Roof Rain Load and Secondary-Drainage Flow (ASCE 7 Ch. 8)
The load that collapses flat roofs: standing water at the blocked-primary case. ASCE 7 Ch. 8 puts the rain load at 5.2 psf per inch of head - R = 5.2 x (static head to the secondary inlet + hydraulic head above it at design flow) - and the optional IPC design flow Q = 0.0104 x area x rainfall sizes the secondary drainage. Doubling the head to the overflow doubles the load, which makes the secondary inlet height a structural decision, not just a plumbing one. A flat roof must also pass the ยง8.4 ponding-instability check. A load and flow aid, not a stamped roof-drainage design.
Formula and source
rain_load_psf = 5.2 x (static_head_in + hydraulic_head_in); design_flow_gpm = (roof_area > 0 && rainfall > 0) ? 0.0104 x roof_area_ft2 x rainfall_in_hr : null.
ASCE 7 Ch. 8 rain load (R = 5.2 x (ds + dh)) and the IPC roof-drainage design flow (Q = 0.0104 x A x i), by name; the 5.2 psf/in (62.4 pcf / 12) and 0.0104 gpm/(ft^2 in/hr) are named constants.
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