20.6 BMP F150 - Roofs for Detention

Detention ponding on roofs of structures may be used to meet flow control requirements provided all of the following are met:

The roof support structure is analyzed by a Washington State Licensed Professional Engineer to address the weight of ponded water and meets the requirements of the applicable building code.

The roof area subject to ponding is sufficiently waterproofed to achieve a minimum service life of 30 years.

The minimum pitch of the roof area subject to ponding is 1/4-inch per foot.

An overflow system is included in the design to safely convey the 100-year peak flow from the roof.

A mechanism is included in the design to allow the ponding area to be drained for maintenance purposes, or in the event the restrictor device is plugged.

A signed letter from the property owner is provided stating they understand that the roof is being proposed for detention and a covenant and easement agreement is signed and recorded and includes the roof as part of the detention facility.

Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.