Roof Rain Load and Secondary-Drainage Flow (ASCE 7 Ch. 8)

ScopeASCE 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 + the ponding head the roof's deflection collects, the ASCE 7-22 dp term) - 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. ASCE 7-22 carries ponding as that dp term instead of 7-16's separate instability check. A load and flow aid, not a stamped roof-drainage design.

Formula or tablerain_load_psf = 5.2 x (static_head_in + hydraulic_head_in + ponding_head_in); design_flow_gpm = (roof_area > 0 && rainfall > 0) ? 0.0104 x roof_area_ft2 x rainfall_in_hr : null.

Source and editionASCE 7-22 Ch. 8 rain load (R = 5.2 x (ds + dh + dp), Eq. 8.2-1) 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.

Free accessThe rain-load relation is in the published ASCE 7 Ch. 8; the roof-drainage design flow is the IPC relation; the constants are public. The arithmetic is public.

What governsEstimate. AHJ and licensed professional govern.

API field namesstatic_head_in, hydraulic_head_in, roof_area_ft2, rainfall_in_hr, rain_load_psf, design_flow_gpm

• Rain load R = 5.2 psf per inch x (static head ds + hydraulic head dh + ponding head dp)ASCE 7-22 Eq. 8.2-1
• Design flow optional Q = 0.0104 x tributary area x design rainfall; enter the 15-minute intensity at the risk-category return period for ASCE 7-22 secondary drainage, the 100-year hourly rate for IPC primary drainsIPC / ASCE 7-22 §8.3
• Ponding ASCE 7-22 carries ponding as the dp term in R rather than as a separate instability checkASCE 7-22 Ch. 8