BCAB #1352 - Size of Building Drain, Sentence 4.9.2.(4), B.C. Plumbing Code
May 20, 1994
Re: Size of Building Drain, Sentence 4.9.2.(4), B.C. Plumbing Code
The project in question is a single family dwelling. The building drain is located underground below the concrete floor slab and consists of 3" and 4" pipe.
Reason for Appeal
Sentence 4.9.2.(4) of the 1992 B.C. Plumbing Code states the "...minimum size of ... underground portion of every building drain shall be 4 in."
The appellant contends that Sentence 4.9.2.(4) requires the 4" building drain to "extend to the first significant stack." He considers that after that point "the branches should be sized with regard to the hydraulic load of the fixtures connected to the stack." He refers to Table 4.10.D. and notes that a 3" pipe is sufficient for up to 27 fixture units. the house in question has only 24 fixture units.
Building Official's Position
The building official maintains that the under slab pipe in question is, by definition, the building drain and was "reduced from a 4" diameter to 3" at a horizontal intersecting under slab branch approx. 20' - 25' downstream from the significant stack". This reduction in size contravenes Sentence 4.9.2.(4) which requires the underground portion of every building drain to be at least 4" diameter.
Appeal Board Decision #1352
The code is not clear in defining the upstream end of a building drain. Therefore, the Board, based on known intent and the wording of previous B.C. Plumbing Codes, has interpreted that the building drain in this case is the horizontal run from the outlet to the building sewer to the base of the most remote stack. Because all of this pipe is underground, Sentence 4.9.2.(4) requires it to be a minimum of 4" diameter.
George R. Humphrey, Chair