Roof Uplift and Drainage at a Low-Slope Addition in a High-Wind Zone
A single-story retail building in a coastal high-wind region adds a 60 ft by 40 ft low-slope roof section at 1/4 in. per foot slope. The existing structure is wood-frame with nominal 2x8 rafters at 16 in. on center spanning 14 ft. The new roof uses a mechanically attached single-ply membrane over polyiso insulation and 5/8 in. plywood sheathing. The designer specifies fasteners at 12 in. on center at the perimeter and 12 in. on center in the field, with no enhanced corner or edge zones. Two interior roof drains serve the 2,400 sq ft area, each with a 3 in. leader. The building official questions the wind uplift design and the drainage adequacy, noting the site is exposed with no adjacent shielding.
1. What is wrong with the fastener layout and how should uplift resistance be evaluated?
Read the worked answer
The uniform 12 in. on center spacing ignores the wind pressure distribution on a low-slope roof. Uplift pressures are highest at corners, then at perimeters and edges, and lowest in the field. A single uniform spacing either over-fastens the field or, more likely here, under-fastens the corners and edges, where the code's component and cladding pressures are greatest. The design must be based on the applicable wind speed and exposure, translated into corner, perimeter, and field zones, with fastener spacing and count varied by zone per the membrane manufacturer's approved uplift assembly.
The evaluation should use the manufacturer's tested assembly (for example, a Factory Mutual or equivalent approval) matched to the calculated design pressures, not a generic spacing. The designer should provide zone diagrams, the design pressure for each zone, and the selected assembly's rated capacity, and confirm the plywood sheathing and rafter connections can resist the uplift. The existing 2x8 rafters at 16 in. on center may also need connection upgrades at the perimeter. The submittal should be returned for a complete wind-uplift design with corner and edge enhancements.
2. Assess the drainage design. Is two 3 in. drains for 2,400 sq ft adequate, and what else must be checked?
Read the worked answer
Drainage adequacy depends on the design rainfall intensity, the roof slope, and the hydraulic capacity of the drains and leaders, not just the count. At 1/4 in. per foot, the roof is low-slope and will pond if flow is restricted. The designer must calculate the required flow using the local 100-year, 1-hour rainfall intensity and confirm the roof drains and leaders can pass it. Two 3 in. leaders may be adequate for a small area, but the calculation must be shown; a common failure is sizing by rule of thumb.
The designer must also verify: secondary (emergency) drainage, since primary drains can clog; scuppers or a secondary system are typically required; overflow capacity; strainers; and the structural ability of the roof to support ponding water at the design depth, because ponding adds dead load and can progress. The 1/4 in. per foot slope is minimal and may not drain fully around the drains, so the designer should check deflection and ponding stability. The submittal should include drainage calculations, secondary drainage details, and confirmation that the structure supports the ponding load.