Calculating Maximum Load
How much weight would this storage area hold?
The roof on my 25-ft. by 30-ft. garage has a 4-in-12 pitch, and the rafters are 2x8s spaced at 16 in. on center with 2×8 rafter ties spanning every other rafter running along the 25-ft. dimension. There is no blocking between the rafter ties, and
there are no other truss members, collar ties, or stringers for rafter-tie support. There are 4×8 sheets of plywood on the ties. How much weight would this storage area hold?
— Mike Crawford, Woodridge, Ill.
There are many variables — such as the lumber species, the grade of the lumber, and the age of the lumber — that factor into calculating the amount a floor or ceiling system can carry. Regardless of those variables, though, the rafter ties in your garage are too overspanned to carry any loads. Even the most generous span for 2×8 ceiling joists on 24-in. spacing is 17 ft. Also, the rafter ties are typically slash-cut at the ends to fit under the slooped roof ceiling, which may reduce their bearing capacity further.