Frequently Asked Technical Questions
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.
As an engineer, I have noticed truss designers in some high wind states routinely using “Main Wind-Force Resisting Systems” wind pressure coefficients as opposed to “Components and Cladding” coefficients to design for wind uplift. A roof truss is not a main wind-force resisting system and would have to have a tributary area of more than 1000 sq. ft. before qualifying for the lower Primary Frame coefficients. In my experience this practice is routine. I have to wonder why the design software would even allow the selection of “Primary Frames.” I resolve the problem in my truss specs but apparently few other designers are even aware of the problem.
I am a truss manufacturer in an area of the country that often has some pretty severe winters. It concerns me how little some of the local builders seem to know about snow load design. What are some of the things that need to be considered?
The Corps' guide spec for wood construction requires the drawings to indicate the design forces on each truss member for the worst loading condition. Loading conditions, of course, can include wind, snow build up, and unbalanced loading, to name a few. Many A/E firms submit drawings lacking these member forces, but instead show typical loading conditions. What does the wood truss fabricator want to see – truss diagrams with maximum loads on each member? Or would he prefer to design the truss from many required loading diagrams? Since I already paid my A/E for a design, how much does a truss manufacturer charge for this service?
We are currently developing a project which specifies “Seismic Design Category C.” We are an East Coast truss manufacturer and have not encountered seismic requirements before.
We are planning to add 1/2 in. cement board and 3/8 in. quarry tile to a kitchen floor. We need to know if the floor trusses will handle the additional weight. The floor trusses are 19.2 in. O.C. and the loading numbers are 40-10-0-5. What do these numbers mean?
We are building churches in Europe. These are modified pole barns and we are trying to use local materials and local volunteer labor. Since the truss is the heart of the building, I am looking for a method to test some completed trusses before the building is started to make sure they are strong enough.
We are concerned with SBCA’s BCSI-B1 Summary Sheet which under “Notes” makes a disclaimer. Our concern is if there would be an accident with our trusses and we point out that the bracing was not placed correctly according to SBCA documentation, which is sent with every job. If the accident goes to court, how will our attorney respond when the opposing attorney points out the disclaimer, which infers that the bracing we recommend must be flawed, otherwise it would not be disclaimed?
As the owner for a building designed to have metal plate wood trusses, what documentation should I receive to be assured that the manufactured wood trusses delivered to the site have indeed been manufactured at a licensed and registered wood truss manufacturer under the required written quality control procedures?