B-34 2020-Current: Roof Drainage

Technical Details

Section 1101.12.1 of the California Plumbing Code (CPC) requires roof areas to be drained by roof drains or gutters. Roof drains are defined as “A drain installed to receive water collecting on the surface of a roof and to discharge it into a leader, downspout, or conductor.” Roof drains, gutters, vertical conductors or leaders, and horizontal storm drains for primary drainage shall be sized to accommodate a rainfall rate of 1.5 inches/hour (0.016 gallons per minute / square foot).

Traditional pitched roofs, with gutters and downspouts and without obstructions to allow ponding water, need not include calculations verifying adequate drainage. Structures such as carports, pole barns, storage sheds, and other accessory structures may be exempt from the roof drain or gutters requirement. This exemption shall be at the discretion of the Building Plan Examiner and is limited to roof areas which discharge a minimum of 10 feet from all non exempt structures that are provided with positive foundation drainage, per Section 1803.3 of the 2007 California Building Code (CBC).

Section 1101.12.2 of the CPC requires secondary drainage on roof areas by roof scuppers, an open side, or a secondary roof drain. Section 1611.1 of the 2007 CBC defines the design rain loads to be incorporated into the design of each portion of a roof. This load is the amount of water that will accumulate on each portion of the roof if the primary drainage system is blocked plus the uniform load caused by water that rises above the inlet of the secondary drainage system at its design flow and is represented in the following equation:

• R = 5.2 (d_s + d_h)
• Where:
• R = Rain load on the undeflected roof, in psf.
• d_s = Depth of water on the undeflected roof up to the inlet of secondary drainage system when the primary drainage system is blocked (i.e., the static head), in inches.
• d_h = Additional depth of water on the undeflected roof above the inlet of the secondary drainage system at its design flow (i.e., the hydraulic head), in inches.

For roofs with a slope less than ¼ inch per foot, or with hardware to control the rate of drainage, the design calculations shall include verification of adequate stiffness to preclude progressive deflection, with the primary drainage system being blocked.

In Wildland Urban Interface Fire Areas roof gutters on new structures must be provided with the means to prevent the accumulation of leaves and debris in the gutter.

Applicants proposing to substitute equivalent materials and methods of construction for the required roof drainage requirements must include adequate justification, be in accordance with CBC Section 1.8.7, CPC Section 301.3, and complete the CNI-008 Code Modifications & Alternate Materials, Designs or Methods (PDF).

References