Safety Issues With Design Dead Loads
Question:
Can I safely install 3/4 in. T&G, OSB on 2x4 trusses that are 24 in. O.C.? My roof was installed over 5/8 in. plywood without clips that have caused a lot of sagging and the shingles need replacing. I want to “fix” it one time and install architecture type shingles, but the garage is 24 ft. wide and 28 ft. long without any load bearing walls. My concern is the weight on the trusses. 5/8 in. plywood weighs 52 lbs. and the OSB weighs 78 lbs. for each 4 ft. x 8 ft. sheet. The roof will require about 84 4 ft. x 8 ft. sheets to cover, which equals about 2,184 lbs. additional weight plus the small increase for the different shingles. The roofers here indicate it is not a problem, but I wanted to hear it from someone “outside the box” that is not trying to sell me something. The shingles I'm talking about are asphalt that resembles shake and are a little heavier than the normal three tab type.
Answer:
You are correct in thinking there might be safety concerns.
If your safety concern stems from installing 3/4" OSB on your roof instead of the originally designed 5/8 in. plywood, you should not worry, assuming you are removing the 5/8 in. plywood and replacing it with the ¾” OSB. The design dead loads for top chords of trusses are generally high enough to compensate for any minor differences in sheathing thickness. By replacing the 5/8 in. plywood with the OSB, you are only increasing the design dead load on the top chord of the truss by 0.8125 psf. (We calculate this value by taking the difference between 78 lbs and 52 lbs and dividing it by 4 x 8 feet {[(78-52)lbs/(32) sq. ft.]= 0.8125 psf}.) A small increase like this is negligible. If you want specifics, we suggest that you contact APA-The Engineered Wood Association.
Our concern is regarding the type of shingles that you are planning to install. What is the difference in the existing shingles and the new “architecture type” shingles that you want to install? If you are replacing asphalt or composite shingles with ceramic or concrete tiles then you are increasing the dead load drastically. Trusses that are designed for asphalt or composite type shingles can have dead loads of around 7-10 pounds per square foot (psf) (or 700 to 1000 pounds per 100 square feet). Trusses that are designed for concrete tile can have top chord dead loads of 15 psf or higher. If you have the original design information on the trusses, you should be able to determine the top chord design dead load. If not, you should contact the truss manufacturer to request help on tracking down this information.