Building drains and building sewers

The building drain on Kingsway Tower was the first large piece of drainage Maya helped install. Luis's crew hung it in the ceiling of the lower parking level, a long run of cast iron that collected the bases of nine stacks and carried everything toward the north wall. Outside, the excavator had already opened the trench for the building sewer that would run under the driveway to the city's sanitary sewer in the street. Luis told Maya that these two pipes carry the whole building's waste and are the hardest to fix if they are wrong, so they get the most careful layout of anything in the drainage system.

This lesson covers where the building drain ends and the building sewer begins, how both are sized by fixture units and slope, how inverts and elevations are worked out, how the sewer is laid and protected underground, and how backwater valves and connections to the public sewer protect the building.

Building drain and building sewer

The building drain is the lowest horizontal drainage pipe inside the building. It receives the discharge from the stacks and other drains and carries it to a point just outside the foundation wall. From there the pipe becomes the building sewer, which carries the waste to the public sewer, a private sewage system or another approved point of disposal.

The exact point where one becomes the other is set by the code's definitions. In the IPC and the Ohio Plumbing Code, the building drain extends to a point a set distance outside the foundation wall, and the building sewer starts there. The city usually owns or controls the part of the sewer lateral within the public right of way.

The distinction matters because the two are often regulated differently. The building drain is part of the building's plumbing system and is inspected by the plumbing inspector. The building sewer may also need approval from the city's sewer or water department, may have different material rules for buried use, and may need a separate connection permit where it joins the public sewer.

Sizing by fixture units and slope

A building drain or building sewer is sized from the total fixture units it carries and the slope it is laid at. The steeper the slope, the faster the flow and the more fixture units a given size can carry. Sizing tables therefore have a column for each permitted slope.

Two rules apply on top of the table. First, the building drain and building sewer may not be smaller than the largest stack or branch they receive, and they never get smaller downstream. Second, many codes set a minimum size for the building sewer, because it is buried, long and expensive to dig up, and a blockage there backs up the whole building.

A building drain does not have to be one size along its whole length. The upstream end, which collects only one or two stacks, carries far less than the downstream end and can be sized for its own load. The engineer's drawings show where it increases.

On Kingsway, the drain starts at 4 inches under the first stack, becomes 6 inches after the third and reaches its full size before it leaves the building.

Inverts and elevations

Building drains and sewers are laid out by elevation. The invert of a pipe is the elevation of the inside bottom of the pipe, where the water flows. Engineers show invert elevations at key points on the drawings, such as at the base of each stack, where the drain leaves the building and where the sewer connects to the public main. The plumber's job is to install the pipe so that every point is at or below the design invert and the slope between points is never less than the minimum.

Working out inverts is simple arithmetic: start from a known invert, multiply the run by the slope to get the fall, and subtract. The important part is starting from the right end. Luis taught Maya to start at the downstream end, where the building sewer meets the public sewer, because that elevation is fixed by the city. Working upstream shows whether the building drain can reach every stack base at the required slope, or whether a stack base is too low and must go to a sump.

# Work upstream from the city sewer connection, checking inverts
legs = [("building sewer, city to wall", 80.0, 0.25),
        ("building drain, wall to stack S-1", 60.0, 0.125),
        ("building drain, S-1 to stack S-9", 80.0, 0.125)]
invert = 289.40
print(f"city connection invert {invert:7.2f} ft")
for name, run_ft, in_per_ft in legs:
    invert += run_ft * in_per_ft / 12
    print(f"{name:34s} run {run_ft:5.1f} ft at {in_per_ft:5.3f} in/ft -> invert {invert:7.2f} ft")

city connection invert  289.40 ft
building sewer, city to wall       run  80.0 ft at 0.250 in/ft -> invert  291.07 ft
building drain, wall to stack S-1  run  60.0 ft at 0.125 in/ft -> invert  291.69 ft
building drain, S-1 to stack S-9   run  80.0 ft at 0.125 in/ft -> invert  292.52 ft

Laying the building sewer

A building sewer is buried, and most of its problems come from how it was laid. The pipe needs uniform support along its whole length, or it will sag, form a belly that holds water and solids, and eventually block. Good practice, and the code, require:

• Bedding: a layer of compacted granular material under the pipe, shaped so that the pipe rests evenly along its length, with recesses for couplings or bells so the pipe does not rest on its joints.
• Haunching and initial backfill: granular material placed and compacted around the sides and over the top of the pipe, so the pipe is supported from the sides and protected from rocks.
• Final backfill: the rest of the trench, placed in layers, free of large stones, debris and frozen material.
• Depth: enough cover to protect the pipe from frost and from traffic loads; where cover is limited, insulation or a stronger pipe may be needed.
• Straight lines and constant slope between cleanouts or access points, so a cable or camera can travel the whole length.

Trenches deeper than a set depth need protective systems such as shoring, sloping or a trench box before anyone enters, as HSE and TRD courses covered. Plumbers are injured every year in trench collapses while making sewer connections. On Kingsway the excavation contractor provided a trench box, and nobody went into the trench without the competent person's check that morning.

The Ohio Plumbing Code, following the IPC, sets rules for building sewer materials, depth, cleanouts and the separation between the building sewer and the water service. Straight runs between cleanouts and a constant slope keep the sewer easy to clean.

Separation from the water service

The building sewer and the water service both enter the building, often from the same street. A leaking sewer next to a water pipe is a contamination risk, particularly if the water pressure ever drops. Codes therefore require a minimum horizontal separation between the two, or allow them in the same trench only with the water pipe on a solid shelf above the sewer and to one side. Some codes require a pressure-rated sewer pipe where the separation cannot be met.

On Kingsway the water service comes in on the east side and the sewer leaves on the north, so separation was easy. On a narrow city lot it is often the hardest part of the layout.

Backwater valves

During heavy rain or when a city sewer blocks, the public sewer can fill and back up. If any fixture or floor drain in the building is below the level the sewer water can reach, sewage will flow backward through the building sewer and out of the lowest openings. That is how basements flood with sewage during storms.

A backwater valve is a check valve designed for drainage. It lets waste flow out to the sewer but closes when water tries to flow back in. Most backwater valves have a hinged flap that floats closed, a clear or removable cover for inspection, and must be installed level, in the correct direction, and where they can be reached for cleaning.

Codes require backwater protection where the flood level rims of fixtures are below the level that a surcharged public sewer can reach, such as below the next upstream manhole cover. Many cities, including Columbus, also encourage or require backwater valves in new construction. Codes restrict where the valve may go: often it protects only the fixtures below grade, so that the upper floors can still drain if the valve closes.

On Kingsway, the lowest fixtures in the parking levels drain to the sump and are pumped up to the building drain, so they cannot be flooded through the gravity system. The engineer still placed a backwater valve on the building sewer in an access chamber outside the foundation, with a sign on the parking level wall showing where it is, because a backwater valve that nobody can find never gets cleaned.

Connecting to the public sewer

The connection to the city sewer is usually made by the city or by a contractor approved by the city, under a separate permit, with its own inspection. The building sewer must reach the city connection at the elevation the city sets, with the correct fitting, and with a test or camera inspection before backfill. A plug left in a new sewer connection, or a connection made into a storm sewer by mistake, is a costly error that may not show up until the building is occupied.

Many cities now require a camera inspection of every new building sewer before acceptance. The recording shows whether the pipe has sags, offset joints, debris or damage, and becomes part of the record of the building.