BCAB #1860

April 15, 2021

Re: Size of trap and trap arm in relation to fixture outlet pipe

Project description

The project is the installation of a freestanding bathtub within a house. It is proposed to use a 2” trap and 2” trap arm to connect the bathtub’s 1 ½” fixture outlet pipe to the drainage system.

Applicable Code requirements

Sentence including excerpt from Table and Sentences and (2) of Division B of Book II Plumbing Systems of the British Columbia Building Code 2018. Except as provided in Sentence (2), the size of fixture outlet pipes shall conform to Table

Excerpt from Table


Minimum Permitted Size of Fixture Outlet Pipe and Hydraulic Loads for Fixtures(1)

Forming Part of Sentences and


Minimum Size of Fixture Outlet Pipe, inches

Hydraulic Load, fixture units

Bathtub (with or without shower)

1 ½

1 ½

Notes to Table

(1) See Note A-Table The hydraulic load from a fixture that is listed in Table is the number of fixture units set forth in the Table. Except as provided in Sentence (1), the hydraulic load from a fixture that is not listed in Table is the number of fixture units set forth in Table for the trap of the size that serves the fixture.

Decision being appealed (local authority’s position)

The local authority has determined that the trap size and trap arm size cannot be increased from the 1 ½” size of the bathtub fixture outlet pipe identified in Table

Further, the local authority has determined the hydraulic load must be determined by the trap size that serves the fixture in accordance with Sentence and Table

Appellant's position

The appellant maintains that Table only determines the minimum size of the fixture outlet pipe for bathtubs and does not restrict an increase in fixture outlet pipe, trap or trap arm sizes.

Since the bathtub is a fixture specifically listed in Table, the appellant considers the hydraulic load for the bathtub is determined based only on that Table and independent of the size of the trap installed.

Appeal Board decision #1860

It is the determination of the Board that Table specifies the minimum size of fixture outlet pipes and hydraulic load for specific fixtures and has no stated restriction on a maximum fixture outlet pipe size. Therefore, the Board considers the use of the proposed 2” trap and 2” trap arm for the bathtub to be compliant with Code requirements for pipe sizing with a hydraulic load of 1 ½ fixture units.

Reason for decision

Sentence and Table are not applicable for establishing hydraulic load as bathtubs are specifically identified in Table

The Board acknowledges that increased pipe sizes from the minimum identified in Table could adversely affect the performance of the drainage system which must be considered as per Article of Division A. However, the Board recognizes that a one pipe size increase is generally accepted by other codes and engineering documents such the 2015 Universal Plumbing Code, Ontario Plumbing Inspectors Association’s interpretation 05-17 and British Columbia Building Code Interpretation Committee’s interpretation 18-0101.

Other Code requirements apply when using the 2” piping system such as vent and cleanout sizing.

Lyle Kuhnert

Chair, Building Code Appeal Board