What interview questions should you ask a drain and sewer technician? (with good answers)
A question bank for employers: technical questions with the answers to listen for, customer and behavioural questions, a practical task, red flags and a scoring method.
The best drain and sewer technician interview questions fall into three groups: technical questions that have right answers, customer and behavioural questions where you are listening for judgment, and a practical task the candidate performs in front of you.
Below is the bank for each group, with the answer a competent drain and sewer technician should give, the answers that are red flags, and a simple way to score candidates against each other.
Before the interview
Write the questions before the candidate is in the room — and write them from your own job description.
The duties you listed in the posting are the test blueprint: if it says solo stoppage calls with camera and locating work, the interview asks about clearing-and-diagnosis judgment, not fixture swaps.
Under the ADA, essential functions are the fundamental duties of the job, not marginal ones — and a written description prepared before advertising or interviewing is evidence of what is essential.
O*NET lists Drain Technician among the reported job titles for Plumbers, Pipefitters, and Steamfitters (SOC 47-2152), next to Plumber and Pipefitter, so your questions should make candidates narrate that stoppage work, not recite definitions.
Before an offer you may ask about an applicant's skills, education, work history and required certifications and licences, and you may ask them to describe or demonstrate how they would do the job's tasks — that is what the practical section below is for.
State the job's physical requirements and ask whether the candidate can meet them.
The EEOC's own examples are lifting a certain amount of weight and climbing ladders; if your work means crawl spaces, basements with a backed-up line and a machine in and out of the van all day, the same principle applies — say what the job demands and ask directly whether the candidate can do it.
Know what you may not ask before an offer.
Not about past job injuries or workers' compensation history — keep "ever hurt your back on a line job?" off the sheet, out of the phone screen and out of small talk.
You may ask about current illegal drug use, but not about lawful prescription medication.
And salary-history questions are banned in Virginia, Hawaii, Rhode Island, D.C. and California — the jurisdictions this research verified, plus others it did not check — so treat that as a floor rather than the full map.
The safe habit everywhere: post the pay range and ask what the candidate is looking for.
Which of these rules binds you depends partly on your size.
Federal EEO laws — Title VII and the ADA — cover private employers with 15 or more employees; the age discrimination law covers those with 20 or more; the Equal Pay Act covers virtually all employers.
A shop with 4 to 14 employees is still covered by the Immigration Reform and Control Act's national-origin and citizenship discrimination rules, which the U.S. Department of Justice enforces.
State laws often reach smaller employers than the federal ones, so check your state's rules too.
Run the same questions, in the same order, for every candidate.
You will compare answers fairly, and your notes will still make sense by the time the last interview of the week is done.
The full sequence — sourcing, registration and licence checks, pay, onboarding — is our guide to how to hire a drain and sewer technician; this page picks up at the interview itself.
Which technical questions test real drain and sewer technician skill, and what are good answers?
Walk me through the last stoppage call you ran — from the arrival to the moment you called the line clear.
What to listen for: A sequence, not a highlight reel: what they asked before they ran anything, how they got access to the line, what they ran first and why, and how they confirmed the line actually drains once the machine comes out.
A working drain tech narrates the diagnosis as easily as the clearing — what told them “clog” rather than “damaged line” — and finishes on the verification step, not “it went down, so we left.”
A candidate whose stories never include the verification step will skip it on your customers' lines.
When do you put the cable down and change plans? What makes you reach for a different method — or stop and call the customer?
What to listen for: Reasoning from the line and the blockage rather than habit: what came back on the cable, what the pipe material and the customer's system will tolerate, and the honest sentence — “clearing it today is not fixing it.”
The discriminator is whether they can say what would change their mind mid-job, and whether they stop before they damage a line they were asked only to clear.
A candidate who resolves every stoppage the same way has either seen only one stoppage or isn't telling you about the rest.
Where does the camera come in? How do you decide a line is cleared versus needs repair?
What to listen for: The camera as a diagnosis tool, not a prop: they pull it after the clear to see what caused the stoppage, they read the picture — where roots have entered, where the line has settled or broken — and they write down what they found and where.
A strong candidate separates the clearing from the repair recommendation without being prompted, and can say what a camera pass would change about the advice they give the customer.
A tech who can't answer that is selling you a clearing and calling it a diagnosis.
Are you NASSCO-trained — PACP, LACP or MACP? What can you actually do with the class?
What to listen for: They know what they hold: NASSCO's classes cover pipeline, lateral and manhole assessment, plus inspector training for cured-in-place pipe work — and a candidate who respects the training describes it as assessment and inspection education, never as a licence.
The strong answer names the class, says what they learned to produce, and offers the certificate.
Be wary of the version where a class has quietly become “certified” or “licensed” — the credential to verify is whatever the issuing board or city actually recognizes.
What licence or registration covered your drain work where you've worked — and what could you do unsupervised under it?
What to listen for: The licence or registration and the authority that issued it, named without looking anything up — and, just as telling, whether they know the answer changes by place.
Our research found no national drain-cleaning licence, and the states we checked handle it differently: Texas registers drain workers with its plumbing board — a drain cleaner-restricted registrant clears obstructions in sewer and drain lines through existing code-approved openings under a responsible master plumber's supervision, and the next rung, the drain cleaner registration, requires at least 4,000 of those supervised hours and adds installing cleanouts and removing and resetting P-traps to clear obstructions, still under a responsible master plumber's supervision.
West Virginia puts unclogging sinks outside its plumbing-work definition altogether.
Elsewhere the sewer scope sits inside plumbing licences and classes: Hawaii licenses plumbing contractors as specialty class C-37, with a C-37a subclass for sewer and drain line contractors; Connecticut's limited classes include P-6 and P-7 for sewer, storm and water work; North Carolina issues a Restricted Limited Plumbing contractor licence for exterior building sewer, exterior water service piping 2 inches or less, exterior backflow preventers on 2-inch-or-less piping and water filtration systems; North Dakota issues separate board licences for sewer and water installers and contractors; and Chicago's city trade licences list drain layer — though what that licence covers is not established by the research, so treat the city's own licensing pages as the authority.
Seven states — Florida, Kansas, Missouri, Nebraska, Nevada, New York and Wyoming — have no state plumber licence at all: there, plumbers are licensed by cities or counties, or through an industry board.
Confirm what the work you sell requires with your state plumbing board, or the city or county where licensing is local, and verify every licence through the issuer's own record, never a photo of a card.
Have you worked the municipal side — collection systems, main lines? What licence did that take?
What to listen for: Different paper for different work — and a candidate who knows it.
In Texas, for example, municipal-side crew supervision sits under a different agency from the plumbing board: the environmental agency (TCEQ) requires anyone directly supervising domestic wastewater collection system operation or maintenance crews to hold a wastewater collection or treatment operator licence — the collection licences come in Classes I, II and III, and Class I needs a high-school diploma or GED, approved training and a $111 application fee, with no work experience; Class II needs two years' experience, part substitutable with college or training; Class III needs five years, or two and a half with a relevant bachelor's degree.
A hire coming from the municipal side holds different credentials than a contractor-side drain tech; a candidate who cannot say which regime they worked under, or who dresses one credential up as the other, hasn't thought about scope.
Other states' collection-operator grades were not part of this research — check with your state's environmental agency where municipal work is in play.
Which customer and behavioural questions matter most?
A customer's line has backed up into the house and your clearing will take another hour. What do you say to them?
What to listen for: The bad news delivered early, in plain language, with a revised time and the reason — not silence until the machine comes out.
A stoppage is the worst hour of that customer's week; listen for whether they check that the customer understood, and whether they offer the next step.
Hiding the slip until the invoice is how an emergency call becomes a lost account.
Tell me about the angriest customer you've ever had to deal with.
What to listen for: A specific story, told calmly: what set it off, what the candidate did, how it ended.
You want de-escalation and documentation, not victory.
Drain work finds people at their worst — a flooded floor is not a rational customer — and a candidate who tells every story as “the customer was unreasonable” will recreate that conversation in your name.
Tell me about a call that came back — a line you cleared that blocked again.
What to listen for: Ownership without self-flagellation, and a change that followed: what they now check that they didn't before — a camera pass, a documented recommendation to the customer, a follow-up call.
A candidate whose every callback story ends “the customer wouldn’t pay for the repair” with no self-examination is telling you how they'll describe your work to your customers.
This truck runs an on-call rotation. Tell me how you've handled after-hours emergency calls.
What to listen for: A real answer with a real night in it: how they triaged the call, what they told the customer about timing, how the ticket got closed out the next morning.
BLS's own description of the trade has most plumbers, pipefitters and steamfitters working full time, including nights and weekends, with plumbers often on call to handle emergencies — if your stoppage truck is the after-hours line, hire someone whose answer shows they have lived that.
Why did you leave your last contractor, and what would the owner say about your work?
What to listen for: A checkable answer: names, the kind of work they ran, a reason you can confirm on a reference call.
Vague on names usually means vague on history.
Ask what their boss would call their weakness — then actually call and compare.
What practical task should a drain and sewer technician candidate do?
At the bench or in the yard: clear this seeded blockage in the test line, and narrate as you go.
What to listen for: Set-up before speed: the machine set up correctly for the line, the cable fed with control rather than force, head changes explained as they happen — and, the discriminator, the verification step at the end: water run, flow confirmed, the work area cleaned before they call it clear.
A candidate who narrates the sequence unprompted has done this on working calls, not just watched it.
Run the camera down this line and tell me what you see.
What to listen for: A methodical pass and a narration that separates what they see from what they conclude — observations first, then the reading of them: where roots have entered, where the line has settled or broken, and what that means for the recommendation.
Watch whether they note distance and location as they go.
The camera report your office can bill from and your estimator can sell from is the skill you are actually buying; a candidate who shrugs “there’s the problem” without locating it has given you neither.
Here's a line with no cleanout where you'd want one. How do you get in?
What to listen for: They start from the drawing and the customer's options, and they ask before they cut: an existing opening elsewhere in the system, a fixture set aside and reset, an access point on the roof side — each explained with a reason and a price before anything is opened.
A candidate who reaches for the sawzall without a word is describing the call you'll be apologizing for.
Quote the handoff: the line is cleared, the camera shows a break outside. Walk me through what happens next.
What to listen for: The boundary discipline that protects both the customer's trust and your margin: the finding documented with the footage, the customer told in plain language what the camera shows, and the repair handed to your estimating process rather than invented on the curb.
Listen for where they stop — clearing is the call they were sent on; a sewer replacement priced from a driveway conversation is your money being spent.
Which answers are red flags?
None of these is automatically a no — but each should stop the interview long enough to verify, and a couple should end it.
- Cannot narrate a recent stoppage end to end. What they ran first, how they got access, what the camera showed, how they verified the clear — a working tech runs through these without notes; a résumé-only candidate stalls at the camera.
- The licence they can't name. They claim a licence or registration but cannot say which one or who issued it. The claim is checkable before you offer — through the issuing board's own record, never a photo of a card — and an unnamed claim cannot be checked at all.
- A class dressed up as a licence. NASSCO's PACP, LACP and MACP are assessment classes and ITCP-CIPP is inspector training — none of them is a licence. A candidate presenting a class as a board credential has misstated what they hold; one lookup at the board that supposedly issued it settles it.
- Hedging on supervision scope. Where a state structures the work, the scope is specific — in Texas, a drain cleaner-restricted registrant clears obstructions through existing code-approved openings under a responsible master plumber's supervision. A registrant there can say who supervises them and what their registration does and does not cover; a candidate anywhere who cannot describe their scope is how work outside a registration's limits lands on your company. Treat the answer as a claim to check with the issuing board, not as the rule itself.
- 'No licence needed for drain work' — said as a national truth. Licensing is state law and, where licensing is local, city or county law: Texas registers even entry-level drain cleaners with its plumbing board, while West Virginia puts unclogging sinks outside its plumbing-work definition. A candidate who cannot tell you which regime they worked under hasn't noticed the difference — your company can't afford to.
- Guess-first diagnosis. Every blockage story ends “so I ran the machine again and it held” with no camera pass or verification step in between. On a sewer line a guess is not a callback — it is a flooded floor with your company's name on the invoice.
- The inspector joke. Brags that inspectors 'just sign it' on their work, or that the permit never got pulled on jobs they've run. However meant, it tells you how the paperwork on your tickets gets done.
- Every past employer is a villain. One bad shop is a story; a row of them is a pattern. Ask what each owner would say, then call and compare.
- Side work on your truck. Talks casually about using your materials, your truck or your shop's name for jobs on the side. However good the hands, this is the behaviour your other techs will copy.
After the interview
Score the same day, on the same sheet.
Four rows — technical answers, customer answers, the practical task, credentials — each on a 1-to-5 scale, with a line of notes on the strongest and weakest answer.
Score before the next candidate sits down: memory grades on charm.
Ask every candidate the same questions in the same order.
That is what makes the scores comparable — and consistent questions and a written scoring sheet also give you a record of how each candidate was judged.
Under the Uniform Guidelines on Employee Selection Procedures, a hiring test that has adverse impact on a race, sex or ethnic group is treated as discriminatory unless it has been validated.
If you take candidates onto real work before you decide — a paid working interview for HVAC and plumbing candidates — or you score applicants on a written test, an off-the-shelf HVAC technician skills test included, have an employment attorney review how you run it.
Weight the rows for your work.
A stoppage-only truck leans on the clearing-judgment and camera answers; a full-line shop leans on the repair-handoff and licence-scope answers.
Same sheet, different weights — just decide the weights before the first interview, not after the first offer.
Verify through the source before you offer.
Our research found no national drain-cleaning licence: confirm the registration or licence the work needs with your state plumbing board — or the city or county where licensing is local — and check it through the issuer itself; our guide to verifying licences covers the lookup step.
And keep the order of the legal sequence straight: after a conditional offer, you may ask disability-related questions and require medical exams, as long as you do it for all entering employees in that job category.
Before the offer, you may ask about current illegal drug use, but not about lawful prescription medication.
If your background check comes from a screening company, the Fair Credit Reporting Act's notice and adverse-action steps apply — and the EEOC separately expects a targeted criminal-record screen that considers at least the nature of the offense, the time elapsed and the nature of the job.
When the sheet says yes, make sure the posting that found them still says the right things — and browse current drain technician jobs to see what candidates see on the board.
This page is employer career information, not legal advice. Federal interview rules are enforced by the U.S. Equal Employment Opportunity Commission, and, for smaller employers (4 to 14 employees), national-origin and citizenship discrimination by the U.S. Department of Justice; state and local laws often add rules and reach smaller employers. Confirm what applies to your company with the agencies for your state and an employment attorney.
More hiring resources
Interviewing drain technicians? Post the opening too.
HVAC and plumbing techs look for their next job on the board built only for the two trades.