8.2 Drainage Joints, Transitions, and Prohibited Connections

Prohibited joints and connections — IPC 707.1

Six items. Learn them as a list, because exam questions ask which one is not permitted.

1. Cement or concrete joints.
2. Mastic or hot-pour bituminous joints.
3. Joints made with fittings not approved for the specific installation.
4. Joints between different diameter pipes made with elastomeric rolling O-rings.
5. Solvent-cement joints between different types of plastic pipe, except as provided in Section 705.16.4.
6. Saddle-type fittings.

Compare the water-supply list in IPC 605.9, which shares items 1, 2, 3, 5 and 6 but does not include the elastomeric rolling O-ring item. Item 4 exists because a rolling O-ring depends on precise annular geometry; between different diameters it cannot seal reliably.

Saddle fittings appear on both lists. Cutting a hole in an existing main and strapping on a saddle is prohibited on the supply side and the drainage side alike.

Joining methods by drainage material

Colorado field note (not exam-scored): Board Rule 1.2(E)(1)(jj) deletes the exception in IPC 705.10.2 — the exception that otherwise permits a one-step cement without primer on certain PVC joints. In Colorado, primer is required on those solvent-cement joints.

Joints between different materials — IPC 705.16

Transitions are where DWV systems leak and where inspectors look first.

Two supporting tables set dimensions for the transition hardware:

• Table 705.18 Caulking ferrules: a 2-inch ferrule has an inside diameter of 2-1/4 inches, a length of 4-1/2 inches and a minimum weight of 1 pound; a 4-inch ferrule is 4-1/4 inches inside diameter, 4-1/2 inches long, and weighs at least 2 pounds 8 ounces.
• Table 705.19 Soldering bushings: of copper or copper alloy; a 2-inch bushing weighs at least 14 ounces, a 3-inch bushing 2 pounds, and a 4-inch bushing 3 pounds 8 ounces.

Drainage slip joints — IPC 705.17

Slip joints shall be made with an approved elastomeric gasket and shall only be installed on the trap inlet, trap outlet and within the trap seal.

Identical in substance to the fixture-side rule of IPC 405.9, which adds that a concealed slip joint requires an access panel or utility space not less than 12 inches in its smallest dimension. A tubular slip-joint connection buried in a finished wall or ceiling violates both sections.

Fittings and changes of direction — IPC 706

IPC 706.1: connections and changes in direction shall be made with approved drainage fittings; connections between drainage piping and fixtures conform to Section 405.

IPC 706.2 Obstructions: fittings shall not have ledges, shoulders or reductions capable of retarding or obstructing flow. Threaded drainage pipe fittings shall be of the recessed drainage type — the reason you cannot use a standard black-iron threaded tee on a drainage line. The section does not apply to tubular waste fittings conveying vertical flow upstream of the trap seal liquid level.

IPC 706.4 Heel- or side-inlet quarter bends:

• Heel-inlet quarter bends are acceptable except where the quarter bend serves a water closet.
• A low-heel inlet shall not be used as a wet-vented connection.
• Side-inlet quarter bends are acceptable for drainage, wet venting and stack venting arrangements.

Reduction in size, and the water closet exceptions

IPC 704.2: the size of drainage piping shall not be reduced in the direction of flow. Three arrangements are expressly not considered reductions:

1. A 4-inch by 3-inch water closet flange.
2. A water closet bend fitting with a 4-inch inlet and a 3-inch outlet, provided the 4-inch leg of the fitting is upright and below — though not necessarily directly connected to — the water closet flange.
3. An offset closet flange.

Pair this with IPC 425.4, which accepts a 4-inch by 3-inch closet bend and requires a 4-by-3 flange to receive the fixture horn where a 3-inch bend is used, and with IPC 1101.5, which applies the same no-reduction rule to storm drainage.

Prohibited fitting patterns

IPC 706.3 carries one limitation stated in the body text rather than in a table:

Double sanitary tee patterns shall not receive the discharge of back-to-back water closets and fixtures or appliances with pumping action discharge.

Exception: back-to-back water closet connections to double sanitary tees are permitted where the horizontal developed length between the outlet of the water closet and the connection to the double sanitary tee pattern is 18 inches or greater.

The 18-inch exception exists because the developed length lets the discharge lose enough velocity that one closet's flush cannot blow across the fitting and siphon the opposing closet's trap.

A transition audit on a remodel

An older house has a 2-inch galvanized steel lavatory waste tied into a 3-inch cast-iron stack. The remodel adds a 2-inch PVC branch.