A leaking pipe under your sink can waste up to 10,000 gallons of water per year if left unchecked, according to the EPA WaterSense program. To fix a leaking pipe under sink, start by turning off the water supply and placing a bucket under the leak. This guide walks you through common repairs for compression fittings, slip joints, and p-traps, helping you save water and avoid costly water damage.
Safety First
Before attempting any repair, turn off the water supply to the sink using the shutoff valves under the cabinet. If those valves are stuck or missing, shut off the main water valve. Open the faucet to relieve pressure and drain remaining water. Place a bucket and absorbent towels beneath the leak to catch drips. Wear safety goggles to protect your eyes from splashing water or debris.
Identify the Leak Source
Knowing exactly where the water is coming from is essential. Under-sink plumbing has multiple connection points that can fail over time. Follow these steps to find the source.
Common Leak Points
- Compression nuts at the shutoff valve and supply line connections – often loosen with temperature changes.
- Slip joint washers between the tailpiece, p-trap, and drain pipe – rubber dries out and cracks.
- P-trap itself – can crack from impact or corrosion, especially on older metal traps.
- Dishwasher drain hose connection – clamp may loosen or hose may develop pinholes.
How to Inspect
Wipe all pipes dry with a clean towel, then run water for a few seconds and watch for drips. Use a flashlight to check the bottom of every fitting. If you don't see active leaking, place a paper towel under each connection and check it an hour later for moisture. For hard-to-reach areas, our leak detection service uses advanced tools and can identify hidden leaks behind walls or under cabinets.
Tools and Materials Needed
Most under-sink leak repairs can be done with basic tools. Assemble the following before you start:
- Adjustable wrench or slip-joint pliers – for tightening or loosening nuts without marring surfaces.
- Bucket and absorbent towels – to catch water and keep the work area dry.
- Plumber's tape (PTFE tape) – wraps around threaded connections to create a watertight seal.
- Pipe joint compound (plumber's putty) – optional, used on metal threads for extra sealing.
- Replacement slip joint washers – available in various sizes (1-1/4" and 1-1/2" are common).
- New compression nuts and ferrules – if the existing ferrule is cracked or deformed.
- Hacksaw and replacement pipe – for cutting out a cracked section (only if you plan a permanent fix).
- Safety goggles and work gloves – protect yourself from sharp edges and water.
Step-by-Step Repair Guide
Repairing a Loose Compression Nut
Compression fittings connect supply lines to shutoff valves and faucets. If you see water dripping from the nut, try tightening it. Use an adjustable wrench to turn the nut clockwise about a quarter turn. Important: Do not overtighten – too much force can crack the ferrule and make the leak worse. If the nut is already tight but still leaks, the ferrule (a small brass or plastic ring inside the nut) is likely worn. Shut off the water, unscrew the nut, remove the old ferrule with needle-nose pliers, and replace it with a new one. Apply three layers of plumber's tape to the threads before reassembling.
Replacing a Slip Joint Washer
Slip joint washers seal the connections between the sink tailpiece, p-trap, and drain pipe. Over time the rubber hardens and begins to drip. To replace one, place a bucket directly under the p-trap. Using your hands or pliers, loosen the slip nuts on both sides of the trap. Carefully slide the trap off and remove the old washer. Clean any debris from the mating surfaces with a rag. Insert the new washer, ensuring it sits squarely and evenly. Reattach the trap and hand-tighten the slip nuts, then give each nut an additional half turn with pliers. Test by running water – if it still leaks, tighten slightly more, but avoid crushing the washer.
Fixing a Cracked Pipe
A cracked pipe requires immediate attention. First, turn off the water at the main shutoff valve (do not rely on under-sink valves if they are old). For a small crack (under 1 inch) on a straight section, apply a temporary repair using epoxy putty rated for plumbing. Clean and dry the pipe, knead the putty, and press it firmly over the crack. Let it cure per the manufacturer's instructions before turning water back on. For a permanent fix, cut out the damaged section with a hacksaw, clean the ends, and install a new piece using slip couplings or compression fittings. However, because local building codes may require specific materials and techniques, many homeowners find it safer to call a professional for pipe repair. The International Association of Plumbing and Mechanical Officials (IAPMO) sets uniform standards that ensure repairs are safe and long-lasting.
Common Under-Sink Leak Causes & Solutions
| Cause | Symptoms | Solution |
|---|---|---|
| Loose compression nut | Water dripping from nut area | Tighten nut clockwise; replace ferrule if needed |
| Worn slip joint washer | Leak at the joint between pipes | Replace washer, hand-tighten slip nuts |
| Cracked p-trap | Water pooling under the bend | Replace p-trap with PVC or metal version |
| Dishwasher drain hose leak | Water at hose clamp connection | Tighten clamp or replace hose |
When to Call a Professional
Many under-sink leaks can be fixed with the steps above, but some situations require a licensed plumber. Call for help if:
- The leak is located inside a wall or under the floor and cannot be accessed without cutting drywall.
- You discover a cracked pipe in a location that requires soldering or special tools you don't have.
- Multiple leaks or low water pressure suggest a larger plumbing issue.
- You are unsure which part to buy or how to safely complete the repair.
Our team can handle everything from simple leak fixes to full pipe replacements. We also offer drain cleaning services to keep your sink's drainage system flowing freely and reduce strain on connections.
Preventive Maintenance Tips
- Inspect under-sink connections every six months for rust, corrosion, or dampness.
- Never hang heavy items (like cleaning caddies) from pipes – the weight stresses slip joints.
- Use a sink strainer to catch food scraps and prevent clogs that can cause backpressure on joints.
- Replace braided supply lines every 5–7 years, as the rubber inner tube can degrade unseen.
- Keep the cabinet under the sink dry by fixing any cabinet leaks and using a drip tray or mat.
If your under-sink leak persists or you'd rather leave it to the pros, MyTownPlumber is here to help. Take advantage of our current plumbing repair offer and get your leak fixed quickly by experienced technicians.