Getting The Most Out of Rooftop Storage

A large number of buildings constructed in Ontario are constructed with flat roofs. Flat roofs are typically designed with shallow slopes which drain to roof drains located throughout the roof. The location of roof drains and the allowable ponding depth of runoff above roof drains are factors that are related to the structural design of the roof which is governed by the Ontario Building Code.


Roof drains are typically laid out to ensure that the maximum ponding that will occur on the roof will not exceed the live loads specified by the OBC thus reducing the requirements of oversizing structural members. To optimize the design, the first row of roof drains are typically located no greater than 15m from the edge of a building and no greater than 30m between each additional row. Targeting a slope of 1 percent, the maximum depth from peak to valley will be 150mm. Based on discussions with several structural engineers, in most instances, the temporary ponding of 150mm of water would be equivalent to or less than the snow load that the roof was designed for, thereby not occurring additional costs.


The most common method for temporary rooftop retention is achieved by installing a flow restrictor on each individual roof drain. The restrictor allows a constant outflow rate based on parabolic weirs within the roof drain. Additional notches or weirs can be specified to increase flows as required based on individual site requirements. For example, a flow controlled roof drain with one notch would provide an outflow of 2.25 l/s for a roof area 30m x 30m with a maximum drawdown time of 5.5 hours. By installing an additional notch, the outflow would increase to 4.50 l/s and would drain within 2.7 hours. During less frequent storm events, scuppers provide an overflow to ensure ponding levels do not exceed the maximum allowable.


There are several benefits to employing temporary rooftop detention. The most obvious benefit of temporary rooftop detention is the cost saving benefit associated with the ability to reduce the size of downstream sewers, manholes and on-site ponding requirements. Depending on site specifics, it may be the only stormwater management option available to achieve municipal requirements.


Although temporary rooftop retention is one of the most cost effective stormwater management practices to reduce peak runoff and meet stormwater quantity objectives, there are approval agencies which do not accept the practice. For example, the Ministry of Transportation have concerns that rooftop controls are too easily accessible and could be removed by the property owner. Therefore, projects within MTO jurisdiction require alternative stormwater practices to achieve peak discharge reduction.


In addition to the above, any building design with rooftop retention being proposed requires coordination between the civil and structural consultant and approval by the structural engineer responsible for the overall building design.


MGM Consulting Inc. has employed temporary rooftop retention for numerous projects completed throughout Ontario.


For additional information on this topic, contact MGM directly.