As today’s architectural design trends toward more open floor plans with vaulted ceilings there is a need for economical rafters that will span long distances while maintaining a shallow profile. In many situations pre-manufactured I-joist rafters fit that need.
As a structural engineer I have inspected many roofs framed with I-joist rafters, but very few that have not needed a correction due to the I-joist bearing. While typically not a problem when the roof is flat and the I-joists bear perpendicularly on the top plate, when the rafters are sloped proper bearing becomes an issue. The simplest way to properly bear the I-joists is to attach a full length beveled block to the top plate. As a structural designer I have not gotten a lot of positive feedback from framers when I specify this method. Beveling the end of the I-joist seems to be preferred over beveled blocking. The problem comes with the compromised stiffness of the I-joist’s web once the bottom flange is beveled.
As shown in the above detail, the web of the I-joist rafter should be reinforced with plywood to increase it’s stiffness at the bearing edge. As always, blocking should be provided between rafters as well as a mechanical clip (such as the Simpson H2.5) to counteract uplift forces.