Interpretation No. 5033 – Horizontal Drainage Pipe Sizing and Grade

Manual Reference Section: 5
Applicable Code: BC Plumbing Code (1998), Part 7, Section 7.4
Status: Approved by VIPCC

Background

This interpretation was published by the Plumbing Officials’ Association of British Columbia in response to questions submitted through the Vancouver Island zone regarding the correct sizing and grading of horizontal drainage piping. Field inspectors had encountered installations where the selected pipe diameter appeared insufficient for the total fixture unit load, and where the installed slope did not align with Code requirements.

Subject Matter

The enquiry centred on how fixture unit values were to be applied when determining the minimum diameter for horizontal branches and building drains. A specific point of confusion involved installations where multiple fixture groups discharged into a common horizontal run, and whether the cumulative fixture unit count required a step-up in pipe diameter at the point of connection or only downstream.

A related question addressed the minimum slope required for horizontal drainage piping of various diameters, and whether any reduction in grade was permitted over short distances where structural conditions limited the available fall.

Ruling

The Association ruled that horizontal drainage pipe sizing shall be based on the total fixture unit load discharged into the pipe at any given point, as determined by the fixture unit values listed in Table 7.4.3.A of the Code. Where multiple branches connect to a horizontal run, the pipe diameter must accommodate the cumulative load from all upstream connections. No portion of a horizontal drain may be reduced in diameter in the direction of flow.

Regarding minimum slope, the ruling confirmed that pipes with a diameter of 75 mm or less shall be installed at a grade of not less than 1 in 50 (2 percent). Pipes exceeding 75 mm in diameter shall maintain a minimum grade of 1 in 100 (1 percent). These grades apply uniformly along the full length of each horizontal section. Localised reductions in slope to accommodate structural elements were not permitted, as insufficient grade could result in solids deposition and recurring blockages.

Scope of Application

This interpretation governed the sizing and installation of horizontal sanitary drainage piping in new construction. It did not alter the provisions for vertical stacks, subsoil drainage, or storm drainage piping, each of which remained subject to separate Code subsections within Part 7.