10.4 Building Sewers, Private Disposal, and Special Drainage Conditions

What the building sewer is

IPC 202 defines the building sewer as that part of the drainage system extending from the end of the building drain and conveying its discharge to a public sewer, private sewer, individual sewage disposal system or other point of disposal. The building drain is the lowest horizontal piping inside the building, receiving the discharge of soil, waste and other drainage pipes and extending to a point 30 inches (2 feet 6 inches) outside the building wall, where the building sewer begins.

That 30-inch dividing line matters because the material tables, sizing tables and cleanout rules change on either side of it.

Rules that apply to the building sewer

IPC 701.3 Separate sewer connection: a building in which plumbing is installed, and each building having plumbing on more than one lot, shall have a separate connection for each lot or building where required by the administrative authority.

IPC 703.6: where the public sewer is a combined system for both sanitary and storm water, the storm sewer connects independently to the public sewer.

Private sewage disposal

IPC 701.2 permits connection to an approved private sewage disposal system where no public sewer is available. The design of the on-site wastewater treatment system itself is not an IPC subject.

In Colorado, C.R.S. 12-155-118(6)(b)(III) places the performance, location, construction, alteration, installation and use of on-site wastewater treatment systems under article 10 of title 25 — the CDPHE and county public health framework — where located within a property line, and expressly outside the definition of plumbing. The building sewer that runs to such a system remains plumbing up to the boundary the statute sets.

IPC 305.4.1 Sewer depth is a bracketed section: building sewers connecting to private sewage disposal systems must be installed not less than a locally determined depth below finished grade at the septic tank connection, and building sewers generally not less than a locally determined depth below grade. The model code leaves the numbers blank for the adopting jurisdiction, and Colorado does not fill them in statewide — the local building department sets the depth.

Colorado's five-foot rule

C.R.S. 12-155-103(8)(a) defines "plumbing" as including the listed items located within the building or extending five feet from the building foundation, excluding any service line extending from the first joint to the property line:

• all potable water supply and distribution pipes and piping;
• all plumbing fixtures and traps;
• all drainage and vent pipes;
• all water conditioning appliances connected to the potable water system;
• all building drains, including their joints, connections, devices, receptacles and appurtenances;
• all multipurpose residential fire sprinkler systems in one- and two-family dwellings and townhouses that are part of the potable water supply;
• all medical gas and vacuum systems in health-care facilities.

Two practical consequences for a Colorado journeyworker:

1. Beyond five feet from the foundation, the statutory definition of plumbing stops. C.R.S. 12-155-118(6)(b) further excludes work in water and sewer systems owned by counties, cities, towns and water and sanitation districts, and work by contractors employed by those entities connecting to the plumbing system within a property line.
2. The code still governs the installation, and the permit and inspection requirements of C.R.S. 12-155-120 and Board Rule 1.6 still apply to the plumbing and gas piping work that is within scope. Do not read the five-foot line as permission to skip the permit on the part that is in scope.

Neither the statute nor the Board Rules are scored on the PSI examination, which is based solely on the model 2021 IPC and IRC.

Special drainage conditions

Health care drainage

IPC 713 (computerized design) and IPC 702.6 (chemical waste) both bear on health care work, and IPC Section 609 adds health-care-specific water supply rules. Two drainage-side points:

• Chemical waste — including laboratory and imaging waste — discharges to a separate drainage system independent of sanitary drainage (702.6), with materials from the chemical-resistant rows of Tables 702.1 and 702.2.
• IPC 803 requires corrosive liquids, spent acids and harmful chemicals to be neutralized to a pH between 6.0 and 8.0 by an approved dilution or neutralizing device before discharge, with the device automatically provided with sufficient neutralizing medium.

Condensate

IPC 314 governs condensate disposal. Condensate piping is of approved corrosion-resistant material, not smaller than the drain connection on the appliance, and sloped not less than 1/8 inch per foot (1 percent) in the direction of discharge. Condensate is a clear-water waste and discharges to an approved plumbing fixture or disposal area — that is, indirectly, never by direct connection.

Vacuum drainage

IPC 715: designed to the manufacturer's instructions with plans submitted for approval; tested at 19 inches of mercury; gravity fixtures in the system still get traps and cleanouts per Chapters 7 and 10; the water supply fixture unit value for a vacuum-type water closet is 1.

Putting a site drainage package together

A small commercial building with 185 dfu connects to a public sewer 220 feet away, with 30 inches of available fall.