Toilet Tank Leaking at Bolts: Replace Bolts and Gasket

Drips under the back of the toilet usually get blamed on the wax ring, but the source is often higher up. A toilet tank leaking at bolts means water is escaping where the tank bolts pass through the bottom of the tank into the bowl, usually because the rubber washers have hardened or the metal bolts have corroded. The fix is a tank-to-bowl kit with new bolts, washers, nuts, and a spud gasket, and most homeowners can do it in about an hour.

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Before buying parts, confirm the leak is really coming from the bolts. Condensation on a cold tank, a loose supply line, and a cracked tank all leave water in the same spot on the floor. A few minutes of detective work with a dry paper towel and some food coloring can save you from replacing the wrong part.

Do not try to stop the leak by cranking the nuts tighter. Vitreous china cracks under uneven pressure, and a cracked tank means buying a new toilet. Replacing the hardware is the reliable, low-cost repair.

Is Your Toilet Tank Leaking at Bolts? How to Diagnose It

Dry the entire toilet with a towel: tank, bolts, supply line, and the floor around the base. Then check each possible source one at a time.

Check the bolts

Reach under the tank and wrap a dry paper towel around each bolt head and nut where it sits below the tank. Flush a couple of times and check again after 10 minutes. A damp towel at one bolt, with drips forming on the nut, points directly to a failed bolt washer.

Rule out a toilet leak from tank to bowl at the gasket

The large spud gasket seals the opening between the tank and bowl. A failed spud gasket leaks during and right after flushing, usually showing up as water running down the back of the bowl between the bolts rather than at a single bolt. The repair is the same kit, so you replace both at once.

Rule out condensation

In humid summer months, cold water in the tank causes sweat to form over the whole outer surface. If the entire tank is beaded with moisture and drips fall from the lowest points, that is condensation, not a bolt leak. An insulating tank liner, a mixing valve, or better bathroom ventilation addresses it.

Rule out the supply line and shutoff

Run a dry towel along the braided supply line, the fill-valve nut under the tank, and the shutoff valve stem. Drips at those points mean you need a new supply line or a tightened nut, not new bolts.

Rule out a cracked tank

Add a few drops of food coloring to the tank water and do not flush. If colored water appears on the outside of the tank, look closely for a hairline crack. A cracked tank cannot be reliably repaired and must be replaced.

Tools and Materials for Toilet Tank Bolt Replacement

• Tank-to-bowl kit matching your toilet (bolts, rubber washers, metal washers, nuts, and spud gasket); check the toilet brand and model under the tank lid
• Adjustable wrench or deep socket
• Long flathead screwdriver
• Large sponge, towels, and a small bucket
• Penetrating oil and a hacksaw blade or mini hacksaw for corroded bolts
• Rubber gloves
• Optional: new braided supply line if the old one is more than about five years old

Choose brass or stainless steel bolts. Older zinc-plated bolts rust quickly in tank water, which is often why the original hardware failed in the first place. Some kits are universal, but two-piece toilets from certain manufacturers use proprietary gaskets, so matching the model number saves a second trip.

Step-by-Step: Installing Toilet Tank Bolts and a New Gasket

1. Shut off the water. Turn the stop valve behind the toilet clockwise until it stops. If it will not close fully, shut off the main water supply.
2. Drain the tank. Hold the handle down to flush and empty as much water as possible. Sponge out the remaining water from the bottom of the tank so it does not spill when you lift the tank.
3. Disconnect the supply line. Unscrew the coupling nut under the tank by hand or with a wrench. Keep a towel underneath for the last bit of water.
4. Remove the old bolts. Hold the bolt head inside the tank steady with a screwdriver while turning the nut underneath with a wrench. If a bolt spins or the nut is seized, apply penetrating oil, wait 10 minutes, and try again. Badly corroded bolts can be cut between the tank and bowl with a hacksaw blade; tape the blade to protect the china.
5. Lift off the tank. Lift straight up and set the tank on a towel, preferably on its back. A typical tank weighs 25 to 40 pounds when empty.
6. Remove the old spud gasket. Pull the gasket off the flush valve outlet under the tank. Clean the tank bottom and the bowl’s mating surface with a rag, removing mineral deposits and old rubber residue. Inspect both surfaces for cracks.
7. Install the new spud gasket. Push the new gasket over the flush valve nut on the underside of the tank, with the beveled or tapered side facing the bowl.
8. Insert the new bolts. Slide a rubber washer onto each bolt against the head, then insert the bolts from inside the tank through the holes. Add any extra washers or nuts on the underside according to the kit instructions; many kits use a rubber washer inside and a rubber-plus-metal washer stack outside.
9. Set the tank on the bowl. Lower the tank so the bolts drop through the holes in the bowl and the spud gasket seats into the bowl opening.
10. Tighten evenly. Thread a metal washer and nut onto each bolt from below. Tighten by hand, then alternate between bolts, turning each a half turn at a time while holding the bolt head with the screwdriver. Stop when the tank sits level and firm and does not rock. On most toilets, the tank will not touch the bowl; there is a small gap by design.
11. Reconnect and test. Reattach the supply line, open the valve slowly, and let the tank fill. Flush several times, then check each bolt with a dry paper towel. Recheck after an hour and again the next day.

Toilet Bolt Replacement Tips That Prevent Cracks

The most common mistake during toilet bolt replacement is overtightening. China does not flex, so a nut driven too far on one side can crack the tank or the bowl ear. Tighten alternately in small increments, and stop at snug. If a bolt drips after testing, give it a quarter turn more rather than a full turn.

Level matters, too. If your floor is out of level, the tank may want to lean. Tighten so the tank sits parallel to the wall rather than forcing it perfectly plumb against an uneven bowl. And if the bowl itself rocks on the floor, fix that separately with toilet shims and by snugging the closet bolts at the base, because a rocking bowl will eventually loosen the tank connection again.

Troubleshooting After the Repair

• Still dripping at one bolt: Tighten a quarter turn. If it continues, drain the tank and check that the rubber washer inside the tank is centered and not pinched.
• Leak between tank and bowl during flush: The spud gasket may be upside down, the wrong size, or not fully seated. Re-seat it or confirm you have the correct gasket for your model.
• Tank rocks side to side: Tighten the bolts evenly a little more. If it still moves, the gasket may be too thick for the toilet, or the bolts are not the right length.
• Water on the floor at the base: If the bolts are dry, the wax ring may be leaking and the toilet needs to be pulled and reset.
• Toilet runs constantly: A disturbed flapper or fill valve may need adjusting after the tank was moved.

Cost of Repair

A tank-to-bowl bolt kit costs roughly the price of a takeout lunch, and a complete kit with a spud gasket runs a bit more. A new braided supply line is similarly inexpensive. If you hire a plumber, expect a service call in the low hundreds of dollars, depending on your area and whether other parts need replacing. If the tank is cracked, the repair becomes a replacement tank or a new toilet, which costs considerably more. Water damage to subfloor from a long-running leak can push costs far higher. A toilet tank leaking at bolts is a cheap fix when caught early, so do not let it drip for weeks.

When to Call a Plumber

Call a licensed plumber if the stop valve will not shut off, if the supply pipe in the wall is corroded, if the tank or bowl shows a crack, or if water has soaked the floor long enough to soften the subfloor. Soft, spongy flooring around the toilet or stains on the ceiling below suggest a long-term leak that may need subfloor repair along with the plumbing. A pro is also worth the call if your toilet is a one-piece model with no tank bolts, since those leaks involve different components.