Most Likely Causes (Ranked)
The large rubber gasket (also called the spud washer) between the tank and the bowl seals the flush valve opening. It compresses over time and hardens, losing its seal. Water leaks between the tank and bowl during and after every flush. This is the most common tank leak and the first thing to check.
Two or three bolts hold the tank to the bowl, each with a rubber washer. These washers compress and deteriorate, and the brass bolts corrode from constant water contact. As the washers thin, the tank can shift slightly, breaking the seal of both the bolt washers and the tank-to-bowl gasket.
Porcelain tanks crack from impact (dropped lid, leaning on the tank), overtightened bolts, or manufacturing defects. Hairline cracks below the waterline leak constantly. Cracks above the waterline only leak when the tank is refilled to that level. A cracked tank cannot be reliably repaired.
In humid climates, cold water in the tank causes heavy condensation on the exterior that drips to the floor. This mimics a leak and can actually cause the same water damage. It's especially prevalent in summer or in bathrooms without ventilation.
The fill valve passes through the bottom of the tank with a rubber gasket and lock nut. If this gasket deteriorates or the nut loosens, water leaks at the fill valve opening — typically a slow seep that runs down the outside of the tank bottom.
How to Fix It
Replace Tank-to-Bowl Gasket and Bolts
$10-15 DIY / $150-250 Pro- 1Turn off water supply. Flush and sponge out remaining water from the tank.
- 2Remove the tank: unscrew the two bolt nuts from under the bowl. Lift the tank off.
- 3Remove the old spud washer and the old bolt washers. Clean the porcelain surfaces.
- 4Install the new spud washer on the flush valve. Install new bolts with new rubber washers (inside the tank).
- 5Set the tank back on the bowl. Tighten bolts alternately, a little at a time, until snug. Don't overtighten — porcelain cracks.
- 6Turn on water, let fill, flush, and check for leaks.
Tighten Tank Bolts
Free- 1Try tightening the existing bolts: hold the bolt head inside the tank with a screwdriver, tighten the nut from below with a wrench.
- 2Alternate between bolts — tighten each a quarter turn at a time.
- 3Stop immediately if you feel the porcelain flex. Overtightening cracks the tank.
- 4If this doesn't stop the leak, the washers need replacement (see above).
Replace the Toilet
$100-350 DIY / $250-500 Pro Installed- 1If the tank is cracked, the entire toilet should be replaced (tank and bowl are matched sets).
- 2Shut off water, flush, sponge out water, disconnect supply line.
- 3Remove tank bolts, then remove the two tee bolts at the base. Rock the toilet gently and lift off.
- 4Scrape the old wax ring, install a new one. Set the new toilet, press down firmly, and tighten tee bolts alternately.
- 5Reconnect supply line and test.
When to Call a Professional
Never overtighten porcelain — it cracks suddenly and completely. Snug is enough.
A cracked tank is a replacement, not a repair. Epoxy patches fail under the constant water pressure.
If condensation is severe, consider an anti-sweat valve ($30-60) that mixes warm water into the cold supply to prevent sweating.
Water pooling around the base from a tank leak can be mistaken for a wax ring failure — check both.
How to Prevent It
Check tank bolt tightness annually (gentle check, not retorquing).
Never lean on or sit on the toilet tank — this stresses the bolts and gasket.
Replace the fill valve gasket when replacing the fill valve (they come together in kits).
Use a fan or keep the bathroom door open after showers to reduce humidity and condensation.
Frequently Asked Questions
Sometimes — if you can find a matching tank for your bowl model. Check the manufacturer's website. Universal tanks don't exist because mounting holes and flush valve openings vary by model.
Wipe the tank dry. If it's uniformly damp again within minutes (especially on a humid day), it's condensation. If water appears only at a specific point (bolt, gasket, crack), it's a leak.
Not dangerous in the injury sense, but a crack can suddenly worsen and dump 1.5 gallons on the floor. And the constant water promotes mold and floor damage.
Tank gasket and bolt replacement: $150-250. New toilet installation: $250-500 (including a basic toilet). Just diagnosis: $75-150 service call.