Types of storage system in warehouse determines volumetric utilization of warehouse space. Carefully selected storage structure can increase space utilization by making to store product high and partitioning space with sub-regions that can be used to store similar products.

Basis the handling units and outbound pick profile following are most commonly used types of storage system in warehouse. Types of storage system also determines warehouse process and use of material handling equipment.

1. Pallet storage structure
   • Block stacking
   • Selective rack or single deep rack
   • Double deep rack
   • Drive in or drive through rack
   • Pallet flow rack
2. Carton storage structure
   • Shelving
   • Gravity flow rack

Block Stacking

In this storage method, pallet blocks are stacked on top of each other and stored in lanes on warehouse floor. The depth of lane is number of pallets stored back to back away from the pick aisle. The depth of lane is allocated for only one SKU.

Block stacking is suitable for very fast moving and high throughput operation. Advantages of block stacking are lesser investment whereas disadvantages are under utilization of vertical height.

Selective Rack or Single deep rack

Selective racks store pallet one deep as shown in below figure. Each pallet is independently accessible, making SKU storage flexible. Selective rack gives complete freedom to retrieve any individual pallet but requires relatively more aisle space to access the pallets. The aisle space is for this storage is minimum 3.1 meter.

Selective racking is suitable for fast moving and high throughput operation. Advantages of selective racking are better utilization of vertical height whereas disadvantages are comparatively more investment.

Double Deep racking

The double-deep pallet rack consists of two single racks that are placed together one behind other and so pallets are stored two deep. This reduces the number of aisles required but this type of racking is susceptible to honeycombing. Besides, a double reach forklift is required to store and retrieve pallets from the racking. ​

This type of racking is suitable, when SKU depth is high.

Drive in or drive through rack

This type of racking system allows truck to drive within rack frame to access interior loads. With drive-in rack the storage and retrieval functions are performed from the same aisle. With drive-through rack the pallets enter from one end of the lane and retrieved from the other, so that product can be moved according to First-In-First-Out (FIFO). Drive in/ through rack may be thought of as floor-storage for product that is not otherwise stack-able.

This type of racking is suitable, when SKU depth is very high, in the range of 5 to 10 pallets of same SKU. This video beautifully explains concept of drive in racking

Pallet Flow Rack

Pallet flow rack is deep lane rack in which the racking is slanted and lines with rollers, so that when a pallet is removed, gravity pulls the remainder to the front. This enables pallets to be put-away at one side and retrieved from the other. Once a pallet is removed the next pallet moves into the position of the pallet that was removed. This racking solution is suitable for warehouses that have high throughput 

Shelving

Simple shelving is the most basic storage structure being used. The shelves are shallow 0.6 meters are typical, for example, but 36 0.9 deep shelf is sometimes used for larger cartons. Because the shelves are shallow, any quantity of a SKU must spread out along the pick face. This reduces SKU-density and therefore tends to reduce pick density increasing travel time. SKUs which occupy more than one shelf of bin-shelving are candidates for storage in another mode that will enable greater SKU-density. With shelving, both picking and put away must be done from the same pick-face.

This type of storage structure is suitable for slow moving SKUs

Gravity Flow Rack

Flow rack is a special type of shelving with shelves that are tilted, with rollers, to bring cases forward for picking as shown in below figure. The shelves may be 3–10 deep. This means that only one case of a product need be on the pick face with high SKU-density, which increases the pick-density, decreases travel.

Flow rack is restocked from the back, independently of stock retrieving. This means that restocking never interferes with picking, as against static shelving, for which picking, and restocking must alternate. However, additional aisle space is required to access the back of flow rack for restocking.