Question: An interesting situation has come up where some engineers are being told that the International Building Code (IBC) no longer allows horizontal diaphragms where sheathing panels are applied with the long axis parallel to framing as is the case with panelized roof assemblies.

Answer: APA Form L350: Diaphragms and Shear Walls, Table 2; and IBC, Table 2306.3.1 illustrate the six standard diaphragm panel layout cases. Although illustrations seem to indicate that panel axis is important the actual issue is direction of continuous panel joints with reference to load. The orientation of sheathing over framing is immaterial to horizontal blocked diaphragm design or load capacity.

Earlier editions of model building codes, e.g. 1997 UBC, included a note following the six case illustrations stating: Framing may be oriented in either direction for diaphragms, provided sheathing is properly designed for vertical loading. For some reason, likely an oversight, the note was omitted during the development of the consolidated IBC.

APA has not changed its position on this subject. The case orientation of diaphragms has nothing to do with panel/framing orientation. Rather the issue is entirely related to the concept of continuous panel joints relative to the applied load. Reference APA Form L350: Diaphragms and Shear Walls, Table 2, Notes: Design for diaphragm stresses depends on direction of continuous panel joints with reference to load, not on direction of long dimension or strength axis of sheet. Continuous framing may be in either direction of blocked diaphragms.