Step 6: Permanent bracing of top chord plane (large buildings)

If plywood floor or roof sheathing is properly applied with staggered joints and adequate nailing,
a continuous diaphragm action is developed to resist lateral movement at the top chord, and
additional bracing in the plane is generally not required. Some metal roofing materials may act as
a diaphragm when properly lapped and nailed, but selection and use of these materials is at the
discretion of the building designer. If purlins are used, spaced not to exceed the buckling length at
the top chord, diagonal bracing should be applied to the underside of the top chord to prevent
lateral shifting of the purlins. The diagonal bracing should be installed on both sides of the ridge
line in all end bays. If the building exceeds 18 m in length, this bracing should be repeated at
intervals not exceeding 6 m.