International Building Code Section 1611


International Building Code 1611.1

Design rain loads. Each portion of a roof shall be
designed to sustain the load of rainwater that will accumulate
on it if the primary drainage system for that portion is blocked
plus the uniform load caused by water that rises above the inlet
of the secondary drainage system at its design flow.
R= 5.2 (ds + dh)

International Building Code 1611.2

Ponding instability. For roofs with a slope less than 1/4
inch per foot [1.19 degrees (0.0208 rad)], the design calculations
shall include verification of adequate stiffness to preclude
progressive deflection in accordance with Section 8.4 of ASCE
7.

International Building Code 1611.3

Controlled drainage. Roofs equipped with hardware
to control the rate of drainage shall be equipped with a secondary
drainage system at a higher elevation that limits accumulation
of water on the roof above that elevation. Such roofs shall
be designed to sustain the load of rainwater that will accumulate
on them to the elevation of the secondary drainage system
plus the uniform load caused by water that rises above the inlet
of the secondary drainage system at its design flow determined
from Section 1611.1. Such roofs shall also be checked for
ponding instability in accordance with Section 1611.2.