Drain, Waste & Vent (DWV) — Plumber (306A) Practice Questions

Free Drain, Waste & Vent (DWV) practice questions

1. 1.The minimum slope for a 3" horizontal drain pipe is:

   • A.1/8" per foot (1%)
   • B.1/4" per foot (2%) for pipes 3" and smaller✓
   • C.1/2" per foot
   • D.No slope needed

   Why: NPC: pipes 3" and smaller = 1/4" per foot (2%). Pipes 4" and larger = 1/8" per foot (1%). Proper slope ensures solids are carried, preventing blockages.

2. 2.A cleanout must be installed at:

   • A.Every joint
   • B.Every change of direction, base of stacks, and at regular intervals per code✓
   • C.Only outdoors
   • D.Only at fixtures

   Why: Cleanouts required at: changes of direction, base of every stack, upstream end of horizontal runs, every 15m (50ft) of horizontal drain, and where the building drain exits.

3. 3.A vent terminating through a roof must extend at least:

   • A.50mm above roof
   • B.150mm (6") above roof surface, and specific clearances from windows/air intakes✓
   • C.Flush with roof
   • D.300mm below roof

   Why: Vent terminals: minimum 150mm above roof (some codes say 300mm in snow regions). Must be specific distances from windows, doors, and air intakes to prevent sewer gas entry.

4. 4.What fitting is NOT allowed to change direction from horizontal to vertical in DWV?

   • A.90° long sweep elbow
   • B.Sanitary tee
   • C.Short-pattern 90° elbow (vent ell)✓
   • D.Wye with 45° elbow

   Why: Short-pattern 90° elbows (vent ells) are ONLY for vent piping. In drain piping, they create turbulence and block flow. Use long-sweep 90s, combos, or wye + 45° for drains.

5. 5.A sump pump with discharge into the sanitary sewer requires:

   • A.No special connections
   • B.A backwater valve to prevent sewage backup into the sump✓
   • C.Direct connection only
   • D.An air gap is optional

   Why: Backwater valves prevent sewage from backing up through the sump pump discharge during sewer surcharges. Some codes require an indirect connection (air gap) instead.

6. 6.What causes a double-trapped fixture to drain slowly?

   • A.Normal condition
   • B.Air lock between the two traps — the second trap prevents proper venting of the first✓
   • C.Pipe too large
   • D.Water too cold

   Why: Double trapping creates an air lock between the traps. Air can't escape, preventing proper drainage. This is why double trapping is prohibited by code.

7. 7.A horizontal-to-horizontal change of direction in a drain line should use:

   • A.Short 90° ell
   • B.Two 45° elbows (or a long-sweep 90°)✓
   • C.A sanitary tee
   • D.A standard 90°

   Why: Horizontal-to-horizontal direction changes require two 45° elbows, a long-sweep 90°, or a combination wye + 45°. Short 90s and tees cause turbulence and blockages in horizontal runs.

8. 8.A backwater valve on a building drain prevents:

   • A.Forward flow
   • B.Sewage from backing up into the building during main sewer surcharging✓
   • C.Sewer gas only
   • D.Grease buildup

   Why: Backwater valves (check valves on the drain) prevent sewage from the overtaxed municipal sewer from flowing backwards into the building. Required by many jurisdictions.

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