Missouri Plumbing Contractor Exam
Practice Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of a plumbing vent system?
A) Increase water pressure
B) Prevent sewer gases from entering the building
C) Heat water lines
D) Reduce pipe corrosion
Answer: B) Prevent sewer gases from entering the building
Rationale: The vent system allows air into the drainage system,
maintaining atmospheric pressure and preventing sewer gases from
entering occupied spaces.
2. Which pipe material is commonly used for water supply lines in
modern residential plumbing?
A) Cast iron
B) PVC
, C) PEX
D) Terra cotta
Answer: C) PEX
Rationale: PEX is flexible, corrosion-resistant, and widely used for
residential water distribution systems.
3. What is the minimum slope for a 2-inch drain pipe according to
general plumbing codes?
A) 1/16 inch per foot
B) 1/8 inch per foot
C) 1/4 inch per foot
D) 1 inch per foot
Answer: B) 1/8 inch per foot
Rationale: Drainage pipes typically require a minimum slope of 1/8
inch per foot for 2-inch lines to ensure proper flow.
4. What device prevents backflow in a plumbing system?
A) Trap
B) Check valve
C) Cleanout
D) Air chamber
Answer: B) Check valve
Rationale: A check valve allows flow in one direction only, preventing
backflow contamination.
,5. What is the purpose of a P-trap?
A) Increase water flow
B) Prevent freezing
C) Trap sewer gases
D) Filter sediment
Answer: C) Trap sewer gases
Rationale: A P-trap holds water that forms a seal, preventing sewer
gases from entering the building.
6. Which fitting is used to change pipe direction 90 degrees?
A) Tee
B) Coupling
C) Elbow
D) Reducer
Answer: C) Elbow
Rationale: Elbows are used to change the direction of piping systems,
commonly at 45° or 90°.
7. What is the function of a cleanout?
A) Increase water pressure
B) Provide access for drain cleaning
C) Prevent leaks
D) Regulate temperature
Answer: B) Provide access for drain cleaning
, Rationale: Cleanouts allow plumbers to access and clear blockages in
drainage systems.
8. Which tool is used to cut copper pipe?
A) Hacksaw
B) Pipe wrench
C) Tubing cutter
D) Chisel
Answer: C) Tubing cutter
Rationale: Tubing cutters provide clean, precise cuts on copper
tubing without deforming it.
9. What does DWV stand for?
A) Drain Water Valve
B) Domestic Water Volume
C) Drain Waste Vent
D) Direct Water Vent
Answer: C) Drain Waste Vent
Rationale: DWV systems handle drainage, waste removal, and
venting in plumbing systems.
10. What is the main purpose of vent piping?
A) Increase drainage speed
B) Maintain trap seal integrity
C) Carry potable water
Practice Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of a plumbing vent system?
A) Increase water pressure
B) Prevent sewer gases from entering the building
C) Heat water lines
D) Reduce pipe corrosion
Answer: B) Prevent sewer gases from entering the building
Rationale: The vent system allows air into the drainage system,
maintaining atmospheric pressure and preventing sewer gases from
entering occupied spaces.
2. Which pipe material is commonly used for water supply lines in
modern residential plumbing?
A) Cast iron
B) PVC
, C) PEX
D) Terra cotta
Answer: C) PEX
Rationale: PEX is flexible, corrosion-resistant, and widely used for
residential water distribution systems.
3. What is the minimum slope for a 2-inch drain pipe according to
general plumbing codes?
A) 1/16 inch per foot
B) 1/8 inch per foot
C) 1/4 inch per foot
D) 1 inch per foot
Answer: B) 1/8 inch per foot
Rationale: Drainage pipes typically require a minimum slope of 1/8
inch per foot for 2-inch lines to ensure proper flow.
4. What device prevents backflow in a plumbing system?
A) Trap
B) Check valve
C) Cleanout
D) Air chamber
Answer: B) Check valve
Rationale: A check valve allows flow in one direction only, preventing
backflow contamination.
,5. What is the purpose of a P-trap?
A) Increase water flow
B) Prevent freezing
C) Trap sewer gases
D) Filter sediment
Answer: C) Trap sewer gases
Rationale: A P-trap holds water that forms a seal, preventing sewer
gases from entering the building.
6. Which fitting is used to change pipe direction 90 degrees?
A) Tee
B) Coupling
C) Elbow
D) Reducer
Answer: C) Elbow
Rationale: Elbows are used to change the direction of piping systems,
commonly at 45° or 90°.
7. What is the function of a cleanout?
A) Increase water pressure
B) Provide access for drain cleaning
C) Prevent leaks
D) Regulate temperature
Answer: B) Provide access for drain cleaning
, Rationale: Cleanouts allow plumbers to access and clear blockages in
drainage systems.
8. Which tool is used to cut copper pipe?
A) Hacksaw
B) Pipe wrench
C) Tubing cutter
D) Chisel
Answer: C) Tubing cutter
Rationale: Tubing cutters provide clean, precise cuts on copper
tubing without deforming it.
9. What does DWV stand for?
A) Drain Water Valve
B) Domestic Water Volume
C) Drain Waste Vent
D) Direct Water Vent
Answer: C) Drain Waste Vent
Rationale: DWV systems handle drainage, waste removal, and
venting in plumbing systems.
10. What is the main purpose of vent piping?
A) Increase drainage speed
B) Maintain trap seal integrity
C) Carry potable water
