The roof perimeter edge is one of the most critical components of a roof system when considering wind resistance. It is the first line of defense against catastrophic roof blow-offs, where increased wind uplift pressures occur due to building aerodynamics, particularly in corner locations. Oftentimes overlooked during the design of roof replacement projects, the as-built conditions of the existing perimeter edge must be investigated and examined to determine compliance with building code requirements and association industry standards. Traditionally, the roof perimeter edge consists of multiple layers of continuous 2x wood blocking with end joints staggered and fastened together with annular ring nails or bolts. The fasteners used for anchoring the wood blocking to the roof structure are typically post-installed and vary between adhesive or mechanical anchors depending on the material of the substrate. The anchors at the roof perimeter must be able to resist tensile forces due to wind uplift, shear forces due to wind loads acting perpendicular to the axis of the anchors, and the combination of both forces acting simultaneously. Listed below are a few guidelines when designing or installing anchors for roof perimeter wood blocking:
- Verify the material and condition of the substrate at the roof perimeter to ensure that the anchors are fastened to a sound, structural element of the roof deck.
- Spacing and embedment depth of the anchors should be designed to ensure that the anchors provide adequate resistance to wind uplift and other applicable loads; reduce the anchor spacing at the corners of the roof.
- When the width of wood blocking is greater than six inches, the spacing of anchors should be staggered in two rows across the width of wood blocking. Countersinking of bolts on the lowermost wood blocking must be avoided due to reduced pull-through resistance.
- Verify that the material of the anchor is compatible with other components such as edge metal fascia to prevent galvanic reactivity.
- Wood blocking must be continuous at all layers and adequately sealed to prevent air infiltration beneath layers of wood blocking.