The Ontario Building Code | Hydraulic Loads from Roofs or Paved Surfaces
7.4.10.4. Hydraulic Loads from Roofs or Paved surfaces
(1) Except as provided in Sentence (2), the hydraulic load in litres from a roof or paved surface is the maximum 15 min rainfall determined in conformance with MMAH Supplementary Standard SB-1, "Climatic and Seismic Data", multiplied by the sum of,
(a) the area in square metres of the horizontal projection of the surface drained, and
(b) one-half the area in square metres of the largest adjoining vertical surface.
(2) Flow control roof drains may be installed provided,
(a) the maximum drain down time does not exceed 24 h,
(b) the roof structure is designed to carry the load of the stored water,
(c) one or more scuppers are installed not more than 30 m apart along the perimeter of the building so that,
(i) the scuppers are designed to handle at least 200% of the 15-minute rainfall intensity, and
(ii) the maximum depth of controlled water is limited to 150 mm,
(d) they are located not more than 15 m from the edge of the roof and not more than 30 m from adjacent drains, and
(e) there is at least one drain for each 900 m2.
(3) Where the height of the parapet is more than 150 mm or exceeds the height of the adjacent wall flashing,
(a) emergency roof overflows or scuppers described in Clause (2) (c) shall be provided, and
(b) there shall be a minimum of two roof drains.