🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Building & Construction › Plumbing & Pipefitting › Drain, Waste & Vent (DWV) Systems: Layout, Installation & Inspection › Lesson 10
BLD-PLUM06-03 · Lesson 10 · 18 clock hours · lab

Rough-In Installation Lab — Residential DWV Assembly (Part 2: Stack & Vents)

Objective. Students will extend the Lesson 9 rough-in vertically, installing a soil stack with vent-through-roof stub, individual vent for the lavatory, and a wet vent arrangement where code permits, all sized to code.

Rough-In Installation Lab — Residential DWV Assembly (Part 2: Stack & Vents)

Why the Stack and Vent Matter

The horizontal drain work you completed in Part 1 carries waste to the stack. The soil stack does two jobs: it drops solid and liquid waste to the building drain below, and it provides the open-air column that lets sewer gases escape upward and atmospheric pressure enter the system. Without a properly vented stack, trap seals siphon out, sewer gas enters the living space, and the plumbing fails inspection. Every inch of the vertical rough-in you install today protects both public health and your grade.

The Soil Stack

The soil stack in a typical single-bathroom residence is 3 inches in diameter. It rises from the closet bend (which you secured in Part 1), passes through the wall cavity, and exits through the roof — or, in a lab setting, terminates above the top plate as a vent stub. The stack must be installed plumb: the accepted field tolerance is within ¼ inch over any 4-foot section. Use a level on two perpendicular faces of the pipe, or a plumb bob, before you make any final cement joints.

Hanger spacing keeps the stack from sagging or shifting. Both the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC) specify maximum intervals for plastic pipe support. Check your local-edition codebook for the exact interval required in your jurisdiction — intervals for vertical PVC pipe are typically shorter than those for horizontal runs because the pipe must also carry its own weight plus intermittent surge loads. Mark hanger locations on the framing before you drill so the spacing is consistent and easy to verify during inspection.

The Vent-Through-Roof Stub

At the top of the stack, transition to a 2-inch vent pipe (or maintain the full stack diameter depending on code and fixture count). In a real installation this pipe penetrates the roof deck, rises above the roofline, and is flashed watertight. In the lab, you will extend the stub above the top plate to represent that penetration. Cap it loosely — do not glue a permanent cap that would prevent future air-pressure testing.

Individual Vent for the Lavatory

The lavatory drain arm connects to the horizontal branch with a trap. The trap must be protected by a vent so the water seal is not siphoned away when fixtures elsewhere in the system discharge. An individual vent (also called a back vent or revent) rises from the trap arm, runs up the wall, and connects back to the stack at a point at or above the flood-level rim of the lavatory — meaning the connection point on the stack must be higher than the top edge of the basin. This prevents contaminated stack air from entering the vent at a level where backflow could reach the fixture.

The 1½-inch vent pipe must connect above the drain centerline of the horizontal drain. In practice, you make this connection using a sanitary tee or vent tee oriented vertically, with the branch pointing upward. Confirm your connection height with a tape measure before cementing.

Wet Venting

A wet vent serves double duty: the same pipe acts as both a drain for one fixture and a vent for another. Many jurisdictions permit wet venting for bathroom groups under specific conditions. Key requirements include:

For the challenge activity, sketch the revised configuration on paper first, label pipe sizes, and calculate the trap-to-vent distance before you pick up a saw. Your instructor must approve the drawing before you cut any pipe.

Cementing Vertical Joints Safely

PVC primer and cement produce vapors that are immediately dangerous in enclosed spaces. Follow these rules every time:

Inspection Criteria

Before your instructor signs the rough-in log, three things must pass:

1. Hanger spacing at every code-required interval — no unsupported spans.

2. Vent connection height above the drain centerline of the lavatory drain arm.

3. Stack plumb within ¼ inch over any 4-foot section.

All three must pass. A partial pass earns a reinspection after corrections. Document every change in the rough-in log with a description and your initials.

Key terms9

Soil stackThe main vertical drain pipe that receives waste from horizontal branches and carries it down to the building drain, while also serving as the primary vent column open to the atmosphere above the roof.
Vent-through-roof stubThe upper termination of the vent stack that passes through the roof assembly, allowing sewer gases to escape and air to enter the drainage system.
Individual vent (back vent / revent)A vent pipe dedicated to a single fixture trap; it connects the trap arm back to the vent stack to prevent siphonage of the trap seal.
Flood-level rimThe top edge of a plumbing fixture at which overflow would occur; vent connections on the stack must be at or above this height to prevent contaminated air from reaching the fixture.
Wet ventA single pipe that simultaneously serves as the drain for one fixture and the vent for another fixture, permitted by code when specific size and distance requirements are met.
Trap-to-vent distanceThe maximum allowable horizontal distance from a trap weir to the nearest vent connection, as specified in the plumbing code for each pipe diameter, to prevent siphonage.
Hanger spacingThe maximum interval at which a pipe must be supported by a hanger or clamp, as specified by code for each pipe material, size, and orientation.
Plumb toleranceThe acceptable deviation from true vertical; for a DWV stack in field installation the common standard is no more than ¼ inch of deviation over any 4-foot section.
Sanitary tee / vent teeDrainage fittings with a swept branch; when installed with the branch pointing upward they direct flow into the vent riser without creating a trap or restriction.

Worked examples4

1. The lavatory drain arm centerline is 18 inches above the finished floor. The lavatory flood-level rim is 32 inches above the finished floor. A student plans to connect the individual vent back to the stack at 28 inches above the floor. Does this meet the requirement that the vent connection be at or above the flood-level rim height?
  1. Step 1 — Identify the flood-level rim height: 32 inches above finished floor.
  2. Step 2 — Identify the proposed vent connection height: 28 inches above finished floor.
  3. Step 3 — Compare: 28 in < 32 in, so the connection is BELOW the flood-level rim.
Answer: No. The connection at 28 inches is 4 inches below the flood-level rim of 32 inches. The student must raise the vent connection to at least 32 inches above the floor to meet code.
2. A student is installing a 3-inch PVC soil stack and must place hangers no farther apart than the interval listed in the local code. If the code requires support every 4 feet for vertical 3-inch PVC, and the stack rises 9 feet 6 inches from the closet bend to the top plate, how many hanger locations are needed (not counting the base support at the closet bend)?
  1. Step 1 — Convert total rise to inches: 9 ft 6 in = 114 inches.
  2. Step 2 — Convert hanger interval to inches: 4 ft = 48 inches.
  3. Step 3 — Divide rise by interval to find spans: 114 ÷ 48 = 2.375, which rounds up because any partial span still requires a hanger.
  4. Step 4 — Number of hanger locations above the base = 3 (at 48 in, at 96 in, and one final hanger near the top plate within the last partial span).
Answer: 3 hangers are required above the base support: one at 4 feet, one at 8 feet, and one within the final 18-inch partial span near the top plate.
3. A student is checking stack plumb. Using a level, they measure a ¼-inch deviation over a 3-foot section. Does the stack pass the ¼-inch-per-4-foot tolerance?
  1. Step 1 — State the tolerance: ¼ inch maximum deviation over 4 feet.
  2. Step 2 — The measured deviation is ¼ inch, but it occurs over only 3 feet, not 4 feet.
  3. Step 3 — To compare fairly, determine what the allowable deviation is over 3 feet using proportional scaling: (3 ft ÷ 4 ft) × 0.25 in = 0.1875 in.
  4. Step 4 — Measured deviation (0.25 in) exceeds the proportional allowance (0.1875 in) for a 3-foot section.
Answer: The stack does not pass. A ¼-inch deviation over 3 feet is worse than the ¼-inch-per-4-foot standard. The student must realign and re-brace the stack before cementing final joints.
4. A student is designing a wet vent. The local code sets the maximum trap-arm length at 42 inches for a 1½-inch pipe (measured from the trap weir to the vent). The lavatory trap outlet is located 36 inches horizontally from the point where the wet vent rises. Does this configuration meet the trap-to-vent distance requirement?
  1. Step 1 — Identify the code limit: 42 inches maximum trap-to-vent distance for 1½-inch pipe.
  2. Step 2 — Identify the actual distance: 36 inches.
  3. Step 3 — Compare: 36 in ≤ 42 in.
Answer: Yes, 36 inches is within the 42-inch maximum, so the wet vent arrangement meets the trap-to-vent distance requirement for 1½-inch pipe.

The math in this lesson2

Learners round a partial span up to count the hanger locations a soil stack requires.
7.EE.3N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Students install the 3-inch soil stack from the closet bend, securing it with pipe hangers at code intervals, and extend a 2-inch vent stub through the top plate representing the roof penetration. · Students cut and install a 1½-inch individual vent from the lavatory drain arm, running it to the stack at or above the flood-level rim height per code, and verify the connection is above the drain center-line. · Instructor challenges each student pair to redesign one vent as a wet vent on a drawing first, confirm it meets code trap-to-vent distance, then install the revised configuration and document the change.
Materials: Previously assembled horizontal rough-in station, 3-inch and 1½-inch schedule 40 PVC pipe, sanitary tees, vent tees, couplings, pipe hangers and screws, drill, driver, levels, tape measure, IPC/UPC codebooks, PVC primer and cement, PPE as Lesson 9
Check: Instructor verifies hanger spacing (at every code-required interval), vent connection height above drain centerline, and stack plumb (within ¼ inch over 4 feet) before signing the rough-in log. All three criteria must pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Plumbing L1-4 — DWV systems and venting principles
1. What is the primary function of the soil stack in a DWV system?
NCCER Plumbing L1-4 — venting installation requirements
2. When installing an individual vent for a lavatory, at what height must the vent connection be made on the stack?
NCCER Plumbing L1-4 — fitting orientation and venting
3. A student cements a vent tee with the branch pointing downward instead of upward. What is the most likely consequence?
Safety — PPE and hazardous materials handling
4. Which PPE combination is required when applying PVC primer and solvent cement in a partially enclosed lab bay?
NCCER Plumbing L1-4 — pipe alignment and installation standards
5. A student's 3-inch PVC soil stack shows ⅜-inch deviation over a 4-foot section when checked with a level. What action is required?
NCCER Plumbing L1-4 — wet venting design and code compliance
6. What is the key sizing requirement that distinguishes a wet vent pipe from a standard drain pipe of the same nominal size?
NCCER Plumbing L1-4 — planning and documentation practices
7. Before cutting and installing the redesigned wet vent configuration, what must a student do according to the lab procedure?
NCCER Plumbing L1-4 — venting calculations and code tables
8. Which of the following best describes the trap-to-vent distance?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →