Plumber screening questions: license level, service or new construction, and endorsements
On this page
Plumber screening questions should confirm three things before experience: the exact license or registration the candidate holds and which state or city issued it, whether their hours are in service work or new construction, and which endorsements the job needs (gas, backflow testing, medical gas). Then ask for one recent job in sequence. A plumber who did the work describes the order of steps and the materials without being led; one who was a helper describes the finished result.
This page covers plumbing specifically. Electrical licensing and lockout are in electrician screening questions, and refrigerant certification is in HVAC technician screening questions. The difference with plumbing is how much the license ladder, the endorsements and the service-or-construction split vary, so those come first here.
License levels: two states, two different ladders
Most states use some version of apprentice, journeyman and master, but the hours, the exams and the names differ, and some states have no statewide license at all and leave it to cities. Two examples, checked on the regulators' own pages as of September 2026:
| State | How it works | What to ask |
|---|---|---|
| Texas | The State Board of Plumbing Examiners licenses tradesman plumber-limited, journeyman, master and responsible master plumbers, and registers apprentices. A journeyman needs 8,000 hours under a responsible master plumber while registered as an apprentice or licensed as a tradesman, plus a 48-hour approved course or a Department of Labor apprenticeship. Hours worked before apprentice registration do not count. A master generally needs two years as a journeyman. Endorsements include medical gas piping installation and water supply protection specialist. | Which license, the number, the expiry, and any endorsements. For apprentices: registration date and documented hours. |
| Illinois | The Department of Public Health says anyone who installs, alters, repairs or maintains plumbing in the state must be licensed. Apprentices hold an apprentice plumber license sponsored by a licensed plumber or approved program, and need at least four years as a licensed apprentice to qualify for the plumber exam. There is a separate reciprocity application for City of Chicago licensees and a public license verification search. | Illinois plumber or apprentice license number; for Chicago work, whether they hold the city license or went through reciprocity. |
Other states differ again, and some cities license plumbers where the state does not. Get the license type, the issuer, the number and the expiry on the call, and look it up before you submit. A license from another state is the start of a reciprocity or exam conversation, not a match.
Knockout questions
Ask these first, and confirm with the client which are genuinely fixed. A helper order and a service plumber order have different knockouts.
| Question | What a strong answer sounds like | Red flags |
|---|---|---|
| What plumbing license or registration do you hold, from which state or city, and when does it expire? | "Texas journeyman, expires in November, I can send a photo." | "I have my hours." A license from a state where the job is not. |
| If you are an apprentice: when did you register, with whom, and how many documented hours do you have? | A registration date, a sponsor or program, and a number from their records. | Counts years in the trade from before they registered, in a state where those hours do not count. |
| This role is [residential service / new construction / commercial]. How many of your last two years were in that work? | Splits it: "Three years new construction, last eighteen months in service." | "All of it." Or the matching work is from the start of their career. |
| The job needs [gas piping / backflow testing / medical gas] certification. Do you hold it, and from whom? | The endorsement or certificate, the issuer and the renewal date. | "I've done gas lines" with no license or endorsement where one is required. |
| You would drive a company truck. Is your driver's license valid? | A yes, with any restriction mentioned. | Unsure. Note it; the client's driving record check will decide. |
| The schedule is [hours] with [on-call rotation / emergency calls]. Can you work that? | A plain yes or a clear limit. | Agrees, then lists exceptions later in the call. |
Service or new construction: questions that separate them
| Setting | What the work usually involves | A question that tests it |
|---|---|---|
| Residential service and repair | Leaks, clogs, water heaters, fixture replacement, repipes, working alone in occupied homes, pricing work for customers | "Walk me through the last water heater you replaced, from the call to leaving the house." |
| Residential new construction | Underground, rough-in or top-out, and trim or set-out on houses and multifamily, working from plans on a production schedule | "What phase were you on at your last builder, and what did a typical day produce?" |
| Commercial construction | Larger pipe sizes, cast iron and copper, grease interceptors, working from engineered drawings, coordination with other trades and inspectors | "On your last commercial job, which drawings did you work from and what did you install?" |
| Drain and sewer | Cable machines, jetting, camera inspection, locating, excavation and line repair | "Tell me about a main line stoppage you could not clear with a cable. What next?" |
Experience questions with strong answers and red flags
-
"Walk me through the last water heater replacement you did on your own."
- Strong answer: shut off gas or power and water, drained it, checked venting and the relief valve line, connected and tested, relit or powered up, checked for leaks and the permit if one was needed. Mentions tank or tankless and fuel type.
- Red flags: no mention of venting on a gas unit, or of the temperature and pressure relief valve. For service roles that is basic.
-
"What materials have you worked with most in the last year?"
- Strong answer: specific: PEX and its fitting system, copper with solder or press fittings, PVC or ABS drainage, cast iron. Says which they are fastest with.
- Red flags: only PEX in new houses, for a commercial role with copper and cast iron.
-
"Tell me about a job that failed inspection. What was the problem and how did you fix it?"
- Strong answer: a specific item: a test that did not hold, venting, slope, a missing cleanout, and the correction.
- Red flags: "Never failed one" from someone with years of permitted work, or blame on the inspector.
-
"How do you find a leak you cannot see?"
- Strong answer: isolates sections, watches the meter, uses pressure testing, moisture readings or acoustic tools, and opens the wall only when they know where.
- Red flags: "Open the wall until you find it."
-
"On a service call, how do you give the customer a price?"
- Strong answer: a flat-rate book or tablet pricing, options at different price points, and how they handle a customer who says no.
- Red flags: a candidate who refuses to discuss price, for a client where service plumbers present and close work. Tell them plainly how the role works.
-
"Have you run a crew or supervised apprentices? How many, on what?"
- Strong answer: a number, the job type, and what they owned: layout, material lists, inspections.
- Red flags: "lead" means the senior person on a two-person truck. Fine, but not a foreman.
Gas, backflow and medical gas
These are where plumbing orders carry extra legal requirements, and where resumes are least precise. Rules vary widely by state and city, so find out what the client's jurisdiction requires first.
| Area | What to confirm | Question |
|---|---|---|
| Fuel gas piping | Some jurisdictions include gas work in the plumbing license; others require a separate license or endorsement. | "Who licensed you to do gas piping, and how do you test a new gas line before it goes into service?" |
| Backflow prevention testing | Testers are usually certified separately, and the water utility or state often keeps a list of approved testers. In Texas, the water supply protection specialist is a board endorsement. | "Which backflow certification do you hold, from which program, and when does it renew?" |
| Medical gas | Hospital and clinic work usually requires a specific medical gas installer certification; in Texas it is a medical gas piping installation endorsement on the license. | "Which medical gas certification do you hold, and what was the last healthcare project you worked on?" |
Safety, tools, schedule and pay
| Question | What a strong answer sounds like | Red flags |
|---|---|---|
| What do you do before you cut into a line in a building you do not know? | Identifies what it carries, shuts off and drains, checks for gas and electrical nearby, and tests before cutting. | Assumes it is water. |
| Have you worked in trenches? What protection was in place? | Describes shoring, a trench box or sloping and a competent person on site. OSHA's trenching and excavation page covers the requirements. | Normal to work in unprotected trenches. |
| Do you bring your own hand tools, and what does the client need to supply? | A complete set and knows which power tools and machines are usually company-supplied. | An experienced plumber with no tools. |
| The pay is [$X per hour, overtime after 40, commission or spiffs on service work]. What do you need to make a move? | An hourly figure, plus how they weigh overtime, on-call pay and the truck. | Only a headline rate, with no view on weekly hours. |
| Why are you leaving, or why did the last job end? | Builder slowed down, wants service work, wants less travel. | A pattern of walking off jobs, with a new reason each time. |
Ask what they need to earn rather than what they earn now; pay history questions are restricted in a number of states and cities. See salary expectation questions.
How plumbing experience gets overstated
- Years in the trade counted as licensed hours. Registration date matters in states that only count hours after it.
- Helper time as plumber time. Digging and carrying material described as installing.
- One phase described as the whole job. A top-out plumber who never set fixtures, or the reverse.
- New construction described as service. Ask about the last diagnosis they made alone in someone's home.
- Gas or backflow work without the certificate. Doing it under someone else's license is not the same as holding it.
- A license in the wrong place. Valid where they came from, not where the job is.
Checklist and lawful wording
Before you submit a plumber
- License or registration type, issuer, number and expiry recorded and checked on the board's public search.
- Valid where the work is, or the client has agreed to an apprentice under supervision.
- Hours in the client's setting and phase, with the last date.
- Gas, backflow or medical gas certification confirmed where the order needs it.
- One job described in sequence, including testing or inspection.
- Tools, driver's license, on-call and schedule confirmed.
- Pay expectation against the client's rate.
- Consent to submit for this client and role, with the date.
General information, not legal advice. Plumbing licensing is set by states and cities and changes. Confirm requirements with the board where the work is done, and wording with your counsel.
Describe the physical work and ask if they can do it, with or without accommodation: "The job involves crawl spaces, trenches, lifting water heaters with a second person and kneeling for long periods. Can you perform those duties, with or without accommodation?" Do not ask about injuries, conditions or workers' compensation history before an offer; the EEOC's pre-employment guidance explains the limits. Leave union membership out of the screen, and leave criminal history to the client's process; see ban-the-box interview questions.
License numbers, registration dates and endorsement renewals are easy to mix up after a day of trades calls. Interview Signal captures phone screens through your computer's audio, with no bot on the call, so the details are in your notes as the plumber said them.
Questions people ask
What is the difference between a journeyman and a master plumber?
It depends on the state, but generally a journeyman can do plumbing work under a master's supervision and a master can supervise others and, often, pull permits or run a business. In Texas, for example, a master plumber license requires at least two years as a licensed journeyman, or one year plus a completed Department of Labor apprenticeship program.
Do hours worked before registering as an apprentice count?
Not in every state. The Texas State Board of Plumbing Examiners says hours worked before you register as a plumber's apprentice do not count toward the tradesman or journeyman exam. Ask candidates from any state when they registered, not only when they started.
Can a plumber licensed in one state work in another?
Not automatically. Licensing is set by each state and in some places by cities; Illinois, for example, has a separate reciprocity application for City of Chicago licensees. Check the board where the work will be done.
Should I screen service plumbers and new construction plumbers differently?
Yes. Service plumbers diagnose problems in occupied buildings, often alone, and often present prices to customers. New construction plumbers work from plans in phases alongside other trades. Both are skilled, but a strong candidate in one can struggle in the other for months.