Glossary
Specific gravity
Also called: G, wood density, relative density
Reviewed
Definition
Specific gravity (G) is the density of wood relative to the density of water, a dimensionless number. It is the main wood property in pallet connection design: in the Uniform Standard's withdrawal equation, nail holding power scales with G raised to the 2.5 power.
Where specific gravity shows up
- Nail withdrawal: the Uniform Standard (Annex F) formula is W = 1380 x G^2.5 x wire diameter x penetration x FWC x adjustment factors. Because of the 2.5 exponent, wood with 10% higher G holds a nail about 27% better.
- Head pull-through: scales with G to the 2.25 power.
- Weight: denser species weigh more per board foot. Oak runs about 5.25 lb per board foot green against about 3.24 lb for SPF (nails included).
- Species classes: the PDS and Uniform Standard species classes group woods of similar density, e.g. high, medium and low density eastern hardwoods.
Density groups used in pallet specs
| Group | Example species classes |
|---|---|
| Medium and high density | eastern hardwoods (high and medium), oaks, SYP, Douglas fir-larch |
| Low density | aspen, basswood, poplar, SPF, hem-fir, cedars |
Per-species G values come from the USDA Wood Handbook.
Why it matters
Density decides which nail to use (helical for dense hardwoods, annular for softer woods), how heavy the pallet is, and how strong the joints are. See pallet lumber and fastener withdrawal characteristic.
Questions
- Why does wood density matter for pallets?
- Denser wood holds nails much better and is stronger, but it is heavier. Nail holding power rises with specific gravity to the 2.5 power.
Sources
- Uniform Standard for Wood Pallets (NWPCA, now Woodpack Global), 2012 edition