3.3 Stacking, tiering and block storage

1 Why stacks fail

A stack does not usually collapse from being too heavy. It collapses because it went out of plumb, or because the bottom unit deformed and the geometry moved.

The last row is the one that hurts people, because it happens while someone is standing next to it doing their job. Taking a case from the middle of a block stack is the warehouse equivalent of pulling a card from the bottom of a house of cards.

2 Column stacking versus interlocking

Two patterns, two properties, and warehouses routinely choose the wrong one for the load. Column stacking puts each carton directly on the one below, corner over corner — the corners carry nearly all of a carton's compressive strength, so this pattern is far stronger vertically. Interlocking rotates alternate layers so cartons bridge across the joints below — much more stable laterally, at the cost of a substantial share of that vertical strength, because corners no longer land on corners.

The choice follows the risk. Heavy product going high, on a stable base, in a low-traffic area: column stack and wrap it. Light product, mixed sizes, traffic and vibration, or handling by hand: interlock. And where the answer is "both," the honest move is fewer tiers.

3 Block stacking on the floor

Floor stacking — pallets stacked directly on each other, several rows deep — is the cheapest storage in the building and the least forgiving. There is no beam to catch anything, the bottom pallet carries every pallet above it, and access is only from the ends of the block.

1. 1
   Set a maximum tier count per product, and post it. Derived from the bottom unit's compressive strength, not from how high the reach truck can get.
2. 2
   Keep the base clean and level. A single broken pallet slat or a lump of wrap under a corner starts the lean.
3. 3
   Square the block. Straight rows and aligned faces mean a lean is visible from the aisle before it becomes a collapse.
4. 4
   Respect the age of the stack. Cardboard creeps under sustained load and absorbs moisture. A stack that has stood for two months in a humid building is not the stack you built.
5. 5
   Never walk between rows that are leaning toward each other. If it has to be dismantled, take it from the top with equipment, from outside the fall line.

4 Pallets, the part nobody inspects

The wooden pallet under every load is a structural component that is treated as packaging. Broken stringers, missing deck boards, protruding nails and split blocks are the direct cause of loads dropping off forks and of stacks going out of plumb — and they are also the "sharp or protruding objects" that 1910.22(a)(3) requires walking-working surfaces to be free of, once the debris lands on the floor.

A working rule is short: a damaged pallet does not go back into stock, it goes to the repair or disposal pile, and that pile lives outside the traffic aisle. Add the manual handling angle from Part 4 — a wet wooden pallet is around 50 pounds of awkward, splintered load, and pallet handling is a routine source of hand and back injuries in every building that ignores it.

5 Cylinders, drums and things that roll

Round objects get their own habits because they store energy differently. Compressed gas cylinders are secured upright, valve protection in place, away from heat and ignition sources, and separated by content — the compressed gas cylinder rules and the toolbox talk on them cover the detail. Drums are stored on level ground, bungs up, ideally on pallets, and never stacked higher than the manufacturer allows. Pipe, bar stock and coiled material must be stacked and blocked so it cannot spread or tilt, which is exactly what 1926.250(b)(9) requires on a construction site and precisely what 1910.176(b) means by "blocked" in a warehouse.

The general rule for anything cylindrical: it will roll when something touches it, so the question is never "will it move" but "what stops it." Chocks, racks, cradles or a bottom row that is physically blocked at both ends.

6 Segregation, and what must not sit next to what

Warehouses store chemicals whether or not they think of themselves as chemical facilities: cleaning products, aerosols, lubricants, propane cylinders, batteries, paint. Two principles apply even before you get to the flammable liquid standards. First, incompatible materials are separated — the construction standard states it flatly at 1926.250(b)(3), and 1910.176(b)'s "storage shall not create a hazard" carries the same meaning in general industry. Second, quantities of flammable and combustible liquids kept in a general storage area are limited, and what exceeds that goes into a proper storage cabinet or room.

The specifics belong to 1910.106 for flammable liquids, 1910.110 for propane — Part 5 covers the fire prevention plan that ties them together — and to the safety data sheet for each product, which is where incompatibilities are actually written down. The warehouse-level rule is simply: know what is in the building, keep the oxidizers away from the fuels, and never let a chemical area become the overflow aisle.