I am designing a building with wood I-joist rafters, gable roof w/ 5:12 pitch.
I would like to get some feedback from those of you who have experience with
detailing wood I-joist construction. I have some typical details produced by
the I-joist industry. I am planning on using a bird-mouth detail for the
rafter bearing as shown in their details, with 2x4 outlookers to form the
My question has to do with shear transfer from the roof diaphragm. Their
typical detail shows a rimboard as solid blocking between rafters. They also
allow 'V' notches in the blocking for air circulation above the insulation.
I will be using 12 inch deep I-joists. At the face of the exterior wall the
solid blocking is about 9 inches high Since I have a sloped soffit the
sheathing will extend above the top plate of the wall stud framing and be
nailed to the rim board blocking between the rafters. Specifically I am
tying to understand the load path of the shear from the diaphragm to the
shear wall. How will it travel? What makes the diaphragm chord? The top
stud wall plate is quite a distance from the roof sheathing. The blocking
between rafters is not continuous. It seems to me the roof sheathing must be
nailed to the rim board to transfer shear from the roof to the wall, but I
don't know what the shear capacity of the blocking is. I guess it should be
more than sufficient for the per lineal foot shear of the diaphragm into the
stud wall top plate.
Any help in details, method of design, etc. would be greatly appreciated.
Richard Lewis, P.E.
Missionary TECH Team