Interpretation No. 4035 – Storm Drainage System Requirements

Manual Reference Section: 4
Applicable Code: BC Plumbing Code (1998), Part 7, Section 7.4
Status: Approved by LMPCC / VIPCC

Background

This interpretation was issued by the Plumbing Officials’ Association of British Columbia to clarify provisions governing the design and installation of storm drainage systems. Questions had arisen among inspectors and contractors regarding the correct application of pipe sizing criteria, acceptable materials, and connection methods for storm drainage piping as set out in Section 7.4 of the 1998 BC Plumbing Code.

Subject Matter

The enquiry centred on whether storm drainage conductors and leaders serving roof areas could be connected directly to the building’s sanitary drainage system, or whether separate storm drainage piping was required in all cases. A secondary concern involved the minimum pipe diameter for horizontal storm drains receiving discharge from multiple roof drain connections.

Ruling

The Association determined that storm drainage piping shall remain independent of the sanitary drainage system unless the authority having jurisdiction has confirmed that a combined sewer services the property. Where a combined system is permitted, the storm drainage connection to the sanitary building drain must incorporate a trap of adequate size, installed in accordance with Subsection 7.4.2.

For horizontal storm drain piping, the minimum diameter shall be determined using the rainfall intensity data applicable to the geographic region and the projected roof area served, as prescribed in Table 7.4.2.A of the Code. Under no circumstances shall horizontal storm piping be reduced in diameter in the direction of flow.

All storm drainage materials must conform to the standards referenced in Section 7.1, including CSA B602 for mechanical couplings used in non-pressure drainage applications. Joints in below-grade storm piping shall be made using methods appropriate to the pipe material and shall be accessible for inspection prior to backfill.

Scope of Application

This interpretation applied to new construction and to retrofit projects where existing storm drainage systems were being extended or modified. It did not alter requirements for subsoil drainage piping, which remained subject to separate provisions within Section 7.4.