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How to Calculate the Live Load Capacity on a Subfloor

Brian Baer

Live loads are also referred to as "dynamic" or "moving" loads in contrast to static or dead loads, which do not move. The structural mechanics of a live load on a subfloor are such that as the load moves, it creates force in other directions than vertical.

Calculating live loads is critical to design.

Live loads also involve considerations such as impact, vibration and momentum. This type of load is temporary, so the capacity of a live load is usually much higher than that of a dead load.

  1. Find out the size of the subfloor joists as well as the square footage of the subfloor. For instance, if a second floor hallway measures 25 by 4 feet with 2-by-8-inch floor joists, the square footage is 100 square feet (25 x 4).

  2. Find out the maximum expected live load on the subfloor at any one time. For example, assume a typical residential construction will experience no more than 1,000 lbs. of live load.

  3. Tabulate the live load capacity based on the example, and refer to the American Institute of Timber Construction's Live Load Capacity table (see the link in the References section) to calculate the maximum capacity. For example, 100 square feet x 40 lbs. per square foot of live load for a capacity of 4,000 pounds. Therefore, this design is adequate to support the assumed live load.