Block stacking of palletized loads has often gone unnoticed as a risk for plant and warehouse operators. This article offers tips regarding what types of loads might be risky to stack and suggests practices for safe load stacking and unstacking.
In 2022, workers at a British Columbia packaging plant stopped when they heard a thunderous boom. Employees rushed to the scene and were horrified to find a fellow workmate pinned under fallen loads of packaging material. He subsequently died from the trauma.
An online review revealed that other workplace fatalities have also been associated with falling loads and material falling from loads. Employers are required to protect the safety of employees. However, stacked loads present a workplace risk that is frequently overlooked. Stacked loads must be assessed and monitored for preventative and corrective measures. Occupational safety and health regulations typically provide limited guidance on safe block stacking practices.
The stability of stacked loads depends upon various load design factors and MHE operator practices. Because of the range of loads and stacking conditions, it is difficult to make sweeping generalizations. However, some insights into load stackability factors and load stacking best practices may be useful for operations in initiating safe stacking procedures or updating current processes. A practical guide to Gemba walk checklists can also help teams across different industries systematically identify on-site risks and reinforce safety standards during load handling. This article aims to provide some general guidance regarding the stackability of loads and safe practices for stacking them.
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Background: History and Benefits of Bulk Stacking
The continued development of lift truck vertical range over the late 1910s and 1920s allowed for the introduction of load stacking – a practice we now know as block stacking. Stacking allowed for greater storage density in factories and warehouses. Before World War 2, pallet stacking was only practiced by a limited number of companies. In the decades following WWll, the practice became widely adopted.
The development of the pallet followed the development of the modern forklift. Initially, only loads with superior bridging strength (such as lengths of lumber) could be stacked. The operator had to leave the forklift cabin to place spacers or dunnage blocks between loads before lowering the load.
Soon, boards were being nailed across the tops of the spacers. This was the invention of the skid. The skid allowed a greater range of goods to be unitized – such as kegs and crates. It also allowed the operator to stay safely in the forklift cabin. In the 1930s, the invention of the double-faced pallet helped more uniformly distribute the weight of the load above, enabling even more loads to be stacked without fear of crushing.
The Advantages of Block Stacking
Today, the block stacking of palletized loads (unit loads) remains a popular choice for high-turnover inventory that can be safely stacked. Bulk bays can often be accessed more quickly than unit loads stored in selective racking systems, so block stacking is frequently used at bottling plant warehouses for high-volume, fast-moving products such as soft drinks. Block stacking also provides flexibility. There is no permanent storage equipment such as pallet racking, so the warehouse space can be reconfigured to take advantage of different stacking patterns or clear additional space when needed.
Regulations Related to Block Stacking
Let’s look at safety regulations related to block stacking in Canada and the U.S. In British Columbia, where the fatality occurred, employers are guided by WorksafeBC Regulation 4.43
4.43 Stacking materials
(1) Material and equipment must be placed, stacked or stored in a stable and secure manner.
(2) Stacked material or containers must be stabilized as necessary by interlocking, strapping or other effective means of restraint to protect the safety of workers.
The first part of 4.43 speaks to forklift operator practices. The second part touches on appropriate unit load design to achieve stabilization and prevent materials from falling from loads at height.
Additional guidance from the regulations of other Canadian provinces is also useful. Many of those insights are aggregated below. Some of these restrictions are referenced by more than one provincial OHS authority;
-Ensure structural integrity of floor and wall if stacking against the wall
-Stack on a flat, solid surface
-guard against moisture (floor absorption, humidity, puncture of containers, leaky roof)
-Don’t create obstructions in passageways or block emergency exits/emergency equipment
-Be mindful of environmental hazards (wind, seismic)
-Maintain clearance from sprinkler heads; do not block light sources
-Stacking of heavy cylindrical items, bricks, bagged granular products.
Relevant regulations from U.S. OSHA provide similar guidance:
29 CFR 1910.176(b) – Handling materials – general:
- This regulation requires that storage of materials should not create a hazard. Materials must be stacked, blocked, interlocked, and limited in height so that they are stable and secure against sliding or collapse.
29 CFR 1910.176(c) – Handling materials – clearance limits:
- Adequate clearance should be maintained to ensure safe operation. Storage areas must have appropriate clearance to prevent hazards such as falling materials and to allow for safe passage.
29 CFR 1910.176(a) – Handling materials – use of mechanical equipment:
- Employers must ensure that mechanical handling equipment is used correctly, including making sure that safe clearances are maintained for aisles, doorways, and loading docks.
29 CFR 1910.159 – Fire protection:
- While this regulation primarily focuses on fire protection, it includes requirements for maintaining clear access to fire extinguishers and other firefighting equipment, which can be impacted by how materials are stored and stacked.
29 CFR 1910.176(e) – Storage of materials:
- Employers must ensure that storage methods do not create a hazard, such as limiting the height of stacks to ensure stability.
Block Load Stacking Checklist: Loads of Concern
Operators often know from experience which loads can be safely stacked. However, exercise caution before stacking unfamiliar loads or loads with potential issues such as the ones listed below:
- Ensure that loads are safely secured by plastic wrap, strapping or other measures to prevent materials from falling from the unit load.
- Loads must be squarely built. Do not stack leaning or mushrooming loads.
- Avoid stacking on top of loads that do not have a flat horizontal stacking surface, loads with a crowned top, or otherwise irregular geometry.
- Do not stack on top of loads with a no-top-stacking warning or related signage.
- Do not stack on a load if it has a significant pallet underhang. Underhanging pallets will have reduced stability.
- Exercise caution if there is a significant pallet overhang. A Significant overhang can significantly reduce stacking strength and put the exposed part of the load in more danger of accidental impact. Where the overhang is not uniform, it can result in an imbalanced load, which increases risk.
- Avoid stacking loads of wet corrugated boxes. Exercise extra caution when stacking corrugated boxes in high humidity conditions or if they get wet. Moisture can significantly reduce the stacking strength of loads. Corrugated packaging can lose as much as 60% of its stacking strength at 100% humidity.
- Ensure the pallet is in good condition and fit for purpose. Do not stack a pallet if it is damaged. If an outside bottom deckboard is missing, the support will not be uniform and the load will be unstable. Missing interior bottom boards can create more concentrated force and result in the crushing of the product below, potentially leading to stack instability. Guard against the possibility of exposed pallet fasteners or splinters puncturing products and causing instability. For example, leaking resulting from puncturing liquid containers can weaken corrugated boxes.
- Ensure that pallet stacking support systems are rated for your loads. Pallet stacking supports, such as steel tier frames, pole racks, pallet collars, plastic sleeve packs, bulk bins, etc., are ideal for stacking irregular loads such as rolls, tubes, tires, etc. Check with the manufacturer to ensure they have an adequate load rating for the application.
- Liquid surge or rolling or sliding loads. Take extra caution regarding large containers that could become unbalanced due to load shifting, including liquid surge and loads of cylindrical materials that could result in spillage if not adequately secured. In the case of the fatality introduced at the beginning of the article, a slippery coating on the stacked sheet packaging had a similar destabilizing effect.
Safe Stacking Practices for Operators
While part of the process is assessing which loads can be stacked, the next component pertains to stacking the loads safely. Professional practices are critical to the safe stacking of loads. Operators and supervisors should consider the following:
- Assess loads. Based on appearance and experience (and the list above), do loads appear safe to stack? If uncertain, proceed with caution. A best practice, if unsure, is to stack loads in a restricted area to monitor.
- Ensure adequate pallet quality: Watch for pallet damage that could impact stacking stability.
- Identify all stacking scenarios. When assessing the safety of loads to be stacked, consider all stacking scenarios, not only bulk stacking. Aside from bulk stacking, consider other situations where loads are stacked in your facility, such as temporarily when undertaking pallet rack repairs, temporary staging, stacked loads being carried by MHE or resting on pallet racks, and load stacking during transport.
- Enforce an exclusion zone: Ensure pedestrians are not too close to a stacking or unstacking operation. In California, a worker was killed when a forklift unsuccessfully tried to unstack a load on the retail floor, and it fell onto another employee standing too close.
- Align and place loads squarely over the load below.
- Stack to a safe height. In some cases, a load height line might be appropriate.
- Approach bridge stacking cautiously. It can create uneven stresses for loads below.
- Practice LIFO (First In Last Out). Do not try to remove loads from the side of the bay, especially if stacks are butted together in that direction.
- Stack the same product in bays. When possible, bulk stacking bays should contain only one product to avoid the need to double-handle loads.
- Do not obstruct sprinklers or lighting.
- Consider environmental hazards such as seismic risk or windy conditions.
- Leave a space between side-by-side bays so that removing loads from one bay will not risk adjoining loads being pulled over.
- Do not place stacks obstructing pedestrian and MHE traffic, emergency exits, emergency and fire suppression equipment, etc.
- Watch for blind spots. Bulk storage bays on either side of the used one being worked could restrict the line of sight and create the risk of MHE and pedestrian collision. Incorporate this risk into operator and pedestrian practices.
- Take care not to stack loads with corrugated boxes in areas where they could get wet, such as from roof leaks or condensation droplets. Do not stack them on a wet floor that could cause moisture to wick up into them.
- Inspect bulk stacked products daily. Increase inspection frequency if concerns about stack stability.
- Report and immediately correct any stability concerns. Incidents of unstable block stacks should be remedied immediately and reported to the safety committee for follow-up.
- Develop a recovery procedure for safe operations required to unstack unstable loads, including prior risk assessment and traffic control in the immediate area.
In the case of the packaging plant where the fatality occurred, the company has now integrated safe load design and stacking practices into its safety system. The pallet labels printed by the plant computer system now instruct forklift operators on which loads can be safely stacked and which can only be stacked on top or placed in pallet racking.
Ultimately, if your operations utilize block stacking, the risks associated with this practice should be incorporated into your operations’ safety system, including work instructions, training, supervision, daily monitoring, incident tracking and recovery.
Appendix: Definitions Related to Block Stacking
Skid: A skid is a flat platform supported by stringers or blocks. Unlike a pallet, a skid does not have a bottom deck.
Pallet: A pallet is a flat, portable platform for storing, handling, and transporting goods and materials. It features a top and bottom deck or face and vertical members, typically stringers or blocks.
Pallet Underhang: Pallet underhang occurs when the load on a pallet does not cover the entire surface area of the pallet, leaving a gap between the edge of the pallet and the edge of the load. If there is a significant amount of load underhang, the load will be less stable than one that fills the pallet footprint. One company has determined 75% coverage of the footprint (25%) underhang to be the minimum tolerance for loads stacked under other loads. The company does permit the underhanging loads to be placed at the top of a stack.
Pallet Overhang: Pallet overhang occurs when the load on a pallet extends beyond the edges of the pallet itself. While a modest amount of overhang is generally acceptable, it may expose the palletized product to a greater risk of damage and may also reduce overall load stacking strength. Remember that stacking strength for corrugated boxes is concentrated in the corners. When the outside corners are overhanging and not supported by the pallet deck, stacking strength can be reduced by as much as 30%.
Unit Load: A unit load is a single item or multiple items grouped together and secured to enable handling, storage, and transportation as a single unit. The unit load consists of the base (often a pallet), materials on top of the pallet, including goods and packaging, and stretch wrap, strapping or other containment technologies.
Aspect Ratio: The relationship between the base (width or depth) and the height. Taller items with a smaller base are less stable, just as a forklift with a raised load is less stable. A tall singular stack would be more unstable than several stacks placed together.
Load Capacity: The load capacity refers to the weight a unit load or pallet can safely support. Three common forms of load capacity include:
- Free racking load capacity – racking strength of the pallet in free or edge-racked placement.
- Dynamic load capacity – weight that can be safely handled on forks during handling. Typically, dynamic load capacity is greater than racking strength but lower than static load capacity. It may be relevant to load stacking if MHE is handling double-stacked loads, for example.
- Static load capacity – weight that can be safely stacked. The bottom pallet or load must be rated to support the weight of all loads above it. For example, if four 2,000-lb loads are vertically stacked, the bottom pallet must have a static load capacity of at least 8,000 lbs. Static load capacity is typically not a limitation for good condition commonly used pallets such as 48×40 GMA style pallets or CHEP pallets. CHEP, for example, recommends stacking on CHEP pallets to a maximum of 10,000 lbs and 5 loads high.
Disclaimer: The information provided in this article is for general guidance only and should not be construed as professional advice. Operators and supervisors should always follow the specific safety protocols and operational procedures established by their employers and adhere to all applicable local, state, and federal regulations. The practices recommended here are intended to supplement, not replace, the guidelines and requirements set forth by regulatory bodies and organizational policies. Always consult your supervisor or a safety professional if you are unsure about any aspect of forklift operation.
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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.