Most under-sink water damage traces back to one of two small, cheap components: a supply line (the flexible hose from the shutoff valve to the faucet) or an angle-stop shutoff valve itself. Both fail in predictable ways, both cost under $30 in parts, and both are within homeowner scope for most repairs. The reason they cause so much damage is that the leak is often slow and hidden behind cleaning supplies — by the time anyone notices, the cabinet floor is rotted and the subfloor is saturated.
This guide walks you through identifying the type of leak, doing the repair safely, and recognizing the specific moments when a plumber needs to take over.
This guide is organized the way the decision actually plays out in practice: what matters, what does not, and the reasoning behind each recommendation. Numbers and ranges reflect 2026 Connecticut, Massachusetts, and New York conditions and pricing.
Quick answer
Under-sink leaks fall into two categories: supply line failures (cracked hose, loose nut, worn washer) and shutoff valve failures (seized handle, leaking stem, corroded body). Supply line replacement is a 15-minute DIY job costing $10-$25 in parts. Shutoff valve replacement is a 30-60 minute DIY job costing $15-$40 in parts if the existing compression fitting is reusable, and $100-$350 professional if soldering is required. The failure rate of braided supply lines is well documented — most insurance carriers treat a 5-year replacement interval as best practice. If the shutoff valve is seized or the leak is coming from behind the wall, stop and call a plumber.
Field context
The difference between a technical checklist and a guide worth reading is the accumulated pattern recognition of someone who has walked through many homes with the same issue. The catalog of symptoms, causes, and remedies is the same in any reference. What experience adds is distribution: which presentations are common and benign, which are common and serious, and which are rare but so high-consequence that they reorganize the priority list the moment they appear. An experienced eye catches the rare-but-serious items homeowners would not think to look for, and calibrates urgency on the common ones.
The Northeast adds its own layer. Housing stock across Connecticut, Massachusetts, and New York ranges from recently-built to pre-Revolutionary, and the same failure mode presents differently in a 1920s three-decker, a 1960s split-level, and a 2015 subdivision. Climate cycling — humid summers, deep-cold winters, freeze-thaw transitions — stresses materials in ways that matter for what fails first and how quickly. Coastal proximity, well water, oil heat, radiator heat, and regional construction practices each influence the shape of the problem. The sections that follow account for those regional factors where they materially affect the recommendation.
Finally, the recommendations below are calibrated to actual outcomes observed at resale. Issues that routinely surface during buyer inspections and cost money at closing are weighted more heavily than cosmetic items that rarely affect a transaction. Homeowners who think about their home the way an eventual buyer's inspector will think about it tend to make better investments and encounter fewer surprises when they do sell.
Identifying the leak source
Before any repair, confirm what is actually leaking. Supply lines, shutoff valves, faucets, and drains all have different fix paths.
Safe investigation
- Clear everything out from under the sink.
- Dry every surface thoroughly with paper towels.
- Run water for 30 seconds at the faucet.
- Look where water reappears. Wet-then-dry-then-wet is the diagnostic sequence.
- Joint where supply line connects to faucet (top of supply line) — usually a loose nut or worn washer at the faucet tailpiece
- Joint where supply line connects to shutoff valve (bottom of supply line) — loose nut or cracked compression washer
- Shutoff valve body or stem — seized valve, worn stem packing, or corroded body
- Behind the shutoff valve (inside the wall) — stub-out connection, wall elbow, or supply pipe — this is beyond DIY scope
- New braided stainless steel supply line matching the length and fitting size (measure before buying — common sizes are 1/2-inch FIP or 3/8-inch OD compression at each end)
- Bucket and towels
- Adjustable wrench or small crescent wrench
- A second wrench or pair of channel-lock pliers if needed
- Close the shutoff valve. If it does not close (common on older valves), close the main water supply instead. If the main won't close, stop and call a plumber.
- Open the faucet to release pressure and drain the supply line.
- Place the bucket and towels under the supply line.
- Loosen the nut at the top (faucet tailpiece) with your wrench. Hold the tailpiece steady with the second wrench so you don't damage the faucet.
- Loosen the nut at the bottom (shutoff valve). Some water will drain.
- Discard the old supply line. Do not reuse — it has already aged.
- Hand-thread the new line at both ends. Tighten with your wrench, but only snug — do not over-torque.
- Open the shutoff valve slowly. Water should flow with no leaking.
- Check for leaks at both connection points. Tighten another quarter-turn if needed.
- New shutoff valve (quarter-turn ball valve recommended for upgrade)
- Compression fitting sleeve and nut (if compression) — usually comes with the valve
- Teflon tape for threaded connections
- Two wrenches
- Bucket and towels
- Close the main water supply to the house. Any shutoff valve replacement requires killing water at the main.
- Open the faucet to drain pressure.
- Disconnect the supply line from the valve (top nut).
- Examine the valve's inlet side. If it is compression fitting, unscrew the compression nut. If soldered, stop and call a plumber.
- Slide the old valve off. The compression ring (small brass sleeve) stays on the pipe.
- Install the new valve. Compression nut first, then new compression ring (if provided), then valve body. Tighten compression nut to snug.
- Reconnect the supply line at the top of the new valve.
- Restore main water slowly. Check for leaks at both connection points.
- Cycle the valve open and closed several times to confirm operation.
- The valve is soldered to copper pipe (requires torch work)
- The pipe stub coming out of the wall is damaged, pitted, or out of round
- The compression ring is stuck on the pipe and won't come off
- The pipe moves when you turn the wrench (indicates loose in-wall fitting)
- Any sign of active leak from the wall behind the valve
- Any soldered shutoff valve replacement
- Any leak from behind the wall or inside the cabinet back panel
- Any shutoff valve that will not close when you need to work on the system
- Any cabinet or floor showing active water damage
- Any under-sink configuration with mixed old and new plumbing types
- Replace all braided supply lines every 5-8 years as preventive maintenance. Cheapest insurance in the house.
- Cycle every shutoff valve annually to prevent seizing. One minute per valve, 10-20 valves per house.
- Install water leak sensors under every sink and behind the washing machine. A $15 sensor triggers a phone alert when water appears.
- Install a whole-home leak detector with automatic shutoff if you travel or have a vacation home. $200-$900 device prevents the catastrophic-leak scenario entirely.
- Keep under-sink cabinets uncluttered so leaks can be seen early. Use a shallow tray under anything stored there.
- Low Water Pressure: Six Causes From Fixture to Street
- Main Sewer Line Issues: Scoping, Roots, and Repair Ranges
- Frozen Pipe Prevention and Repair
- Galvanized Steel Supply Pipes: Lifespan and Replacement Cost
- Stela Report — pre-purchase property intelligence with disclosure, condition, and risk flags.
- Repair Calculator — modeled cost ranges by category and ZIP, calibrated with regional and complexity multipliers.
- Stela Guides — step-by-step repair walkthroughs reviewed by licensed professionals, with safety callouts and disclosure.
- International Plumbing Code (IPC) Section 605 — water distribution fittings
- American Water Works Association (AWWA) — residential plumbing best practices
- Insurance Institute for Business and Home Safety (IBHS) — water damage prevention
- Plastics Pipe Institute — plastic supply line specifications
- Copper Development Association — copper plumbing repair guidance
Four common leak locations
If water is appearing from behind the wall where the shutoff valve exits, stop immediately. Shut off the main water supply to the house and call a plumber. In-wall leaks compound fast.
Supply line failure modes
Braided stainless steel supply lines (the most common modern type) can fail in several ways:
Braided jacket rupture
The most visible failure — the flexible braided covering splits and water sprays out. Often happens suddenly under full supply pressure. Caught early, this is a $15 fix; caught late, this is a flooded kitchen.
Compression washer failure at either end
The rubber or fiber washer inside the coupling nut hardens with age and no longer seals. Shows up as a slow drip at one of the connection points.
Corrosion at the connection fittings
Older supply lines with brass or chrome-plated fittings can corrode at the nut threads, especially in areas with hard or chlorinated water. Once corrosion starts, the fitting will not re-seal even with a new washer.
Plastic line degradation
Older beige plastic or white plastic supply lines (common in 1970s-1990s installations) become brittle with age and can crack or rupture. Any plastic supply line more than 10 years old is a replacement candidate regardless of visible condition.
Age-based replacement
Industry best practice — and many insurance carrier recommendations — is to replace braided supply lines every 5-8 years as preventive maintenance, regardless of visible condition. The cost is under $15 per line; the cost of a failure event is thousands in water damage.
Shutoff valve failure modes
Shutoff (angle stop) valves are the small quarter-turn or multi-turn valves under every sink, behind every toilet, and at every water heater.
Seized handle
An old multi-turn angle stop that has not been operated in 20 years will often not turn when you need to shut water off for a repair. This is the most common shutoff valve failure and the most frustrating — the valve is not leaking, but it is not usable either.
Stem leak
Water leaks from around the handle when the valve is open. The internal stem packing has worn out. Sometimes fixable by tightening the packing nut behind the handle; often requires valve replacement.
Body leak
Water leaks from the valve body itself, often at the junction between the valve and the supply pipe. Always requires replacement.
Quarter-turn vs. multi-turn
Older homes typically have multi-turn globe-style angle stops. Modern installations use quarter-turn ball valves, which are more reliable, easier to operate, and less prone to seizing. Upgrading old multi-turn stops to quarter-turn during any repair visit is cheap insurance.
Supply line replacement — the 15-minute DIY
If the leak is at a supply line, replacement is within homeowner scope for most installations.
What you need
Step-by-step
If either end won't seal after a quarter-turn extra, the fitting is damaged — replace or call a plumber.
Shutoff valve replacement — 30-60 minute DIY (when reusable fitting)
If the leak is at the shutoff valve, replacement is DIY-capable when the valve connects to a compression fitting (common in modern installations) or a threaded fitting. Soldered connections require a plumber.
What you need
Step-by-step
When to stop and call a plumber
What fixes actually cost in 2026
National ranges. DIY parts are very cheap; professional labor dominates on anything beyond the simplest job.
| Scope | Low end | Typical | High end |
|---|---|---|---|
| DIY braided supply line replacement (parts only) | $10 | $15 | $30 |
| DIY shutoff valve replacement (compression, parts only) | $12 | $25 | $50 |
| Professional supply line replacement (single line) | $100 | $175 | $300 |
| Professional shutoff valve replacement (compression, no solder) | $150 | $275 | $450 |
| Professional shutoff valve replacement (soldered copper) | $200 | $375 | $600 |
| Professional under-sink plumbing repair (multi-component) | $250 | $500 | $1,000 |
| In-wall supply line repair (small drywall cut + patch) | $400 | $850 | $1,800 |
| Water damage repair (cabinet, floor, drywall) — estimate only | $800 | $2,500 | $8,000+ |
The asymmetry is striking: a $15 supply line replaced on schedule prevents thousands in water damage. A $275 shutoff valve replaced preemptively prevents a midnight emergency plumber call at 2x rates.
When to call a professional
Call a licensed plumber for:
Stela Home earns no referral fees from contractor connections.
Preventing the next leak
Diligence and documentation
Diligence on an issue like this comes down to two practices that repeatedly separate homeowners who handle it well from those who do not. The first is verification over assumption. Condition findings should be confirmed by the relevant specialist — a structural engineer for structural concerns, a licensed plumber or HVAC technician for systems findings, an environmental consultant for hazardous materials, a certified arborist for tree-related concerns. The $400-$800 specialist-inspection fee is almost always cheaper than the decision that would be made without that information.
The second is documentation. Receipts, service records, permit paperwork, before-and-after photographs, and contractor contact details all belong in one organized place. The Connecticut, Massachusetts, and New York homes that sell cleanly are the ones with a clear paper trail; the homes that get nickel-and-dimed at the buyer's inspection are the ones where nobody can document what was done, when, by whom, or under what permit. The documentation habit also creates continuity across ownership — future homeowners inherit not just the house but the record of how it has been maintained, which shapes how they care for it in turn.
Bottom line
The common thread across every category covered in this guide: condition verification beats assumption, documentation beats memory, and early attention to small problems beats deferred response to large ones. The homeowners who come through inspections with the fewest surprises are the ones who have treated their house as a set of known systems with known service histories rather than a collection of things that mostly work until they don't.
Related Stela Home coverage
How Stela Home helps
Three Stela Home tools work together on this kind of decision: