Why is my pallet load leaning? A pallet load can lean because of a damaged or uneven pallet, off-center weight distribution, weak or crushed cartons, an unstable stacking pattern or inadequate load containment. The best clue is often where the lean begins. If the bottom of the load is already out of square, start with the pallet and lower layers rather than simply adding more stretch wrap.
It looked straight when it left the palletizer. By the time someone notices it in the warehouse, however, the load has developed a definite lean. Or perhaps it arrives at the customer’s dock looking like it is one hard stop away from falling over. Adding more stretch wrap might help, but it doesn’t necessarily address the underlying problem.
Start at the bottom
The pallet is the foundation of the unit load. A broken deck board, damaged block or stringer, excessive pallet deflection or an uneven deck can change how the packages above it are supported. If the pallet itself is suspect, our guide to when a wood pallet should be taken out of service provides a useful starting point.
Research at Virginia Tech’s Center for Packaging and Unit Load Design has demonstrated that pallet deformation can create uneven support beneath corrugated boxes and significantly affect their compression strength. The research on pallet stiffness and box strength illustrates why a pallet’s performance cannot be considered separately from the packaging it supports.
Weight distribution matters as well. A load with considerably more weight on one side may look acceptable while stationary but behave differently when it is lifted, transported or subjected to lateral forces. Pallet capacity also depends on how the pallet is supported and how the load is distributed, as explained in How Much Weight Can a Pallet Hold?
Then look at the bottom layers of product. Are cartons beginning to crush? Is product overhanging the pallet? Are there gaps in the pattern that allow packages to move? In a mixed-SKU unit load, placing robust, larger containers in the lower layers and more fragile containers higher up can help prevent carton deformation, provided the resulting load remains stable.
Overhang deserves particular attention because it can leave portions of the packaging unsupported while also increasing the effective dimensions of the load. See How Much Pallet Overhang Is Too Much? for the handling and product-support implications.
The pallet pattern may be part of the problem
A pattern designed primarily to maximize the number of cases on a pallet doesn’t necessarily produce the most stable unit load. Stacking patterns, package dimensions and the strength of the individual cases all interact. The best pattern therefore depends on the product and packaging rather than on a universal rule about how cartons should be stacked.
Stretch wrap or another containment system then has to keep that unit together. More film isn’t automatically the answer. Film selection, pre-stretch, containment force, wrap pattern and how effectively the load is tied to the pallet can all influence performance. If the lower layers are already deforming or the load is poorly balanced, additional wrap may conceal the problem without correcting it.
When did the lean start?
This is one of the most useful diagnostic questions. If the load is already leaning when it leaves production, look closely at the pallet, package, stacking pattern and wrapping process. If it leaves square and arrives leaning, transportation becomes a stronger suspect.
ISTA has documented field testing in which palletized bottled-water loads developed substantial lean during transportation. Video and test work showed the importance of lateral movement and the interaction between the product and stretch-wrap system. Sharp braking and turns can contribute to a leaning load, while appropriate dunnage or bracing may help control movement. Our companion article, Why Do Pallet Loads Shift During Transportation?, examines those forces and the distinction between product moving on the pallet and the entire pallet moving within the trailer.
Some packaging failures are less obvious. Corrugated trays held together by interlocking tabs, for example, can sometimes work loose under vibration and rough handling. If a tray collapses into the one below it, the resulting loss of support can produce a lean even when the original pallet pattern was sound.
Leaning can also develop through environmental exposure. Corrugated cartons gradually lose strength in humid conditions. It is not uncommon to see unit loads of iced corn or broccoli begin to lean after a few days in the cooler as moisture weakens the packaging. In those cases, adding stretch wrap may do little to address the loss of carton strength.
Photographs can be surprisingly useful. Take one after palletizing, another after wrapping and, where practical, another after trailer loading. Compare those with the load at destination. The sequence may help establish whether the problem began with palletizing, developed during storage or emerged in transit.
A leaning pallet load is usually a symptom rather than the problem itself. Find where the lean begins, and you are much closer to finding the cause.
Find answers to more questions about pallet condition, load stability, overhang and transportation 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.