Most Likely Causes (Ranked)
Single-handle bathroom faucets use a cartridge (ceramic disc or pressure-balancing). Two-handle faucets use stems with washers or ceramic cartridges. After 5-10 years, internal seals harden, and ceramic discs develop microscopic scoring. The faucet drips from the spout even when off. Bathroom faucets often fail sooner than kitchen faucets because they're turned on and off more frequently throughout the day.
The O-rings that seal the spout to the body wear from swiveling. Bathroom faucets with swivel spouts (less common than kitchen) and widespread faucets with separate spout and handles are prone to this. Water seeps around the base of the spout during use.
The pop-up drain mechanism connects through the back of the drain tailpiece with a pivot rod. The pivot rod seal (a nylon ball and rubber gasket) leaks when the gasket degrades. This is technically a drain leak, not a faucet leak, but it's closely associated with the faucet.
Braided supply lines from the shut-off valves to the faucet tailpieces develop leaks at the fittings. The internal rubber hose degrades from constant pressure. The connections at the faucet may loosen from vibration.
Low-quality faucets with zinc alloy (zamak) bodies can develop cracks from stress, overtightening during installation, or dezincification corrosion. The crack may be internal and not visible. Brass-body faucets are much more durable.
How to Fix It
Replace Cartridge (Single-Handle)
$15-35 DIY / $150-250 Pro- 1Turn off both shut-off valves under the sink. Open the faucet to drain.
- 2Remove the handle: find the set screw (usually under a decorative cap) and remove with an Allen key.
- 3Remove the cartridge retaining nut or clip. Pull the cartridge straight out.
- 4Match the cartridge at the hardware store. Moen 1225/1255, Delta RP46074, and Pfister 974-044 are the most common bathroom cartridges.
- 5Insert new cartridge (note orientation — a mark or tab indicates hot side). Reassemble and test.
Replace Stems and Washers (Two-Handle)
$5-15 DIY- 1Turn off water. Remove handle (decorative cap → screw → handle).
- 2Unscrew the packing nut with a wrench. Pull the stem out.
- 3Replace the rubber washer at the bottom (held by a brass screw). Also replace the O-ring on the stem body.
- 4If the brass seat (inside the faucet body) is pitted, regrind it with a seat wrench ($10) or replace if removable.
- 5Reassemble, turn on water, and test both handles.
Replace Pop-Up Drain Gasket
$5-10 DIY- 1Under the sink, locate the pivot rod entering the back of the drain tailpiece.
- 2Unscrew the retaining nut. Pull the pivot rod out.
- 3Replace the nylon ball and rubber gasket inside the opening. Apply plumber's grease to the new gasket.
- 4Reinstall the pivot rod and tighten the nut just until snug.
When to Call a Professional
If the shut-off valves under the sink are seized (won't turn), don't force them — you'll break the valve. Turn off the main water supply instead and plan to replace the valves.
Supply line bursts are a leading cause of water damage. Replace braided lines over 5 years old.
Don't reuse old O-rings — they may look fine but have taken a compression set and won't seal again.
Moen, Delta, and Kohler all offer lifetime warranties on their cartridges. Call before buying a replacement — they often send one free.
How to Prevent It
Don't crank handles beyond the stop point — it grinds the washer into the seat.
Replace supply lines every 5-7 years ($8-15 per line).
Fix drips immediately — delay only increases the repair scope.
Lubricate cartridges with silicone grease during replacement to extend their life.
Frequently Asked Questions
Look for a logo on the faucet body or handle. Under the sink, the supply line packaging may have a label. Moen has a flat handle nut, Delta often has a ball mechanism, Kohler has a distinctive hex shape. Or post a photo on a plumbing forum.
Repair if: it's a quality brand (Moen, Delta, Kohler) under 10 years old. Replace if: it's a no-name brand, 15+ years old, or parts are discontinued.
Water pressure rises at night when neighborhood usage drops. Higher pressure pushes past worn seals. This means the seal is close to full failure — fix soon.
1 drip per second = 3,000+ gallons/year. That's $25-60 on your water bill, plus the faucet damage worsens.