8.2 Drainage Pipe Materials, Drainage Fittings & Prohibited Fittings

8.2 Drainage Pipe Materials, Drainage Fittings & Prohibited Fittings

Exam Focus: The longevity and hydraulic efficiency of a sanitary drainage system depend entirely on selecting code-approved piping materials and installing directional fittings that maintain self-cleansing velocity without inducing turbulence or blockages. The Michigan Journeyman Plumber licensing exam heavily tests MPC Table 706.3 (Fittings for Changes in Direction), the strict restrictions governing sanitary tees, the proper torque standards for hubless cast iron couplings, and the explicit list of prohibited fittings under MPC Section 706.4.

---

1. Approved DWV Piping Materials & Standards (MPC Section 702)

Sanitary drainage, waste, and vent (DWV) piping must withstand structural loads, corrosive sewer gases (hydrogen sulfide and sulfuric acid), thermal expansion, and household chemical exposure. Under MPC Section 702 (Tables 702.1, 702.2, and 702.3), only listed materials complying with designated ASTM or CISPI standards are approved for installation:

+-----------------------------------------------------------------------------+
|                   APPROVED SANITARY DWV PIPING MATERIALS                    |
+-----------------------------------------------------------------------------+
|                                                                             |
|  [1] POLYVINYL CHLORIDE (PVC):                                              |
|      - ASTM D2665 (Solid Wall Schedule 40)                                  |
|      - ASTM F891 (Coextruded Cellular Core / Foam Core)                     |
|      - Joining: Purple primer (ASTM F656) + PVC solvent cement (ASTM D2564) |
|                                                                             |
|  [2] ACRYLONITRILE-BUTADIENE-STYRENE (ABS):                                 |
|      - ASTM D2661 (Solid Wall Schedule 40)                                  |
|      - ASTM F628 (Coextruded Cellular Core)                                 |
|      - Joining: ABS solvent cement (ASTM D2235, no primer required)         |
|                                                                             |
|  [3] CAST IRON SOIL PIPE:                                                   |
|      - ASTM A74 (Hub & Spigot: Service SV and Extra Heavy XH)               |
|      - CISPI 301 / ASTM A888 (Hubless / No-Hub)                             |
|      - Joining: Neoprene compression gaskets or CISPI 310 shielded bands    |
|                                                                             |
|  [4] COPPER DWV TUBE:                                                       |
|      - ASTM B306 (Light-gauge copper DWV, yellow stripe)                    |
|      - Joining: Solder (lead-free <0.2% lead) + ASME B16.29 fittings        |
|                                                                             |
|  [5] GALVANIZED STEEL PIPE:                                                 |
|      - ASTM A53 (Schedule 40 standard weight)                              |
|      - Must be installed at least 6 inches above ground (above grade only)  |
|      - Joining: Recessed threaded drainage fittings (ASME B16.12)           |
+-----------------------------------------------------------------------------+

Material Comparison & Jobsite Rules

1. Schedule 40 PVC (ASTM D2665 / F891):

   • Highly resistant to acids, alkalis, and sewer gases.
   • Joining Standard: Requires two-step solvent welding using ASTM F656 purple primer followed immediately by ASTM D2564 solvent cement. Clear primer is permitted only where acceptable to the local AHJ, but Michigan inspectors routinely enforce purple primer for visual verification of pipe surface etching.
   • Cellular core (foam core / ASTM F891) is approved for DWV installations above and below ground, but is restricted in certain commercial fire-rated plenum spaces.

2. Schedule 40 ABS (ASTM D2661 / F628):

   • Excellent impact strength at cold temperatures (resists cracking during harsh Michigan winters).
   • Joining Standard: Single-step solvent welding using ASTM D2235 ABS solvent cement. ABS does NOT use primer.
   • Transition Warning: Direct solvent cementing of PVC to ABS using standard cements is strictly prohibited. When transitioning between PVC and ABS, installers must use an approved ASTM D3138 transition cement (limited to one joint between building drain and building sewer) or an approved mechanical shielded transition coupling.

3. Cast Iron Soil Pipe (ASTM A74 & CISPI 301):

   • The industry standard for commercial, institutional, and high-rise construction due to its non-combustible rating, fire-resistance, and superior acoustic dampening (reduces water rush noise in walls).
   • Hub & Spigot (ASTM A74): Available in Service (SV) and Extra-Heavy (XH). Joined using preformed ASTM C564 neoprene compression gaskets inserted into the hub, or traditional molten lead and oakum caulked joints.
   • Hubless / No-Hub (CISPI 301 / ASTM A888): Plain-end pipe joined with elastomeric neoprene sleeves encased in corrugated stainless steel shield assemblies complying with CISPI 310.
   • Coupling Torque Standards: Standard CISPI 310 couplings with 5/16-inch hex screws must be tightened in alternating sequence using a calibrated torque wrench to 60 inch-pounds (in-lbs). Heavy-duty couplings complying with ASTM C1540 (incorporating 3/8-inch hex heads and wider shields with 4 to 6 bands) must be torqued to 80 or 100 in-lbs in accordance with manufacturer listings.

4. Galvanized Steel Pipe (ASTM A53):

   • Standard Schedule 40 threaded steel pipe. Must be installed with recessed cast iron or malleable iron drainage fittings (ASME B16.12) that provide a smooth interior waterway with a 1/4-inch-per-foot pitch built into the fitting tap.
   • Corrosion Restriction: Galvanized steel drainage piping shall not be used underground and must be kept at least 6 inches (152 mm) above ground level.

5. Copper DWV Tube (ASTM B306):

   • Thin-walled drawn copper tubing marked with an identifying yellow stripe. Permitted strictly for aboveground non-pressure sanitary drainage, waste, and vent lines. Prohibited underground or in corrosive soil environments.

---

2. Directional Fittings & Flow Dynamics (MPC Table 706.3)

In gravity drainage systems, liquid waste moves in horizontal pipes as open-channel flow, carrying suspended organic solids. When wastewater transitions between vertical and horizontal planes, centrifugal forces and abrupt velocity changes occur. Fittings must provide smooth hydraulic sweeps to prevent turbulence, splashing, solids deposition, and stack backpressure.

+-----------------------------------------------------------------------------+
|                   DIRECTIONAL FITTING RULES (MPC TABLE 706.3)               |
+-----------------------------------------------------------------------------+
|                                                                             |
|  [1] HORIZONTAL TO VERTICAL (Drop into stack / vertical pipe)               |
|      - Sanitary Tee: PERMITTED                                              |
|      - Long-Turn 90° Sweep: PERMITTED                                       |
|      - Combo Wye & 1/8 Bend: PERMITTED                                      |
|      - 45° Wye: PERMITTED                                                   |
|                                                                             |
|  [2] VERTICAL TO HORIZONTAL (Base of stack dropping into horizontal drain)  |
|      - Long-Turn 90° Sweep: PERMITTED                                       |
|      - Combo Wye & 1/8 Bend: PERMITTED                                      |
|      - 45° Wye with 1/8 Bend: PERMITTED                                     |
|      - Sanitary Tee: >>> STRICTLY PROHIBITED <<<                            |
|      - Short-Turn 90° (Quarter Bend): >>> STRICTLY PROHIBITED <<<           |
|                                                                             |
|  [3] HORIZONTAL TO HORIZONTAL (Branch entering horizontal main)             |
|      - Combo Wye & 1/8 Bend: PERMITTED                                      |
|      - 45° Wye: PERMITTED                                                   |
|      - Long-Turn 90° Sweep: PERMITTED                                       |
|      - Sanitary Tee: >>> STRICTLY PROHIBITED <<<                            |
|      - Short-Turn 90° (Quarter Bend): >>> STRICTLY PROHIBITED <<<           |
+-----------------------------------------------------------------------------+

Master Fitting Application Matrix (MPC Table 706.3)

The Hydraulic Logic of the Sanitary Tee Restriction

Exam Core Concept: A sanitary tee has a short, abrupt curvature designed solely to guide falling water downward into a vertical stack where gravity rapidly accelerates the flow.

When a sanitary tee is illegally installed vertically-to-horizontally (such as at the base of a vertical soil stack), wastewater falling down the stack at 10 to 15 feet per second slams into the abrupt short radius of the tee. This creates a severe hydraulic jump, where kinetic energy converts to high turbulent pressure, throwing solids out of suspension, splashing wastewater up adjacent lower fixture drains, and blowing trap seals. Vertical-to-horizontal transitions mandate a long-sweep fitting (Combination wye and 1/8 bend or long-turn 90) to gently guide high-velocity vertical flow into a smooth horizontal transition.

When a sanitary tee is illegally installed horizontally-to-horizontally, incoming branch waste enters at a near 90-degree angle without directional momentum, hitting the opposite pipe wall, swirling in eddies, and depositing solids that quickly create a complete pipe blockage.

---

3. Prohibited Fittings & Assemblies (MPC Section 706.4 & 707.1)

The Michigan Plumbing Code strictly outlaws several fitting patterns and mechanical arrangements that create sanitation hazards or structural compromises:

+-----------------------------------------------------------------------------+
|                        PROHIBITED DRAINAGE FITTINGS                         |
+-----------------------------------------------------------------------------+
|                                                                             |
|  [1] DOUBLE SANITARY TEE ON HORIZONTAL RUNS                                 |
|      - Effluent washes across barrel into opposing fixture branch.          |
|                                                                             |
|  [2] SADDLE HUBS / BAND SADDLES                                             |
|      - Cutting a hole and strapping a branch induces edges that snag solids |
|                                                                             |
|  [3] RUNNING THREADS                                                        |
|      - Straight bolt threads lack the tapered interference seal of NPT.     |
|                                                                             |
|  [4] HEEL-INLET QUARTER BENDS (USED AS DRAINAGE)                            |
|      - Waste entering heel induces turbulence and siphonage across throat.  |
|                                                                             |
|  [5] REDUCING FITTINGS IN DIRECTION OF DRAINAGE FLOW                        |
|      - Decreasing pipe diameter creates an internal dam / immediate choke.  |
+-----------------------------------------------------------------------------+

Detailed Breakdown of Prohibitions

1. Double Sanitary Tee on Horizontal Piping (MPC 706.4):

   • A double sanitary tee (often called a cross) is permitted only on vertical stacks to receive the discharge of two opposing, back-to-back fixtures at the same elevation (such as back-to-back lavatories or water closets, provided fixture distances are code-compliant).
   • Installing a double sanitary tee in a horizontal drain line is strictly prohibited. When one fixture discharges, the wastewater shoots straight across the barrel of the tee into the trap or waste arm of the opposing fixture, contaminating the opposite fixture and creating severe siphonage.
   • Approved Horizontal Alternative: To connect two opposing fixture branches horizontally, the plumber must install a double combination wye and 1/8 bend, a double wye, or two individual wyes staggered along the horizontal run.

2. Saddle Fittings (MPC 707.1):

   • Clamping a saddle fitting onto an existing drainage pipe (by drilling or cutting a hole in the pipe wall and securing a branch with metal bands) is strictly prohibited. Saddles leave rough, jagged pipe edges protruding into the waterway that snag toilet paper and solids. A full tee or wye fitting must be cut into the line using approved shielded couplings.

3. Running Threads (MPC 707.1):

   • Running threads (straight mechanical threads cut continuously along a pipe nipple without a taper) cannot maintain a watertight seal and are prohibited in drainage and vent systems. Tapered National Pipe Threads (NPT) rely on thread deformation and sealant to create an airtight, gas-tight wedge seal.

4. Heel-Inlet Quarter Bends as Drainage (MPC 706.4):

   • Quarter bends (90-degree elbows) manufactured with a low-heel or high-heel auxiliary inlet can never be used to receive drainage waste from a fixture. Drainage entering the heel inlet crashes into the main stream, generating extreme turbulence and siphoning the trap of the heel-connected fixture.
   • Sole Legal Exception: A heel inlet is permitted to serve exclusively as a vent connection, provided the vent connects above the horizontal centerline of the drainage line.

5. Enlargement / Reduction of Pipe Size (MPC 704.2):

   • The diameter of sanitary drainage piping must never decrease in the direction of flow. Reducing the pipe size creates an internal shoulder or dam that traps solids and causes chronic blockages.

---

4. Realistic Exam Application Scenarios

Scenario A: Sizing and Fitting Selection at the Base of a Multi-Story Soil Stack

Exam Scenario: An apprentice plumber in Detroit is installing the base-of-stack piping for a 4-inch cast iron soil stack serving a 5-story apartment building. The stack drops vertically through the ground-floor ceiling and transitions horizontally to connect to the building drain in the basement. The apprentice plans to install a 4-inch sanitary tee with a cleanout plug in the end run.

Will this installation pass inspection by the local plumbing inspector?

Code Analysis:

1. Fitting Violation: The transition from vertical to horizontal at the base of a stack represents a change of direction from vertical to horizontal. Under MPC Table 706.3, a sanitary tee is strictly prohibited in vertical-to-horizontal drainage transitions.
2. Hydraulic Justification: Wastewater falling through 5 stories acquires massive kinetic energy. Impacting the short radius of a sanitary tee will cause a severe hydraulic jump, explosive turbulence, and chronic clogging at the foot of the stack.
3. Corrective Requirement: The installer must replace the sanitary tee with an approved long-sweep fitting: either a 4-inch combination wye and 1/8 bend or a 4-inch long-turn 90-degree sweep, equipped with a 4-inch cleanout installed at the base of the stack.

Scenario B: Back-to-Back Fixture Rough-In in Commercial Restrooms

Exam Scenario: A journeyman is roughing in two back-to-back water closets in a commercial office building in Troy, Michigan. The horizontal branch runs inside a 6-inch plumbing chase between the men's and women's restrooms. To save space, the installer uses a 4-inch double sanitary tee laid on its back horizontally to receive both water closet waste lines simultaneously.

What code violation has occurred, and how must the fitting assembly be corrected?

Code Analysis:

1. Code Violation: Under MPC Section 706.4, double sanitary tees are prohibited on horizontal drainage lines.
2. Operational Failure: Flushing the water closet in the men's room will propel effluent across the barrel of the horizontal double sanitary tee, washing waste and contaminated water straight into the women's toilet bowl.
3. Corrective Assembly: The journeyman must remove the horizontal double sanitary tee and install a 4-inch double combination wye and 1/8 bend, or install two individual 4-inch wyes staggered horizontally along the run.