A small drip under the kitchen sink doesn’t feel like an emergency. But then you open the cabinet and find warped wood, mold creeping along the back wall, and a puddle that’s been growing for who knows how long. Kitchen sink leaks are sneaky. They hide behind cleaning supplies and trash bags, and by the time you notice them, the damage is already rolling.
Before you grab a wrench or call a plumber, knowing the right questions to ask about a kitchen sink leak saves you from misdiagnoses, unnecessary part replacements, and the kind of repair bill that makes you question homeownership. This guide covers what to check before the plumber arrives, what to ask during the visit, and how to make sure the fix actually lasts.
Before You Contact a Plumber
- Turn off the water supply under the sink to stop the leak from getting worse.
- Clear everything out from under the cabinet so the plumber can actually see the pipes.
- Take photos or a short video of the leak and any visible damage.
- Pay attention to *when* it leaks. Only when the faucet runs? When the dishwasher drains? All the time? That detail matters more than you’d think.
- Note any recent work. A new garbage disposal install or faucet swap can loosen connections.
During the Visit: Key Questions to Ask Your Plumber
1. Can you identify exactly where the leak is coming from?
Not all leaks are obvious. Water travels along pipes and pools somewhere else entirely, so the wet spot you see might not be where the problem actually is. A good plumber will trace it back to the source, whether that’s a drain line, supply line, faucet connection, or failed seal.
Be skeptical of anyone who glances under the sink and immediately starts quoting you. A proper diagnosis involves running water, checking connections one at a time, and sometimes a pressure test. “It could be a few things” without any diagnostic work isn’t an answer. It’s a guess.
2. What caused the leak in the first place?
This is the question that separates a real fix from a temporary patch. Corrosion, a bad installation, mineral buildup, a cracked fitting: each cause has a different solution. If the plumber just tightens something and leaves without telling you why it failed, you’ll be calling them again in three months.
Tip: If corrosion is the culprit, ask whether your water chemistry or pipe material is contributing. Hard water eats through certain fittings faster than others.
3. Is the leak affecting other plumbing components?
Water under a kitchen sink doesn’t stay in one place. It can reach dishwasher connections, the garbage disposal mounting, the P-trap, and even the wall cavity behind the sink. A plumber who fixes the obvious drip without checking adjacent parts is doing half the job.
You want to hear something like: “I’ll inspect the faucet base, valves, and disposal connections while I’m under here.” That’s thoroughness. Fixing one connection while a second one is about to fail is a waste of everyone’s time.
4. Do you recommend repairing or replacing any parts?
Sometimes a reseal does the trick. Sometimes the shutoff valves are 15 years old and barely functional, and replacing them now saves you a bigger headache later. The honest answer depends on the age and condition of what’s under there.
Watch for this: Pushing a full replacement when a $12 fitting would solve the problem. But also watch for the opposite, a band-aid fix on parts that are clearly at the end of their life. A good plumber explains the tradeoff and lets you decide.
5. How long will the repair take, and will the water be shut off?
Most under-sink repairs take 30 to 90 minutes. But if your water’s going to be off for the duration, you’ll want to plan around that. Fill a pot for cooking, use the bathroom first, whatever you need to do.
A straightforward answer sounds like: “About 45 minutes, and I’ll only need to shut off the water for 15 of those.” Vague timelines or surprise shutoffs are a sign of someone who hasn’t thought through the job.
6. Do you guarantee your work?
A warranty on the repair tells you the plumber stands behind it. One year on parts and labor is reasonable for this kind of work. Get it in writing. Verbal promises evaporate the second the leak comes back.
Specific Questions for a Leak Under the Kitchen Sink
7. Is the leak from the drain or the supply line?
This distinction matters. Drain leaks only happen when water is flowing through. You’ll see dripping when the faucet runs or the dishwasher drains. Supply line leaks happen under constant pressure and can drip all the time, even when nothing’s running.
Fixing the wrong line means the leak comes right back. A plumber who doesn’t clarify which type you’re dealing with before starting work isn’t being careful enough.
8. Could the garbage disposal or dishwasher connection be causing it?
A surprising number of under-sink leaks start at the disposal flange or the dishwasher drain hose connection, not the pipes themselves. These joints get loose over time, especially if the disposal vibrates during use.
If your plumber doesn’t check these connections, ask them to. A loose disposal mount or a cracked dishwasher hose is a 10-minute fix that gets missed constantly.
9. Are the shutoff valves working properly?
Old shutoff valves corrode, seize up, and sometimes leak themselves. If you can’t turn the valve under your sink without a pair of pliers and a prayer, it’s a problem. Corroded valves also make future repairs harder. Imagine needing an emergency shutoff and the valve won’t budge.
Replacing them during this visit, while the plumber’s already under there, is usually cheap and saves you a separate service call later.
10. Do you see signs of water damage or mold?
This is the question most people forget, and it’s one of the most important. A plumber who’s under the cabinet anyway should check the cabinet floor, the back wall, and the area around the drain opening for swelling, discoloration, or mold.
Catching water damage early, before it spreads to subfloor or drywall, can save you thousands. If the plumber spots something, ask whether it needs professional remediation or if drying it out is enough.
11. Should the P-trap or drain assembly be replaced?
The P-trap is the U-shaped pipe under every sink. Plastic P-traps crack over time. Metal ones corrode. The slip-joint washers that seal the connections dry out and fail. If your P-trap is original to the house and the house isn’t new, replacing the whole assembly during the repair is often worth the $20-30 in parts.
12. Can you show me how to check for future leaks?
A plumber who takes 60 seconds to show you what to look for is a plumber who cares about more than the invoice. The basic check: run water for 2-3 minutes, then feel along every fitting and connection with a dry hand. Do this once a month. You’ll catch the next leak before it causes damage.
13. What materials are you using for the repair?
Brass compression fittings last longer than plastic push-fit connectors. Quality plumber’s putty and thread sealant matter. You’re not expected to be an expert on pipe fittings, but you should know what’s going into your home, especially if you’re paying for the parts.
14. Do you charge a service call fee?
Most plumbers charge $50-$150 for the diagnostic visit. Some apply it toward the repair cost if you hire them. Others don’t. Neither is wrong, but you should know which one you’re dealing with before they show up. Ask on the phone, not at the door.
15. Will this fix affect my garbage disposal warranty?
If the repair involves resealing or adjusting the disposal connection, ask whether that impacts the manufacturer’s warranty. Most competent plumbers can work around the disposal without voiding anything, but it’s worth confirming, especially on newer units still under coverage.
16. What can I do to prevent this from happening again?
Leaks under kitchen sinks are one of the most preventable plumbing problems. Simple habits make a real difference: tighten fittings once a year, don’t overload the disposal, check for drips monthly, and consider installing a small leak detector ($15-$30) that alerts you before water damage starts.
A plumber who offers zero prevention advice is focused on the next service call, not on actually helping you.
What to Mention or Send Beforehand
- Photos or videos showing where and when the leak appears.
- Any prior plumbing repairs or part replacements under the sink.
- Brand and model of your faucet, garbage disposal, and dishwasher.
- When the problem started and whether it’s constant or intermittent.
- Where your water shutoff valves are located and whether they work.
Typical Cost Range and Factors
Minor repairs (resealing a joint, tightening connections): $125-$350.
Part replacements (valves, P-trap assembly, supply lines): $300-$700.
Costs vary based on:
- Type of leak: Drain leaks are usually simpler than pressurized supply line repairs.
- Accessibility: Cramped cabinets or back-wall plumbing adds labor time.
- Parts replaced: Shutoff valves, fittings, or disposal seals all have different price points.
- Water damage: If the leak caused cabinet or flooring damage, remediation is extra.
- Emergency timing: After-hours or weekend calls typically run 1.5x to 2x standard rates.
Red Flags vs. Green Flags
| Red Flag | Green Flag |
|---|---|
| Quotes a price without inspecting the leak | Performs a full inspection before quoting |
| No license or insurance when asked | Provides credentials without hesitation |
| Pushes replacements you didn’t ask about | Explains options with honest tradeoffs |
| Leaves the area wet or messy | Cleans up and tests for additional leaks |
| Can’t explain what caused the problem | Walks you through the diagnosis in plain English |
Money-Saving Tips
- Check whether the repair is covered under your home warranty or homeowner’s insurance.
- Bundle small plumbing tasks (slow drain, dripping faucet) into one visit to save on the service fee.
- Ask about flat-rate pricing. It’s often cheaper than hourly for straightforward repairs.
- Keep your photos for reference. Recurring leaks may qualify for free follow-up under warranty.
- Install a $15 to $30 leak detector under the sink. Early alerts prevent expensive water damage.
Quick Reference Checklist
Pull this up on your phone when the plumber arrives, or before you call:
- Can you identify exactly where the leak is coming from?
- What caused the leak in the first place?
- Is the leak affecting other plumbing components?
- Do you recommend repairing or replacing any parts?
- How long will the repair take, and will the water be shut off?
- Do you guarantee your work?
- Is the leak from the drain or the supply line?
- Could the garbage disposal or dishwasher connection be causing it?
- Are the shutoff valves working properly?
- Do you see signs of water damage or mold?
- Should the P-trap or drain assembly be replaced?
- Can you show me how to check for future leaks?
- What materials are you using for the repair?
- Do you charge a service call fee?
- Will this fix affect my garbage disposal warranty?
- What can I do to prevent this from happening again?
Helpful Tools and Resources
This specialized tool reaches the tight space behind the sink to tighten faucet mounting nuts. If you ever need to swap a faucet or tighten a loose connection, it's the only tool that fits back there.
A waterproof cabinet liner catches small drips and spills before they damage the cabinet floor. It also makes leaks visible sooner so you can act before the wood warps.
A staple for sealing sink drains and faucet bases. A single tub costs a few dollars and lasts for years. Useful for resealing a drain flange if your plumber recommends it.
These resources can help you understand your leak, compare repair costs, and stay ahead of future plumbing problems.
- EPA Fix a Leak Week: The EPA’s annual campaign includes guides on finding and fixing common household leaks. Useful for understanding which leaks you can handle yourself and which ones need a pro.
- HomeAdvisor Plumbing Cost Guide: Detailed cost breakdowns for common plumbing repairs by region, so you know whether the quote you’re getting is in the right ballpark.
- State Contractor License Lookup: Search for “[your state] plumber license lookup” to verify your plumber’s credentials before they show up. Takes two minutes and costs nothing. The BBB is also worth checking for complaint history.
Frequently Asked Questions
How do I know if my kitchen sink leak is serious?
If water is pooling in the cabinet, the wood is warped or soft, or you smell mold, call a plumber now rather than later. Constant dripping under pressure (even when the sink isn’t running) is more urgent than a slow drain leak. Either way, don’t ignore it. Water damage compounds fast.
Can I fix a small joint leak with plumber’s tape myself?
For a minor drip at a threaded connection, yes. Teflon tape or pipe joint compound can work as a temporary fix. But if corrosion, pressure, or a cracked fitting is the real issue, tape is a band-aid. If the leak persists after a DIY attempt, call a professional before you end up with a bigger problem.
How long does a typical under-sink repair take?
Most repairs take 30 to 90 minutes, depending on accessibility and whether parts need replacing. If the plumber needs to order a specific valve or fitting, it might stretch to a second visit.
Do plumbers charge extra for emergency leak calls?
Yes. After-hours, weekend, and same-day emergency calls typically cost 1.5x to 2x the standard rate. If the leak isn’t actively flooding, it may be worth waiting for a regular appointment to save $100-$200.
Should I replace both supply lines if only one is leaking?
Usually, yes. Supply lines age at the same rate. If one has failed, the other is likely close behind. Replacing both during the same visit costs marginally more and saves you from a repeat service call in a few months.
Glossary
P-trap: The U-shaped pipe under your sink that holds a small amount of water to block sewer gases from coming up through the drain.
Compression fitting: A connector that joins two pipes using a threaded nut and a metal or plastic ring (ferrule) that compresses to create a watertight seal.