This page is for contractors hiring plumbers into their own shop, not for homeowners looking for one.
Plumbing is the trade where the gap between a resume and reality is widest, because the work splits into genuinely different disciplines that all get the same job title. A new construction plumber with twelve years of experience can be excellent and still struggle on their first week of service calls, where the job is diagnosis in someone's occupied home with a customer standing behind you. Both people apply for the same posting.
What a license tells you, and what it doesn't
Plumbing licensure varies more than any other trade. Some states license at the state level with clear journeyman and master tiers, some leave it to municipalities, and some do both. That variation is worth understanding before you evaluate a candidate's credentials, and it means the first question is which authority issued the license rather than whether one exists.
Where a license exists, it establishes documented hours and a passed exam covering code, which is real. It does not establish diagnostic ability, and it does not tell you which of the plumbing disciplines the candidate actually lives in.
A question that reveals how a candidate thinks
Recall questions in plumbing are especially useless because code varies by jurisdiction and any competent plumber looks it up. Ask for reasoning instead.
A customer says they're getting lukewarm water at every fixture, and the hot water runs out faster than it used to. The water heater is eight years old, gas, and it's lighting and staying lit. Where do you start?
Check the thermostat setting, then rule out a crossover before touching the heater. Correct
The thermostat is the cheapest possible cause and a candidate who skips past it is telling you something about how they work. Then the crossover: every fixture being affected is the clue. Shut the cold supply to the water heater and open a hot tap; if water keeps flowing, cold is entering the hot side somewhere and the heater is not the problem at all. Usual culprits are a worn single-handle cartridge, a failed tempering or thermostatic mixing valve, a failing check valve, or an open bypass. This is standard field practice rather than a documented procedure, but it costs five minutes and changes the entire diagnosis.
Quote a replacement, since eight years is about when they go. Incorrect
Might even be right. It is also the answer that replaces a heater for a customer whose actual problem was a $40 cartridge, and you will hear about it. The repair-versus-replace conversation does belong on the table at eight years, but as part of the diagnosis rather than instead of it.
Flush the tank, because sediment is the usual cause of a heater losing capacity. Incorrect
Sediment does produce this complaint, but the way the symptom arrived narrows it. Sediment arrives gradually and comes with a rumbling or popping sound during heating as water works through the mineral layer, and hard water areas see it much faster. A sudden onset over days points elsewhere.
Replace the dip tube, since that fits an eight-year-old heater. Incorrect
The right suspect reached too early. A dip tube cracked near the top dumps incoming cold into the upper part of the tank where the hot water is about to leave, so the water goes lukewarm fast rather than fading gradually, and it is common in the eight to twelve year range. The confirming evidence is small white plastic fragments in aerators and shower screens. A candidate who checks those has done this job. One who orders the part first has not ruled out a crossover.
The strong answer starts with two things that cost almost nothing and change the entire diagnosis.
Check the thermostat setting. It is the cheapest possible cause and a candidate who skips straight past it is telling you something about how they work.
Rule out a crossover. Every fixture being affected is the clue. Shut the cold supply to the water heater and open a hot tap; if water keeps flowing, cold is entering the hot side somewhere and the heater is not the problem at all. The usual culprits are a worn single-handle cartridge, a failed tempering or thermostatic mixing valve, a failing check valve, or an open bypass. A useful supporting question for the customer is whether the problem changes depending on which fixture was used last, because a crossover often fluctuates that way while a failing heater does not.
If there is no crossover, the symptom set points inside the tank, and the way the symptom arrived narrows it further.
A dip tube cracked near the top dumps incoming cold into the upper part of the tank where the hot water is about to leave, so the water goes lukewarm fast rather than fading gradually. The tell-tale is onset: it appears suddenly, over days rather than months, and it is common on heaters in the eight to twelve year range, which fits this call. A candidate who mentions checking aerators and shower screens for small white plastic fragments has definitely done this job, because that is the confirming evidence.
Sediment produces the same complaint but arrives gradually, and it comes with a rumbling or popping sound during heating as water works through the mineral layer. Hard water areas see it much faster.
A gas control or thermostat problem is the remaining path on a unit that is lighting and staying lit.
A good candidate will also note that an eight year old heater puts the repair-versus-replace conversation on the table as part of the job rather than an afterthought.
The weak answer is "sounds like the water heater's going, I'd quote a replacement." That candidate might even be right. They are also the one who will replace a heater for a customer whose actual problem was a $40 cartridge, and you will hear about it.
Six areas a plumbing assessment should cover
Diagnosis on service calls. Working backward from a symptom, isolating between supply, fixture, and drain, testing before condemning. This is the single highest-value area for service roles and the one most tests ignore.
Code fluency. Which model code applies where you work, and more importantly whether the candidate knows what triggers a code question. Venting requirements, fixture unit loading, backflow prevention, required clearances, what needs a permit and an inspection.
Drain, waste, and vent. Why a trap siphons, how to read a venting problem from the symptoms, cleaning versus cabling versus jetting versus camera, recognizing when the problem is the line and not the fixture.
Water heaters and gas. Conventional and tankless, combustion and venting safety, gas line sizing, sediment and expansion issues, code requirements at replacement, which are often where the profit and the liability both live.
Materials and methods. Joining methods across copper, PEX, CPVC, and cast iron, when each is appropriate, transitions, and repair practice in older buildings. This is where new construction experience translates or does not.
Customer-facing judgment. Explaining a repair to a homeowner in their house. Presenting options honestly when the expensive one is also the right one. What they do when they find a previous plumber's dangerous work.
How contractors evaluate plumbers today
The ride-along or working day. Most accurate, least scalable, and impossible for employed candidates during business hours. Also inconsistent as a measurement: one candidate rides a day of straightforward water heater swaps and the next gets a genuinely hard drain diagnosis.
Hands-on shop test. Cutting, joining, and assembling to a spec in the yard or shop. Very good at revealing craft, and blind to diagnosis and customer skill, which is where most service hires actually fail.
Code quizzes. Cheap and consistent, and largely duplicating what a licensing exam already measured. They also punish plumbers who have worked under a different model code than yours, which is a bad filter since those candidates are often strong.
Hiring and finding out. The default. In plumbing the callback cost is high and the customer damage is immediate, since the failure usually happens inside someone's home, which is worth running the number on before you treat it as the cheap option.
Weight the test by role
| Role | Weight the test toward | Common failure mode to screen for |
|---|---|---|
| Service plumber | Diagnosis, water heaters, customer conversation, repair-versus-replace | New construction plumbers who are skilled installers but weak diagnosticians |
| Drain technician | DWV systems, cabling and jetting judgment, camera interpretation, upsell integrity | Recommending a line replacement that a proper cleaning would have solved, or missing one that was needed |
| New construction | Layout, rough-in accuracy, code compliance to inspection, production pace | Speed at the cost of inspection failures and callbacks |
| Commercial service | Larger systems, backflow, boilers where applicable, working in occupied buildings | Residential depth without commercial systems exposure |
| Apprentice | Fundamentals, safety, how they behave when they don't know something | Guessing instead of asking |
| Lead / foreman | All of the above plus evaluating others' work | Strong plumber who cannot inspect or teach |
Scroll sideways to see all three columns.
For the interview itself, here are the questions to ask plumbing candidates, organized the same way.
Tell the installer from the diagnostician before you hire.
TradeGrader sends candidates a role-specific plumbing assessment and returns a comparable report for each one. Free trial, no card required.
Start Free TrialHow TradeGrader tests
TradeGrader runs a live, adaptive assessment that adjusts to how the candidate answers, so strength gets probed further and guessing gets identified quickly. Questions are scenario-based and scored against cited code and real field situations rather than definitions.
The report shows where the plumber is strong, where they are guessing, how they compare to others assessed for the same role, and a learning path targeted at the specific gaps found. Because scoring is consistent across candidates, ranking applicants is an actual comparison.
Frequently asked questions
Should I test differently for service versus new construction?
Yes, and this matters more in plumbing than in any other trade. They are different jobs. A single test applied to both will either screen out good installers or pass through plumbers who cannot diagnose.
How do I test a candidate licensed under a different model code?
Test reasoning rather than citation. A plumber who understands why a vent is required will adapt to your code in weeks. One who memorized citations will not.
How long should a plumbing skills test take?
Twenty to forty minutes. Longer and employed candidates abandon it.
What about drain technicians who aren't licensed plumbers?
Test them on DWV systems, equipment judgment, and honesty about what they are recommending. The upsell integrity question matters more in this role than in any other in the trade.