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Calculating Live Loads

Published September 05, 2017 by John A. Bologna,

The IRC defines live loads as “those loads produced by the use and occupancy of the building or other structure and do not include construction or environmental loads such as wind load, snow load, rain load, earthquake load, flood load or dead load.” In simple terms, the live load for the floors in a home includes your client (the weight of your client’s body and any other bodies in a room), furniture, appliances, and anything else a client puts on the floor.

Floor live-load requirements come directly out of the code books. Table R301.5 (or Table 5301.5 in the Massachusetts building code, in my jurisdiction) lists the minimum uniformly distributed live load for residential construction in a variety of situations. For one- and two-family dwellings, the code specifies a uniform live load of 40 pounds per square foot (40 psf) for “rooms other than sleeping rooms (bedrooms)” and decks. The code also specifies minimum uniform loads of 30 psf for sleeping rooms (which are unlikely to experience live loads as big as, say, the living room), 20 psf for uninhabited attic spaces, and 50 psf for “passenger vehicle garage” floors.

Note that some of the engineered-wood-product design software on the market uses the commercial code values. So if you use that software to calculate floor framing, the result will be a more conservative design that incorporates more-robust framing members.

The key phrase here is “minimum requirements.” If you or your client wants to install special equipment, such as a large hot tub, that may be particularly heavy, it would be advisable to consult with an engineer to have the floor framing sized appropriately.

For long joist spans, deflection is often the primary factor controlling the design. Accounting for deflection can result in deeper sections (larger joists) to maintain code-prescribed deflection limits. And while the prescriptive building code does take deflection into consideration (as do canned programs used by lumber suppliers), other factors, such as floor vibration and long-term creep, must also be taken into consideration for larger joist spans. Creep is a permanent sag or deflection that can develop in floor framing members after having sustained weight on them for a long period of time. Similarly, large open rooms (with long joist spans) can have multiple uses that may create vibration concerns. A person sitting quietly may be annoyed by someone else’s exercising activity on the same floor.

These more subtle, but equally important, issues are not spelled out in the code, however. If there is ever any question whether a particular design will be adequate for handling all the loads it needs to, consult an engineer.

This article originally appeared in The Journal of Light Construction