Storage Loading
Question:
How much weight can a wood truss hold, when it’s made with 2x4 material and spans 36 ft. with a 4/12 pitch. I would like to hang things from the ceiling and am wondering if it is safe. I live in an area where there are heavy snowstorms. Also, how much exposure to the weather can a truss handle before there is a problem?
Answer:
The storage load you propose adding to these trusses is called bottom chord live load (BCLL). Trusses are generally not designed for this kind of loading unless specifically required by the customer or the building code. Do you have the original truss drawings from the truss manufacturer? If so, they will list the BCLL. If not, but if your structure was designed for a jurisdiction that uses IRC 2012/2015, then table R301.5 lists uniform live load requirements. Most residential trusses are designed for a 10 psf live load in areas where a 42 in. x 42 in. rectangle would fit. All shallower areas would be designed for 0 psf. If you don’t know what code applies to your jurisdiction, you should assume the trusses are designed using 0 psf bottom chord live load.
Trusses are also built with a bottom chord dead load (BCDL) which accounts for loads from ceiling finishes like drywall, insulation, mechanical and lighting fixtures. BCDLs are typically 5-10 psf. It sounds like you are describing a garage that has no finished ceiling. Assuming a 10 psf BCLL and a 5-10 psf BCDL does not account for a lot of storage. You might be able to store lightweight things like bicycles, skis, etc., but not piles of building materials or engine blocks. If the integrity of the system is of concern, you might want to add lateral bracing to the bottom chord plane of the trusses, as shown below. This bracing will help stabilize the bottom chord and help spread out concentrated storage loads.
