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BRANCH OPINION

Flow Control Roof Drainage

Following the publication of the 1997 Ontario Building Code (OBC), the Housing Development and Buildings Branch has received a number of inquiries concerning the application of Sentence 4.1.7.3.(3) to roofs utilizing flow control roof drains.

There are currently no uniform provincial standards for drainage systems that restrict the flow of storm water from roofs for the purpose of storm water management. Sentence 4.1.7.3.(3) of the OBC was introduced to address the potential for additional loads on roof structures due to storm water management measures. This Sentence states:

(3) Except as provided for in Sentence 4.1.7.1.(1) and except where a roof is intended to provide rain water retention, loads due to rain need not be considered to act simultaneously with loads due to snow.

Sentence 4.1.7.3.(3) of the 1995 National Building Code, does not refer to roofs intended to provide rain water retention. The design of flow control roof drains is specifically addressed by Sentences 4.10.4.(2) and (3) of the 1995 National Plumbing Code (NPC). These Sentences state:

(2) Flow control roof drains may be installed provided:

1. the maximum drain down time does not exceed 24 h,
2. the roof structure has been designed to carry the load of the stored water,
3. one or more scuppers are installed so that the maximum depth of water on the roof cannot exceed 150 mm,
4. they are located not more than 15 m from the edge of the roof, and not more than 30 m from adjacent drains, and
5. there is at least one drain for each 900 sq m.

(3) Hydraulic loads in litres per second for flow control roof drains and restricted paved area drains shall be determined according to rain intensity-duration frequency curves as compiled by Environment Canada, using 25-year frequencies.

The purpose of Sentence 4.1.7.3.(3) of the OBC is to draw the attention of the designer to roofs that provide for rain water retention, requiring live loads created over and above those resulting from conventional roof drainage systems, to be recognised and dealt with in the design.

Furthermore, it is not the intent of Sentence 4.1.7.3.(3) of the OBC, to require the addition of the rain loads, as provided for in Sentences 4.1.7.3.(1) and (2), to the snow load, as provided for in Article 4.1.7.1.

It is the opinion of the Housing Development and Buildings Branch, that the structural design of a roof incorporating flow control roof drains, need not be treated any differently from a conventionally drained roof, if the roof drainage system conforms to the following:

1. the maximum drain down time does not exceed 24 h,
2. one or more scuppers are installed, so that the maximum depth of water on the roof cannot exceed 150 mm,
3. drains are located not more than 15 m from the edge of the roof and not more than 30 m from adjacent drains, and
4. there is at least one drain for each 900 sq m.

Where the roof drainage system exceeds the limitations contained above or where the roof is designed for permanent storage or retention of storm water, the potential for additional structural loading for rain and snow has to be considered. In all cases, it is the responsibility of the designer to ensure the safety of the building structure.

___________________________________
Ali Arlani
Code Development & Training Section
Housing Development & Buildings Branch
Issued: April 28, 1999