How to interview a plumber: what to ask before you put someone in a truck

How do you interview a plumber?

You interview a plumber in three layers. First, verify the license — plumbing is state-licensed at apprentice, journeyman, and master levels, and the level controls what work they can legally do for you, so pin it down before anything else. Second, map their real experience against your actual work mix, because "plumber" covers a service tech clearing drains, a new-construction rough-in specialist, and a remodel plumber, and those are different jobs. Third, run scenario questions that force them to narrate a diagnosis and a customer conversation out loud. The resume already told you the years; the interview is for reasoning, code honesty, and how they'll behave in a stranger's home with water running somewhere it shouldn't.

The stakes are the standard ones: SHRM puts average cost per hire around $4,700, and replacing someone who doesn't work out runs 90–200% of salary all-in — the full damage math is in the cost of a bad hire in the trades. And with the Bureau of Labor Statistics projecting about 44,000 plumbing openings a year through 2034, the good candidates are interviewing at more than one shop. Run a sharp interview, and run it fast.

Layer one: the license, before anything else

Plumbing licensure is state-by-state and level-by-level, and the differences are not paperwork trivia. An apprentice generally has to work under supervision. A journeyman can work independently but usually can't pull permits. A master can pull permits and, in many states, is what your business license itself hangs on. Hire at the wrong level and you've bought permit problems, failed inspections, and insurance exposure — with your company's name on all of it.

So before the interview gets interesting:

• Which license, which level, which state. Then verify it with the state board yourself. Cards can be expired, from a state you don't operate in, or aspirational.
• Any disciplinary history. Most boards publish it.
• Gas certification, if your work includes gas. Gas lines and gas appliance connections often carry separate certification requirements. Do not take "yeah, I do gas" at face value — ask what certification, issued where, current through when.
• Driving record. They're in your truck, in your logo, in customers' driveways. Check early.

If your market pays differently than you think, that surfaces here too — get comp expectations on the table in the first call. The national median for plumbers is $63,800 as of May 2025, but local markets vary a lot; check plumber salary by state so you're negotiating from data.

Layer two: map their experience to your work, not to "plumbing"

This is the layer most owners skip, and it's where most plumbing mis-hires happen. The trade splits into worlds that barely overlap:

• Service and repair — diagnosing in occupied homes, talking to homeowners, pricing on the spot.
• Drain and sewer — augers, jetters, camera work, and the judgment call between "cleared it" and "this line is broken."
• New construction rough-in — speed, print-reading, inspections, almost no customer contact.
• Remodel — tying new work into old houses full of surprises.

A ten-year new-construction plumber can be genuinely excellent and still flounder in residential service, because the job is half diagnosis and half kitchen-table conversation — skills his last decade never exercised. So ask:

• "Break down your last year of work: what percentage service calls, drains, new construction, remodel?"
• "When did you last run service calls solo in occupied homes?"
• "Have you quoted work at the kitchen table, or did an office price everything?"
• "What's your camera and jetting experience, honestly?"

You're not scoring the answers as good or bad. You're checking the overlap between what they've done and what your calendar looks like — and deciding how much retraining you're really signing up for.

Layer three: scenarios that force a diagnosis out loud

Now make them think in front of you. Give real situations and listen for ordered reasoning — simple and likely causes first, evidence before conclusions. (This is the same principle as our HVAC interview question list, which pairs every question with what a good and bad answer sounds like — the format transfers; the content below is the plumbing version.)

"A homeowner reports low water pressure through the whole house. Walk me through your first fifteen minutes." Good answer: establish whether it's really whole-house or one fixture, hot and cold or just hot, then move in an order — pressure-reducing valve, main shutoff partially closed, pressure reading at a hose bib, softener or filter housing, then supply-line questions. A pretender jumps to "probably needs a repipe."

"No hot water. Gas water heater. What's your sequence?" Good answer: a sequence at all — status of the pilot or igniter, thermocouple or gas control, venting, and the age-and-condition conversation about repair versus replace. Listen for how they'd present that choice: options and honest tradeoffs, not an automatic upsell to replacement.

"You're on a routine drain call and the camera shows a bellied, cracked clay line. The homeowner expected a $300 ticket and you're about to talk about thousands. Go." This is the money question for service plumbers, because it tests diagnosis and the conversation at once. Good answer: show the evidence — camera footage, locate marks — explain in plain language, give real options including the do-nothing-for-now option with its risks, and don't pressure. Red flag: either eagerness to close big on fear, or ducking the conversation entirely and low-balling to escape the room.

"Tell me about a job you refused to do without a permit, or a shortcut a customer pushed for that you turned down." Good answer: a specific story and a polite, firm no with the reason — that's someone who'll protect your license when nobody's watching. Red flag: "you have to be flexible" or any winking about how inspections work in the real world. A plumber who'll skip a permit to keep a price down is skipping it on your company's liability.

"Tell me about a callback or a mistake that cost real money — a leak you missed, a fitting that let go. What changed afterward?" Every working plumber has one. The good answer owns it specifically and names the process change. "That doesn't really happen to me" means they hide mistakes, and water damage is a terrible place to discover that.

Then watch them work

If the conversation clears the bar, run a paid working interview — a half-day ride-along with your best senior plumber, who gets a veto. You'll learn more from how they protect floors, how they narrate a diagnosis to a homeowner, and what their fittings look like than from any answer in a conference room. Reference calls close the loop; the two questions that matter are "would you send them alone into a customer's home?" and "would you hire them back?" Silence after the second one is an answer.

Two process notes. Move fast between steps — screen, interview, and working interview inside two weeks, because slow processes are how shops lose candidates to no-shows and ghosting. And remember the interview can only work with the candidates the top of the funnel delivered; if the applicants are wrong, fix the job post and the broader process in how to hire plumbers before you blame the interview.

This layered screen — license verification, work-mix mapping, scenario questions with written listen-fors — is roughly what we run at HireAligned for our plumbing customers, before candidates ever reach the owner's calendar. Openings get answered from a pre-screened bench instead of a cold start, which is the difference between a phone call and a two-month scramble. If you'd rather have the whole thing handled, here's how done-for-you works.