2021 IPC Chapter 7
This lesson follows waste out of the building. It leaves the fixtures through branches and stacks (sized in Lesson 9), enters the building drain, crosses the foundation, and runs through the building sewer to the public sewer. Chapter 7 says how that pipe meets the sewer, how much it must fall, which fittings are allowed at each turn, and where you must be able to reach in and clear it. Houston enforces the Uniform Plumbing Code (Lesson 1), so these are 2021 IPC numbers to check against the adopted code.
The building drain is the lowest piping of the drainage system inside the building, and it runs 30 inches of developed length past the exterior wall. The building sewer starts there and carries the discharge to the public sewer, a private sewer or another approved point of disposal. A building subdrain is any part that cannot drain to the sewer by gravity and needs a pump (Lesson 9).
Sanitary drainage connects to a public sewer (701.2). Where none is available, it goes to a private sewage disposal system under state or local rules. A building on a lot that abuts a street, alley or easement with a public sewer needs its own connection (701.3), although several buildings on one lot may share a building sewer. Where the public sewer is combined, the sanitary sewer connects independently (703.6). Sanitary and storm sewers may share one trench (703.3), and a sewer near a water service follows Section 603.2. Pipe on filled or unstable ground must be one of the ABS, cast-iron, copper, PVC or polypropylene standards in Table 702.3 (703.2). When an entire existing system is replaced, drains under slabs and sewers that will be reused must be examined internally for slope direction, breaks, obstructions and size (703.4). In rehab work, that means a camera before you reuse an old line.
| Pipe size | Minimum slope (inch per foot) | Fall in 100 feet |
|---|---|---|
| 2 1/2 inches or less | 1/4 | 25 inches |
| 3 to 6 inches | 1/8 | 12.5 inches |
| 8 inches or larger | 1/16 | 6.25 inches |
| Upstream of a grease interceptor, any size | 1/4 | 25 inches |
Horizontal drainage piping is installed in uniform alignment at uniform slopes. Too flat, and solids stay behind; too steep, and water outruns them. The slope is a minimum, so a designer may go steeper.
Drainage pipe may not shrink in the direction of flow (704.2). Three things are not counted as a reduction: a 4 by 3 inch water closet flange, a closet bend with an upright 4 inch leg, and an offset closet flange. A horizontal branch connects to the base of a stack, or to a horizontal stack offset, at least 10 stack diameters downstream (704.3). Drainage for future fixtures ends in a cap or plug (704.4).
Fittings must be approved drainage fittings without ledges or shoulders that catch solids (706.1, 706.2). Table 706.3 controls turns: a sanitary tee is allowed only for horizontal-to-vertical changes, while a wye, a long sweep and a combination wye and eighth bend work for any change. A double sanitary tee may not take back-to-back water closets unless the developed length from each closet outlet to the tee is at least 18 inches (706.3). Cement, concrete, mastic and hot-pour joints and saddle-type fittings are prohibited (707.1). Joints by material are in Section 705.
Horizontal drainage pipes in buildings and building drains need cleanouts at intervals of not more than 100 feet, measured along the developed length from the cleanout opening to the next access fitting or the end of the line. Manholes may replace building drain cleanouts at up to 400 feet. Building sewers under 8 inches follow the same 100 feet. A sewer of 8 inches or more needs a manhole within 200 feet of the junction and then every 400 feet. The junction of the building drain and sewer needs a cleanout at the junction or within 10 feet of developed length upstream, and it must be reachable without pulling a water closet (708.1.3).
A change of horizontal direction of more than 45 degrees needs a cleanout. If several such turns fall within 40 feet of developed length, the first cleanout serves all of them (708.1.4). A fixture drain serving a nonremovable trap needs no cleanout between the trap and its vent.
A cleanout is the size of the pipe it serves, but not larger than 4 inches on bigger pipe. A removable P-trap with slip or ground joints can serve a drain one size larger, cleanouts on stacks may be one size smaller, and a removable trap or integral-trap fixture counts as a cleanout equivalent. A cleanout is arranged to clean in the direction of flow, with test tees and approved two-way installations excepted. Clear space in front of the opening is at least 18 inches for pipe of 6 inches and smaller and 36 inches for 8 inches and larger (708.1.10).
Cleanouts may not be in concealed locations: inside plenums, in walls, in floor and ceiling assemblies, below grade, or in a crawl space where the height along the path is under 24 inches (708.1.11). At a finished wall, the opening face sits within 1 1/2 inches of the surface. Below grade, the cleanout is extended so the top of the plug is at or above grade; in a floor with no trim cover, the plug is countersunk flush. Plugs may not be covered with mortar or plaster, and a threaded cleanout opening may not be used to add a fixture or extend piping unless another equal-size cleanout with access is installed (708.1.12).
Worked example. A clinic annex has a 4 inch building drain that runs 45 feet from the stack base to the wall, turns 90 degrees, and leaves a 4 inch sewer 120 feet long to the public sewer. Slope: 4 inch pipe needs 1/8 inch per foot, so the fall over the sewer is 120 x 1/8 = 15 inches. The pipe at the wall sits at least 15 inches higher than at the connection. Cleanouts: one at the junction (or within 10 feet upstream), one at the 90 degree turn, and one more on the sewer because 120 feet is more than 100 feet from the first.
In the field
A rehab bid replaces a failed 4 inch sewer under a community center parking lot, where the old cleanout was buried when the lot was repaved. The new work needs a cleanout at the junction and another within 100 feet, each brought up so the plug is at or above grade, with a traffic-rated assembly in the drive. They are easy to leave out of an estimate, so list them as their own line items.
1. Under the Chapter 2 definition, the building drain ends how far beyond the exterior walls of a building?
2. A 4 inch horizontal branch drain in a Group B office is being roughed in. Under Table 704.1, its minimum slope is:
3. A 3 inch waste line from a restaurant kitchen sink runs to a grease interceptor. Under Section 704.1, the minimum slope of that piping upstream of the interceptor is:
4. An inspector checks a 4 inch building drain that runs 60 feet in a straight line and finds a total fall of 5 inches. Under Table 704.1, the drain:
5. A horizontal branch connects to the base of a 4 inch drainage stack. Under Section 704.3, how far downstream of the stack must the connection be at minimum?
6. Where must the cleanout at the junction of the building drain and building sewer be located under Section 708.1.3?
7. An inspector finds two cleanouts on a 4 inch building sewer spaced 130 feet apart along its developed length. What is the finding?
8. A campus uses an 8 inch building sewer. Under Section 708.1.2, where must the first manhole be located?
9. A 4 inch horizontal drain makes two 90 degree changes of direction within 40 feet of developed length. How many cleanouts does Section 708.1.4 require for those turns?
10. Under Section 708.1.10, what clear space is required in front of the opening of a cleanout on 4 inch piping?
11. A cleanout for a 3 inch drain is proposed in a crawl space where the height from the floor to the nearest obstruction along the path is 20 inches. Under Section 708.1.11, this location is:
12. A plumber uses a sanitary tee to turn a 4 inch drain from vertical to horizontal. Which is correct under Table 706.3?
13. Which of the following is NOT counted as a reduction in pipe size in the direction of flow under Section 704.2?
14. Two small buildings on the same lot each have plumbing fixtures and the lot abuts a street with a public sewer. Under Section 701.3, may they connect to one common building sewer?