Pallet stacking patterns: column, interlock, pinwheel and brick

A stacking pattern is the arrangement of cases on each layer of a pallet and how the layers relate to each other. It decides three things: how many cases fit, how strong the stack is in compression, and how stable the load is when it is moved. There is always a trade-off between strength and stability. This guide explains the common patterns and how to choose one.

#The four common patterns

#Column stacking: maximum strength

In column stacking, each box sits exactly on top of the box below, so the corners and walls line up from the top layer to the pallet. Corrugated boxes carry most of their load in the corners and vertical panels. When those line up, the box's rated compression strength is available to the stack.

Column stacking is the default for loads that are:

• stored for weeks in block stacks or racking;
• double stacked in trucks;
• packed in boxes with little product support (the product does not help carry the load).

The weakness of a column stack is that it can separate into independent columns that lean or tip, especially at the edges. Stretch wrap, corner boards, strapping and a tier sheet every few layers hold the columns together. See stretch wrapping pallets.

#Interlocking: maximum stability

In an interlocked (brick or row) pattern, alternate layers are turned so each box bridges the seam between two boxes below. The layers act as one block and resist separating.

The cost is strength. A box that sits across two boxes below is supported on only part of its bottom panel, and its corners no longer bear on corners. Packaging researchers and the corrugated industry report that interlocking can remove a large share of a box's stacking strength compared with column stacking, often cited as up to about half. The exact loss depends on the box, the pattern and the product inside.

Use interlocking when the load is light relative to the box strength, when it will be handled many times, or when the product itself carries load (rigid cans or bottles, for example).

#Pinwheel patterns

A pinwheel places boxes around the center of the pallet, each turned 90° from its neighbor, often leaving a small open square in the middle. Rotating the pattern on each layer interlocks the load. Pinwheels are useful when neither box dimension divides evenly into the pallet length or width. They combine some of the stability of interlocking with a reasonable case count.

#A compromise: column with interlocked top

Many shippers column stack most of the load and interlock only the top one or two layers. The columns keep most of the compression strength, and the interlocked top ties them together where stretch wrap has the least grip. Adding a tier sheet between layers has a similar effect.

#Worked example: 16 × 12 in cases on a 48 × 40 pallet

A 16 × 12 in case has a footprint of 192 in², and a 48 × 40 pallet is 1,920 in², so up to 10 cases per layer is possible.

The 10-case layout uses two orientations in one layer. Flipping it end for end on the next layer turns it into an interlocked pattern; repeating it exactly keeps it a column stack. With 10 inch tall cases stacked 5 high, the Ti-Hi is 10 × 5 = 50 cases and the load is 50 in tall plus the pallet.

The cases per pallet calculator tests these layouts for any case and pallet size.

#Ti-Hi and cube utilization

Ti is cases per layer, Hi is layers. Ti-Hi is how most manufacturers, retailers and warehouses communicate a pallet pattern. Cube utilization is the volume of product divided by the volume available (pallet footprint × allowed height). Higher utilization means fewer pallets and fewer trucks, but not at the cost of overhang or crushed boxes.

#Rules that apply to every pattern

• No overhang. Boxes that hang past the pallet edge lose support at their bottom edges and lose compression strength. See pallet overhang.
• Limit underhang. A load much smaller than the pallet wastes space and lets the load shift. Keep the load within about 1 in of the edges where the pattern allows.
• Keep the top flat. A flat top layer lets you double stack and protects the top boxes.
• Heaviest on the bottom. For mixed loads, put heavy, strong cases low. See mixed pallets.
• Mind the pallet gaps. On pallets with wide deckboard gaps, small or soft cases can sag into the gaps. Align case edges over deckboards or use a slip or tier sheet. See deckboard thickness and spacing.
• Humidity and time reduce box strength. Corrugated boxes lose strength in humid conditions and under sustained load, so a stack that survives one day may fail after weeks in storage.

#Checking stacking strength

For a quick check, compare the load on the bottom box with its rated compression strength. The load on the bottom box is the weight of everything above it in that column. Packaging engineers apply a safety factor to the box's laboratory compression strength to cover humidity, time, pattern, overhang and handling. The right factor depends on conditions; ask your box supplier or packaging engineer. The unit load design guide shows how the pallet, box and pattern interact.

#Patterns for automated palletizers

Robotic and conventional palletizers build patterns from a library, and they work best with patterns that repeat. If a load is built by machine:

• keep to one or two layer patterns and alternate them;
• avoid patterns that need a box to be placed into a tight pocket between others;
• allow a small gap (often a fraction of an inch) between boxes so the gripper can release cleanly;
• tell the pallet supplier: palletizers need flat, consistent pallets with no protruding nails and boards in known positions. See pallets for automation.

#Specifying a pattern

When you send a pallet spec or a load to a customer, state the case dimensions and weight, the Ti-Hi, the pattern (column or interlocked), the load height, and any tier sheets or corner boards. It helps the pallet manufacturer too: a column-stacked load of rigid cases bridges gaps between deckboards and helps the pallet, while bags do not.