Wood species
Elm wood
Elm is a hardwood; this page covers the elm, led by American elm (Ulmus americana). At 12% moisture content it has a Janka hardness of 830 lbf (3,692 N) and it weighs about 35 lb/ft³ (560 kg/m³), 2.9 lb (1.32 kg) per board foot.
Names and species on this page: American elm, white elm, rock elm, slippery elm, red elm.
Key numbers
- Janka hardness, 12% MC
- 830 lbf (3,692 N)
- Green: 620 lbf (2,758 N)
- Weight at 12% MC
- 35 lb/ft³ (560 kg/m³)
- 2.9 lb per board foot
- Specific gravity (oven-dry weight)
- 0.50 at 12% MC volume
- 0.46 on green volume
- Shrinkage, green to oven-dry
- 9.5% tangential
- Radial 4.2%, volumetric 14.6%
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Why: Measured by the USDA Forest Products Laboratory; elm's density is worked from its specific gravity with the Handbook's own equation.
How much elm moves
A 12 in wide flatsawn board of American elm that goes from 6% to 12% moisture content swells about 0.24 in (about 1/4 in, 6.2 mm). Quartersawn, the same board moves about 0.1 in (about 3/32 in, 2.6 mm). Flatsawn faces move 2.3 times as much as quartersawn ones, because the wood shrinks 9.5% around the growth rings but only 4.2% across them.
Worked with the Wood Handbook's dimensional change coefficients for American elm (Table 13-5: 0.00338 tangential, 0.00144 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
- Soft elm (American, slippery) is moderately heavy, hard and stiff with high shock resistance; hard elm (rock and others) is heavier. All elm bends very well.
- Uses
- Today mostly furniture and decorative panels; hard elm where strength matters.
The Handbook reports that Dutch elm disease and phloem necrosis have killed huge numbers of American elm, which the IUCN now lists as endangered.
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 American 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) | 7,200 psi (50 MPa) | 11,800 psi (81 MPa) |
| Stiffness (MOE) | 1,110,000 psi (7.7 GPa) | 1,340,000 psi (9.2 GPa) |
| Crushing strength (compression parallel to grain) | 2,910 psi (20 MPa) | 5,520 psi (38 MPa) |
| Compression perpendicular to grain | 360 psi (2 MPa) | 690 psi (5 MPa) |
| Shear parallel to grain | 1,000 psi (7 MPa) | 1,510 psi (10 MPa) |
| Tension perpendicular to grain | 590 psi (4 MPa) | 660 psi (5 MPa) |
A dash means the Handbook prints no value; none is filled in.
The elm
The Handbook tests each species separately; they are sold together as elm. Across the 3 species measured, Janka hardness at 12% runs from 830 to 1,320 lbf.
| Species | Janka, 12% MC | Weight, 12% MC | Tangential shrinkage |
|---|---|---|---|
| American elmUlmus americana | 830 lbf | 35 lb/ft³ | 9.5% |
| Rock elmUlmus thomasii | 1,320 lbf | 44 lb/ft³ | 8.1% |
| Slippery elmUlmus rubra | 860 lbf | 37 lb/ft³ | 8.9% |
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
Why these numbers
Why these numbers?
The situation
The USDA Forest Products Laboratory measured American elm at 830 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 × 0.50 × 1.12.
Arithmetic: exact by definition , sources [1]
The number
About 35 lb/ft³, or 2.9 lb per board foot (1/12 ft³), at 12% moisture content.
Arithmetic: exact by definition
Sources