood species
Rock elm wood
Rock elm is a hardwood (Ulmus thomasii). At 12% moisture content it has a Janka hardness of 1,320 lbf (5,872 N) and it weighs about 44 lb/ft³ (706 kg/m³), 3.7 lb (1.67 kg) per board foot.
Also called: cork elm, Ulmus thomasii.
Key numbers
- Janka hardness, 12% MC
- 1,320 lbf (5,872 N)
- Green: 940 lbf (4,181 N)
- Weight at 12% MC
- 44 lb/ft³ (706 kg/m³)
- 3.7 lb per board foot
- Specific gravity (oven-dry weight)
- 0.63 at 12% MC volume
- 0.57 on green volume
- Shrinkage, green to oven-dry
- 8.1% tangential
- Radial 4.8%, volumetric 14.9%
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Why: Measured by the USDA Forest Products Laboratory; rock elm's density is worked from its specific gravity with the Handbook's own equation.
How much rock elm moves
A 12 in wide flatsawn board of rock elm that goes from 6% to 12% moisture content swells about 0.21 in (about 7/32 in, 5.2 mm). Quartersawn, the same board moves about 0.12 in (about 1/8 in, 3 mm). Flatsawn faces move 1.7 times as much as quartersawn ones, because the wood shrinks 8.1% around the growth rings but only 4.8% across them.
Worked with the Wood Handbook's dimensional change coefficients for rock elm (Table 13-5: 0.00285 tangential, 0.00165 radial per 1% change in moisture content), which hold between 6% and 14%. Individual boards vary: the Handbook gives shrinkage a coefficient of variation of about 15% (p. 4-11).
Color, grain and working
- Color
- Light brown heartwood, often tinged red, and nearly white sapwood.
- Working and character
- A hard elm: somewhat heavier than the soft elms (American and slippery). Elm has excellent bending qualities.
- Uses
- Hard elm is preferred where strength is needed; elm lumber and veneer go mostly into furniture and decorative panels.
Adapted in our own words from the Wood Handbook, Chapter 2, p. 2-6 to 2-7.
Strength
Bending strength is the modulus of rupture (MOR); stiffness is the modulus of elasticity (MOE). Clear, straight-grained samples of rock elm tested by the Forest Products Laboratory (Wood Handbook Table 5-3b). Knots and cross grain make lumber weaker than these clear samples (Handbook Chapter 5).
| Property | Green | 12% MC |
|---|---|---|
| Bending strength (MOR) | 9,500 psi (66 MPa) | 14,800 psi (102 MPa) |
| Stiffness (MOE) | 1,190,000 psi (8.2 GPa) | 1,540,000 psi (10.6 GPa) |
| Crushing strength (compression parallel to grain) | 3,780 psi (26 MPa) | 7,050 psi (49 MPa) |
| Compression perpendicular to grain | 610 psi (4 MPa) | 1,230 psi (8 MPa) |
| Shear parallel to grain | 1,270 psi (9 MPa) | 1,920 psi (13 MPa) |
| Tension perpendicular to grain | — | — |
A dash means the Handbook prints no value; none is filled in.
Dust and health
- Wood dust is classed as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer, chiefly for cancers of the nose and sinuses.
- NIOSH recommends keeping wood dust exposure to 1 mg/m³ of total dust, as quoted in OSHA's sawmill guidance.
This is what the named authorities publish, not medical advice.
Compare with
Buying and finishing rock elm
Work it out for your project
- Rank woods for furniture with the species picker
Weigh rock elm against other woods on hardness, weight, movement and color, every figure from the Wood Handbook.
- Plan a finish for furniture in rock elm
The finishes whose makers name that use, and a dated coat schedule from each product’s own sheet.
Why these numbers
Why these numbers?
The situation
The USDA Forest Products Laboratory measured rock elm at 1,320 lbf of side hardness at 12% moisture content, on clear, straight-grained samples (Wood Handbook Table 5-3b).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
What that means
Side hardness (Janka) is the load needed to press a 0.444 in (11.28 mm) steel ball halfway into the wood across the grain; a higher number dents less (Handbook p. 5-5).
One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]
So
Density at 12% is water's density × specific gravity × (1 + 0.12) (Handbook Eq. 4-14): 62.43 × 0.63 × 1.12.
Arithmetic: exact by definition , sources [1]
The number
About 44 lb/ft³, or 3.7 lb per board foot (1/12 ft³), at 12% moisture content.
Arithmetic: exact by definition
Sources