Can loaded pallets be double-stacked? Yes, provided the lower load and pallet can safely support the additional weight, and the combined stack remains stable. Product and packaging strength, pallet condition, load distribution, stack height and the handling environment all matter. A pallet load that is safe by itself isn’t necessarily safe with another loaded pallet sitting on top of it.
Double-stacking pallet loads can make a lot of sense. If you have the available height, putting two loads in the same footprint can improve warehouse space utilization and, in the right application, dramatically improve trailer cube.
But there is an important qualification. Just because one pallet load will physically sit on top of another doesn’t mean the two loads were designed to be stacked.
Start With the Bottom Load
When two loaded pallets are stacked, everything underneath the upper pallet has to carry the additional weight. That includes the bottom pallet, of course, but the pallet may not be the weakest link. Often it is the product packaging.
Corrugated cartons in the lower layers have to withstand the weight imposed from above without crushing or deforming. Moisture, storage time and uneven support can reduce their ability to do that. Bags, pails, trays and other packaging formats bring their own considerations.
The top of the lower load also needs to provide suitable support for the pallet above it. A reasonably flat, uniform load is a much better candidate for double-stacking than one with an irregular top surface or product that can shift under concentrated pressure. Significant carton underhang can also affect how the upper pallet transfers its weight into the load below, one of the issues discussed in Should Cartons Overhang a Pallet or Be Set Back From the Edge?
This is why the question shouldn’t simply be, “Can my pallet hold the weight?” It is whether the entire lower unit load can safely support the one above it.
Know What Your Pallet Capacity Really Means
Pallet load ratings can also cause confusion because pallet capacity isn’t necessarily one number.
A pallet sitting fully supported on the floor is loaded differently from one spanning pallet rack beams or sitting on forklift tines. As we discuss in our guide to pallet load capacity, support conditions, pallet design and pallet condition all influence how much weight a pallet can safely carry.
For double-stacking, the condition of both pallets deserves attention. A damaged bottom board, split stringer, broken block or other weakness may not prevent a forklift from moving the pallet across the warehouse, but it could become more significant when another loaded pallet is placed above it. If the pallet itself is questionable, see When Should a Wood Pallet Be Taken Out of Service?
The upper pallet matters too. Its bottom deck or runners determine how its weight is transferred into the load below. Concentrating that weight in areas the lower packaging wasn’t designed to support can create problems even when the total weight doesn’t initially look excessive.
Stability Matters as Much as Weight
Then there is the stack itself. A slightly leaning load might be manageable as a single pallet at floor level. Put another loaded pallet on top and you have raised the center of gravity and increased the consequences if something moves. A load that is already leaning should be investigated before another load is placed above it. See Why Is My Pallet Load Leaning?
Loads with significant pallet overhang, damaged cartons, uneven weight distribution or poor containment are poor candidates for double-stacking. The floor should be level and suitable, and lift truck operators need enough room and visibility to place and retrieve the upper load without striking or destabilizing adjacent stacks.
We’ve gone into load stacking safety issues in considerably more detail in our Block Stacking and Unstacking Loads: Safety Checklist. That discussion grew out of actual workplace incidents involving falling palletized loads, including a fatal accident at a British Columbia packaging operation. Double-stacking shouldn’t be treated simply as a space-saving exercise when employees are working or travelling nearby.
Warehouse Stacking and Trailer Stacking Aren’t Quite the Same
A stack that works well on a warehouse floor doesn’t automatically behave the same way in a trailer. In the warehouse, the stack is primarily stationary. In transportation, it has to deal with braking, acceleration, cornering and vibration. The combined stack also has to clear the trailer interior, and total cargo weight may become the limiting factor before available cube is exhausted.
Where the freight is suitable, however, double-stacking can produce a substantial transportation benefit. As we explain in How Many Pallets Fit in a Truck?, double-stacking can substantially increase the number of palletized loads carried in a trailer when product durability, load height, stability and allowable payload permit it.
Transportation also introduces forces that aren’t present when the same stack is sitting on a warehouse floor. If double-stacked loads are arriving out of position, our guide to Why Do Pallet Loads Shift During Transportation? looks at the interaction between unit-load stability, trailer loading and cargo restraint.
Some supply chains actually design around the requirement to double-stack. Sam’s Club, for example, states in its structural packaging standards that outbound product may be double-stacked and establishes additional compression requirements for the lower pallet load. That’s a useful illustration of the larger point: when double-stacking is part of the supply chain, the package and unit load should be designed for it rather than leaving the decision to chance at the loading dock.
What About Pallet Racking?
This is one place where I would draw a firm distinction. Floor or block stacking loaded pallets isn’t the same thing as putting a double-stacked unit into pallet rack. Rack storage introduces pallet support conditions, load clearances, rack capacities and placement issues that need to be evaluated for that particular system. Don’t assume that because two loads can safely be block-stacked on the floor, the same arrangement is suitable for a rack position.
For floor stacking, the basic assessment is more straightforward. Look at the bottom pallet, the strength and condition of the lower load, how the upper pallet transfers its weight, the stability and height of the combined stack, and how it will be handled.
Double-stacking can be an excellent way to use otherwise wasted cube. But “there’s room for another pallet” isn’t a double-stacking specification. The two pallets, the packaging and the complete unit load need to be designed or verified to work safely together.
Find answers to more questions about pallet capacity, load stability, stacking and safe handling at our Pallet Problems hub.
Rick LeBlanc, EMBA, is the founder and editor of Reusable Packaging News (subscribe to the free newsletter) and editor of Western Pallet Magazine. A supply chain journalist with more than 30 years of experience, he specializes in pallets, reusable packaging, material handling and related operations. Rick is co-author of Pallets: A North American Perspective and Pallets & Progress: A Collected History of Pallets and Palletized Handling 1922–1945. He is also an advisory board member of the Virginia Tech Center for Packaging and Unit Load Design. Read Rick’s full bio.