What Type of Wood Are Pallets Made Of? Treatment & Preservation

The Core Question: What Type of Wood Are Pallets Made Of?

The global supply chain relies on over 3 billion wooden pallets, yet the specific lumber species used in their construction is rarely uniform. To answer what type of wood are pallets made of, you must look at regional forestry outputs, load-bearing requirements, and preservation capabilities. Pallet lumber is broadly categorized into hardwoods and softwoods, with each offering distinct structural and treatment-related properties.

From a preservation standpoint, the species dictates the wood's natural susceptibility to biological degradation. Southern Yellow Pine, while strong, has a high sapwood content that is highly permeable to both moisture and wood-destroying organisms, making rigorous treatment protocols mandatory for international transit.

ISPM 15 and the Reality of Pallet Treatment

When discussing pallet preservation, the International Standards for Phytosanitary Measures No. 15 (ISPM 15) is the governing global framework. The standard mandates that all wood packaging material (WPM) moving across international borders must be treated to eradicate wood-boring pests like the Asian longhorned beetle and pinewood nematode.

It is a common misconception that ISPM 15 treatment preserves the wood against rot or mold. The standard is strictly phytosanitary, not preservative. The two primary approved methods are:

According to the National Wooden Pallet and Container Association (NWPCA), over 95% of globally exported pallets now utilize HT rather than MB, driven by environmental regulations and the phasing out of ozone-depleting substances.

Chemical Preservatives and Modern Alternatives

While ISPM 15 handles pests, it does nothing to stop fungal decay or termites in domestic storage. For pallets stored outdoors or used in high-humidity environments (such as cold-chain logistics or agricultural transport), secondary chemical preservation is required.

Borate Treatments for Insect Resistance

Disodium octaborate tetrahydrate (DOT), commonly known by brand names like Tim-bor or Bora-Care, is the industry standard for domestic pallet preservation against termites and carpenter ants. Borates are water-soluble, meaning they penetrate deeply into green SYP or Oak during a dip-treatment process. However, because they are water-soluble, borate-treated pallets cannot be exposed to rain or prolonged high humidity, as the chemical will leach out, rendering the wood unprotected.

Anti-Mold and Sapstain Dips

Freshly cut pallet lumber is highly susceptible to sapstain fungi (such as Ceratocystis) and mold (such as Aspergillus and Penicillium). To preserve the aesthetic and sanitary quality of pallets destined for food, beverage, or pharmaceutical transport, mills utilize quaternary ammonium compounds or chlorothalonil-based anti-sapstain dips immediately after sawing. These treatments provide a 30- to 60-day chemical barrier against mold spores during transit and initial storage.

Critical Warning: Never use pallets treated with Chromated Copper Arsenate (CCA) or Creosote for indoor furniture, gardening, or agricultural projects. While rare in modern standard shipping pallets, legacy or specialized industrial pallets may contain these toxic heavy metals. Always verify the absence of a greenish tint or harsh chemical odor before repurposing lumber.

Moisture Content and Rot Prevention in Transit

The single greatest threat to pallet longevity is moisture. Wood-decaying fungi require a moisture content (MC) above the fiber saturation point—typically around 28% to 30%—to initiate cellular breakdown. However, surface mold can bloom at an MC as low as 20% if ambient relative humidity exceeds 80%.

To preserve structural integrity and prevent biological growth, the U.S. Forest Products Laboratory recommends maintaining pallet moisture content below 19%. This is where the distinction between Heat-Treated (HT) and Kiln-Dried (KD) becomes vital:

• HT Pallets: Heated to 56°C to kill pests, but the process may only reduce MC to 22-25%. They are safe for bugs but vulnerable to mold if stored in damp warehouses.
• KD Pallets: Dried to an MC of 19% or lower. Kiln drying inherently satisfies the ISPM 15 HT requirement while simultaneously preserving the wood against fungal decay.

Field Testing for Moisture and Treatment

Procurement officers and warehouse managers should utilize pin-type moisture meters (such as the Delmhorst J-2000 or Wagner Meters Orion) to verify incoming pallet quality. Insert the pins into the thickest component of the pallet—usually the 3.5-inch stringer or block. If the reading exceeds 19%, the pallet is at immediate risk of mold proliferation and fastener corrosion.

Extending Pallet Lifespan: Preservation Best Practices

Even well-treated SYP or Oak pallets will fail prematurely if storage protocols ignore basic wood science. Implement the following preservation framework in your facility:

1. Elevate Storage: Never store empty or loaded pallets directly on concrete or soil. Concrete wicks moisture, and soil introduces ground-dwelling fungi. Use rack systems or place a sacrificial layer of pressure-treated lumber at the base.
2. Cross-Stacking for Airflow: When storing empty pallets outdoors or in unconditioned spaces, cross-stack them (alternating the direction of each layer) rather than nesting them. This creates vertical chimneys that promote convective drying after rain events.
3. UV Protection: While UV radiation does not cause rot, it degrades the lignin on the wood surface, causing checking (splitting) and reducing the holding power of the top deckboards. Store pallets under opaque roofing or heavy-duty tarps.
4. Rotate Inventory (FIFO): Pallets stored for over 12 months in fluctuating humidity will experience fastener loosening due to the natural swelling and shrinking of the wood fibers. Strict First-In, First-Out rotation prevents structural fatigue.

Sourcing and Compliance Checklist

Understanding the specific wood species and the precise nature of their treatment transforms pallets from a disposable shipping expense into a managed, predictable asset. By aligning your procurement with ISPM 15 standards, strict moisture thresholds, and proper biological preservation techniques, you safeguard both your supply chain and the products it carries.