palletdesign.ai

Pallet lumber and fasteners

The wood and the nails. Species, grade and moisture set a pallet's weight, strength and cost; the nail sets how long it lasts.

16 pages

Pallet lumber: species, grades and moisture
Pallet lumber is specified by three things: species class (from Annex B of the Uniform Standard), grade, and moisture content at manufacture. In North America the common choices are mixed Eastern hardwoods, oak, southern yellow pine, SPF and Douglas fir, and weight runs from about 3.2 lb per board foot for green SPF to about 5.3 lb for green oak.
Mixed hardwood pallet lumber
Mixed hardwood is the most common pallet lumber in the eastern US: a blend of Eastern hardwood species classes, often 50% high-density (class 1), 25% medium-density (class 3) and 25% low-density (class 7). It weighs about 4.37 lb per board foot green including nails, so a GMA-style 48 x 40 pallet comes in around 59 lb.
Oak pallet lumber
Oak (red and white oak, Uniform Standard species class 21) is one of the densest common pallet woods. palletdesign.ai assumes about 5.25 lb per board foot green and 3.9 lb kiln dried, including nails, and an allowable bending stress of 1,200 psi, so it suits heavy and racked loads at the cost of extra weight.
Southern yellow pine (SYP) pallet lumber
Southern yellow pine (SYP) is the dense softwood group of loblolly, longleaf, shortleaf and slash pine, species class 22 in the Uniform Standard. palletdesign.ai assumes 4.45 lb per board foot green and 3.4 lb kiln dried including nails, and 1,300 psi allowable bending stress, the highest of its five species.
SPF (spruce-pine-fir) pallet lumber
SPF (spruce-pine-fir) is Uniform Standard species class 13, a low-density softwood group. It is the lightest of palletdesign.ai's five species at about 3.24 lb per board foot green and 2.3 kiln dried including nails, with a conservative allowable bending stress of 850 psi, so SPF pallets are light but need larger sections for heavy or racked loads.
Douglas fir pallet lumber
Douglas fir is a western softwood classed with larch as Uniform Standard species class 11, in the medium/high-density sort group. palletdesign.ai assumes about 3.9 lb per board foot green and 3.0 lb kiln dried including nails, and an allowable bending stress of 1,150 psi, giving near-hardwood strength at lower weight.
Hem-Fir pallet lumber
Hem-fir is a western softwood lumber group (western hemlock with true firs) that the Uniform Standard for Wood Pallets classes as species class 12, in the low-density sort group. For pallets it behaves as a light softwood: annularly threaded nails are the usual recommendation, and strength and weight values should come from the manufacturer's PDS lumber data.
Pallet lumber grades
Pallet lumber grades limit defects such as knots, wane, splits, decay and slope of grain. The Uniform Standard sets two minimum quality levels, reusable (about PDS Standard grade) and single-use (about PDS Economy), and PDS grades run Select, Premium, Standard, Utility and Economy, often written as a minimum plus better (for example Standard &BTR).
Pallet lumber moisture content
Moisture content (MC) is the weight of water in wood as a percentage of its dry weight. Pallets are built either green or kiln dried, commonly to 19% (KD19). Drying cuts weight (a GMA-style mixed hardwood pallet drops from about 59 lb green to about 39 lb at KD19) and shrinks parts about 1% for every 5% of moisture lost below about 28%.
Kiln-dried vs heat-treated pallets
Kiln drying (KD) removes moisture from lumber to a target, commonly 19%, to cut weight, shrinkage and mold. Heat treatment (HT) is the ISPM 15 phytosanitary treatment, holding the wood core at 56 °C for 30 minutes, and is marked with the IPPC stamp. They solve different problems, and a KD stamp alone does not make a pallet export compliant.
Nominal vs actual lumber sizes for pallets
Pallet drawings and PDS specs always use actual dimensions at manufacture, not nominal lumber sizes. A nominal softwood 2x4 is 1-1/2 x 3-1/2 in, a 1x4 is 3/4 x 3-1/2 in and a 1x6 is 3/4 x 5-1/2 in, while pallet deckboards called 1x4 are often 5/8 in thick and hardwood stringers are often 1-1/4 or 1-3/8 x 3-1/2 in.
Pallet nails: types, sizes and how many
Most wood pallets are assembled with threaded nails: helically threaded for medium- and high-density hardwoods, annularly threaded (ring shank) for low-density hardwoods and most softwoods. A typical 48 x 40 stringer pallet uses 2-1/4 in x 0.112 in (11-1/2 gauge) nails, 84 in total, set by the Uniform Standard minimum of 2 per joint for boards up to 5-1/4 in wide and 3 up to 7 in.
Pallet nail quality: MIBANT, FWC and thread press-out
Pallet nail quality comes down to stiffness and grip. Stiffness is measured by the MIBANT bend angle (lower is better; 8 to 24 degrees is rated excellent), and grip by the fastener withdrawal characteristic (FWC, minimum 1.9 for reusable pallets) and thread press-out (over 0.020 in is excellent). Virginia Tech found 11-1/2 gauge nails gave about twice the pallet life of collated 12-1/2 gauge nails.
Plywood and OSB pallet decks
Plywood and OSB panels are used as solid pallet decks for small, point or unstable loads. Under the Uniform Standard they must be grade-stamped PS 1, PS 2 or PRP-108 panels with exterior adhesive, laid with the face grain perpendicular to the stringers in pieces at least 24 in wide. Processed wood panels are exempt from ISPM 15, but solid-wood parts on the same pallet are not.
Presswood (wood-composite) pallet blocks
Presswood blocks are wood-composite pallet blocks made from wood fiber and resin under heat and pressure. The Uniform Standard requires a density of 36.9 to 42.1 lb per cubic foot (591 to 675 kg per cubic meter) and tests for water soak, side tine compression (at least 1,400 lbf), nail insertion and nail withdrawal (at least 310 lbf). As processed wood they are exempt from ISPM 15.
Recycled pallet lumber and remanufactured pallets
Recycled pallet lumber is reclaimed from dismantled pallets and reused to repair pallets or build remanufactured and combo pallets. The Uniform Standard classes these as Remanufactured or Remanufactured/Combo, sets repair methods and repair marking, restricts some low-density species to interior deckboards in repairs, and requires repaired or remanufactured pallets to be re-treated and re-marked for ISPM 15.

More reference