Under Sink Plumbing Repair Cost

Common Under-Sink Repairs and Their Costs

The plumbing under a kitchen sink includes both the supply side (hot and cold water lines, shut-off valves) and the drain side (tailpiece, P-trap, branch drain). Each system has its own set of common failure points.

Drain leak at a slip-joint connection ($100 to $200): The most common under-sink repair. Slip-joint connections use compression fittings with rubber or nylon washers to join drain pipes. When the washer deteriorates or the fitting loosens from vibration, water drips at the connection. The fix involves tightening the slip-nut or replacing the washer ($2 to $5 part). The cost is almost entirely the service call and labor.

P-trap replacement ($100 to $250): The P-trap is the U-shaped pipe that holds water to block sewer gases. Plastic P-traps can crack, and metal P-traps corrode over time. Replacement involves disconnecting the old trap, installing a new one ($8 to $25 part), and verifying proper alignment with the tailpiece and wall drain. The job takes 20 to 45 minutes.

Shut-off valve replacement ($50 to $150 per valve): The shut-off valves under the kitchen sink control the hot and cold water supply to the faucet. Gate valves in older homes seize over time and fail to shut off water completely. Replacing each valve with a modern quarter-turn ball valve costs $50 to $150 including the valve ($10 to $30) and labor. Ball valves are more reliable and provide a positive shut-off with a 90-degree handle turn.

Supply line replacement ($75 to $200): The flexible supply lines connecting the shut-off valves to the faucet can develop cracks, bulges, or leaks at the connection fittings. Braided stainless steel supply lines ($10 to $25 each) are the modern standard, replacing older rubber or vinyl hoses that are more prone to failure. Replacing both supply lines and both shut-off valves during a single visit is a common upgrade that costs $200 to $400 total and eliminates the most common sources of under-sink water damage.

Corroded drain pipe replacement ($200 to $600): If the tailpiece, continuous waste arm, or branch drain pipe is corroded beyond repair, the affected section must be replaced. PVC replacement pipes are inexpensive ($5 to $20 per section), but the labor to cut out the old section, fit the new pipe, and ensure proper slope and alignment takes one to two hours. Extensive corrosion that reaches the branch drain inside the wall escalates the project significantly.

Branch drain repair in the wall ($500 to $1,200): When corrosion or a leak extends to the drain pipe inside the wall behind the sink, the plumber must cut into the drywall to access the pipe, repair or replace the affected section, and then the drywall needs patching and painting. The plumbing work itself may cost $300 to $600, with drywall repair adding $200 to $500 depending on the size of the opening and whether tile or backsplash is involved.

Water Damage From Under-Sink Leaks

The enclosed space inside a kitchen cabinet makes under-sink leaks particularly damaging because they often go unnoticed for weeks or months. Water pools on the cabinet floor, warping particleboard and promoting mold growth in the dark, humid environment. By the time the homeowner notices, the damage may have spread to the subfloor beneath the cabinet.

Minor water damage (surface staining, slight warping of the cabinet floor) is cosmetic and does not require structural repair. The cabinet floor can be sealed with a waterproof coating after the leak is fixed. Moderate damage (soft spots, swollen particleboard, visible mold) requires removing the damaged section of cabinet floor, treating the area with a mold-killing primer, and installing a replacement panel. Severe damage (structural weakness, mold spreading to adjacent walls or the subfloor) may require professional water damage restoration costing $500 to $2,000.

Preventing water damage starts with periodic inspection. Open the cabinet doors every few weeks and look for moisture, stains, or musty odors. Check the connections at the shut-off valves, supply lines, P-trap, and disposal (if present). A slow drip that seems insignificant can cause hundreds of dollars in damage over several months of undetected leaking.

Galvanized Pipe Issues in Older Homes

Homes built before the 1970s often have galvanized steel drain and supply pipes. These pipes corrode from the inside out, gradually narrowing the interior diameter and eventually developing pinhole leaks or complete failures at joints and fittings.

Replacing galvanized drain pipes under the sink with PVC is a straightforward upgrade that costs $200 to $500 for the visible section below the sink. The challenge arises when the galvanized pipe extends into the wall and connects to the main drain stack. The visible pipes under the sink may be the tip of the iceberg, and a plumber discovering corroded galvanized pipes should also assess the condition of the pipes inside the wall.

Galvanized supply lines are a more urgent concern because they carry pressurized water. A failure in a supply line can release a significant volume of water in a short time, causing rapid flooding and water damage. If your home has galvanized supply lines under the kitchen sink, replacing them with copper or PEX piping ($200 to $600 for the under-sink section) is a worthwhile preventive upgrade.

When to Bundle Repairs

If you are calling a plumber for any under-sink issue, use the visit as an opportunity to address other aging components. A plumber who is already under the sink can replace shut-off valves, swap old rubber supply lines for braided stainless steel, replace a worn P-trap, and install a new drain basket for incremental labor cost that is much less than a separate service call for each item.

The ideal bundle for an older kitchen includes both shut-off valves (gate to ball valve upgrade), both supply lines (rubber to braided stainless steel), and the P-trap and slip-joint washers. This "under-sink refresh" costs $250 to $500 total and addresses all the most common failure points in a single visit. Compared to calling a plumber three separate times as each component fails individually, the bundled approach saves $100 to $200 in repeat trip charges.

Signs You Need Under-Sink Repairs

Water stains or discoloration on the cabinet floor are the most obvious indicator. Even a small stain suggests an active or recent leak that needs investigation. Musty or mildew odors when you open the cabinet doors often precede visible water damage and should prompt an immediate inspection of all connections.

Visible corrosion on metal fittings (green deposits on brass, white deposits on chromed fittings, rust on steel) indicates deterioration that will eventually lead to leaks. Mineral buildup around connection points shows that water has been seeping past the fitting, even if the drip is too slow to notice during normal use.

Shut-off valves that do not fully stop the water flow when turned are a safety concern. If you cannot shut off the water to the faucet in an emergency (a burst supply line, for example), you would need to shut off the main water supply to the entire house. Replacing failed shut-off valves is preventive maintenance that pays for itself when you need to isolate the kitchen plumbing for any reason.