When it comes to industrial storage rack design, one size never fits all. Case in point? Choosing between selectivity and density. The right answer depends on a number of variables, especially what types of products you’re storing, how fast your inventory moves, and how often you need to access it.
Selective pallet rack provides direct access to every pallet stored, explained John Krummell, President and CEO of Advance Storage Products. The company is a 
of the Rack Manufacturers Institute (RMI).
“With selective rack, you can access any one pallet or carton without moving others,” he said. “It’s a popular choice for warehouses and distribution centers because it offers flexibility and efficiency. That makes it an especially good choice when handling diverse products and frequent inventory turnover.”
The price of all the selectivity, however, is lower storage density. Selective rack requires more floorspace to achieve complete forklift access. There are, however, other rack styles that maximize storage space to increase capacity.
High Density Rack Types
There are multiple types of high-density storage systems, and each has its own sweet spot, said Krummell.
Drive-In / Drive-Through Rack
These racks feature uprights and rails upon which two or more pallet loads rest. They are ideal for storing large quantities of the same product. Drive-in racks offer a single forklift entry point for placing and removing pallet loads. This enables last-in/first-out (LIFO) inventory management.
“Conversely, drive-through racking rows have both an entry and an exit. That supports first-in/first-out (FIFO) inventory management, which is ideal for products with expiration dates,” Krummell explained.
Pushback Rack
These racks incorporate inclined rails with a series of nested carts that roll on top of them. The pallet loads sit on the carts. A forklift loads and removes pallets from the front of the structure for LIFO inventory management. As each new pallet enters the system, it pushes the pallets behind it back into the racking. When a forklift removes the aisle-facing pallet, gravity impels the pallets behind advance to the front. Pushback rack can hold two to six—or even seven—pallets per storage lane.
“Pushback rack works best when you have at least five times the number of pallets as the lane depth,” noted Krummell. “For a 4-deep pushback system, you’d want at least 20 pallets of the same item to ensure decent utilization.”
Pallet Flow Rack
Also known as “gravity flow,” pallet flow rack designs can handle 20 pallets or more in a single lane. These systems utilize gravity wheel or roller lanes that slope downward. Forklifts load pallets from the back and remove them from the front for FIFO inventory management.
Semi-Automated Pallet Shuttle Rack
These systems use motorized shuttles for high-density storage. The shuttles autonomously perform the movements inside the racking to move the pallet loads, which rest on rails. Forklifts never enter the system, which can store 50% or more inventory than standard selective rack.
“These systems are ideal for very high volumes of the same product,” noted Krummell. “They work especially well for facilities handling very limited stock keeping units (SKUs). For example, operations handling bottled water or frozen meat often use these systems.”
Very Narrow Aisle (VNA) Rack
In VNA rack, the rows of racking are placed as closely as possible to minimize aisle widths. While it does not reduce the number of access aisles, VNA rack does enable deployment of more rows. VNA rack combines the access advantages of selective rack with high-density compact rack to increase warehouse capacity. They do, however, require specialized narrow aisle lift trucks to access the inventory.
Deciding Between Selectivity and Density Based on Inventory
Before investing in any racking, the first step to determining if selectivity or density is more important is to evaluate current and projected storage needs, advised Krummell.
“What types of products are you storing? How many SKUs do you carry? How quickly do those products turn? These are the data points you need to help make the right decision,” he said.
For example, food manufacturers often store high volumes of the same items. That typically makes them ideal candidates for high-density solutions.
“But this can change. We’ve seen companies that started with a low-SKU, high-pallet environment only to evolve over five years into high-SKU operations,” Krummell noted. “Suddenly, that dense storage system becomes a bottleneck.”
Balancing Selectivity and Density
In industries like grocery distribution, Krummell reported a noticeable shift away from dense storage.
“SKU counts keep rising. A single grocery store might carry 30,000+ items, and every one of those needs to be stored and accessible at the warehouse,” he explained. “As a result, many grocery distribution operations are favoring selective racking for its ease of access—even if it means sacrificing storage density.”
That said, there’s no need to go all-in on one solution. In fact, many warehouses benefit from a hybrid layout. These operations deploy high-density rack for storage of bulk items, and selective racking for fast-moving or varied SKUs. The goal is to match rack type to product profile.
“To illustrate, we recently worked with a customer who had an older warehouse that hadn’t had an update in 25 years,” recalled Krummell. “Their product mix had evolved, but their racking hadn’t. As a result, the facility was inefficient. Some areas needed more density; others needed more selectivity and easier inventory access. By redesigning the layout to accommodate those different needs, they improved overall operational efficiency significantly.”
Review, Reevaluate, Repeat
Ultimately, storage needs change. Businesses evolve, product mixes expand, and e-commerce forces entirely new picking strategies. Although there’s no hard rule for how frequently to evaluate existing racking, Krummell advised assessing an existing storage strategy whenever there’s a major shift in business operations.
“If you’re handling more SKUs, breaking cases more often, or moving into e-commerce, it’s worth asking if your current setup is still working for you,” he said. “As a real-world example, we worked with a company that originally invested in pushback racking before pivoting to e-commerce. But their warehouse wasn’t designed for broken case picking, so their efficiency plummeted.”
After installing a dedicated case flow system and creating low-density pick areas, the operation increased productivity by 30% in just six months, Krummell noted.
Learn More About Rack System Design
While every operation’s ideal mix of rack selectivity and density is unique, there are resources to help guide the optimal choice. RMI publishes “Considerations for the Planning and Use of Industrial Steel Storage Racks.” It includes guidance to help operations select the rack style that best supports their needs. Additionally, the members of RMI are available for consultations, insights, and recommendations. For more information, visit mhi.org/rmi.
