Toilet Leaking at the Base? 5 Causes and How to Fix Them

Before you start

Stop the water and get the floor dry first. New water on a dry floor is what leads you to the source.

Fargo's water department warns that contact with sewage is a serious health risk.⁠[4] If the water smells like sewage or looks dirty, wear gloves and keep children and pets away. Water that comes up out of a drain, rather than leaking from the toilet, is a clog; see clear a clogged drain.

Find where the water starts

Dry the tank, the bolts, the supply line and the floor with towels. Then flush once and watch with a flashlight. The first wet spot is the one that counts.

1. 01

   Condensation dripping off the tank

   CommonDIYTime: 1 day to confirm

   The whole tank is beaded with water in humid weather.
   Fix it: Condensation dripping off the tank →
2. 02

   Leak at the supply line or fill valve nut

   CommonDIYTime: 15 min

   Drips start at the line or the nut under the tank's left side.
   Fix it: Leak at the supply line or fill valve nut →
3. 03

   Worn tank-to-bowl gasket or bolt washers

   CommonDIY with careTime: 1–2 hr

   Water runs down the back of the bowl from under the tank.
   Fix it: Worn tank-to-bowl gasket or bolt washers →
4. 04

   Failed wax ring or flange

   CommonPro only

   Water seeps out from under the bowl while everything above stays dry.
   What a pro does: Failed wax ring or flange →
5. 05

   Cracked bowl or tank

   Less commonPro only

   A hairline crack weeps water whether you flush or not.
   What a pro does: Cracked bowl or tank →

Water from the lid or around the tank lever is a separate case. Fluidmaster traces it to a flush valve that is the wrong height or blocks water from draining into the bowl.⁠[6]

Condensation on the tank

Fargo's water department names a sweaty tank in humid weather as one cause of water on the floor around a toilet.⁠[1] Oregon State University Extension explains why: when warm, moist air cools, the extra moisture condenses, usually on the first cold surface it meets.⁠[7] A tank full of cold supply water is that surface.

1. 01

   Step 01 of 03

   Look at the whole tank

   • Time: 2 min
   • LED work flashlight

   Run a dry hand over the front, sides and back of the tank. Note whether it is damp all over or wet in one place.

   You'll know it worked when: You know whether water beads cover the tank or come from one spot.

2. 02

   Step 02 of 03

   Dry it and watch

   • Time: 1 day
   • Microfiber towels

   Wipe the tank and floor dry and check again through the day, including after a shower. Water that returns while every fitting stays dry points to condensation.

   You'll know it worked when: You know whether the water comes back with dry fittings.

3. 03

   Step 03 of 03

   Run the bath fan longer

   • Time: 5 min

   OSU Extension calls exhaust fans the first fix for moist bathrooms. It suggests a timer that runs the fan for 15 to 30 minutes, vented straight outside.⁠[7]

   You'll know it worked when: Less water forms on the tank after showers.

If the tank is dry but the fill valve keeps turning on by itself, the problem is inside the tank. See a toilet that runs on its own.

Still not fixed? Next likely cause: Leak at the supply line or fill valve nut →

Leak at the supply line or fill valve nut

The supply line runs from the shutoff valve to the bottom of the fill valve, where a nut and a rubber washer seal it to the tank. Fluidmaster says to check that the fill valve locknut under the tank is snug, and to look for leaks along the supply line.⁠[5] Oatey adds that if the fill valve gasket cracks or the nut loosens, water can leak onto the floor below the tank.⁠[2]

1. 01

   Step 01 of 03

   Dry the line and run a finger along it

   • Time: 5 min
   • Microfiber towels
   • LED work flashlight

   Check both ends of the line and the shutoff valve stem.

   You'll know it worked when: You find the wet spot: the valve, the line or the nut under the tank.

2. 02

   Step 02 of 03

   Hand-tighten the nut under the tank

   • Time: 2 min

   Oatey hand-tightens the nut that secures the fill valve to the tank and warns against overtightening.⁠[2] Korky also tightens the supply connection by hand, with no sealant or tape.⁠[3]

   You'll know it worked when: The connection under the tank stays dry.

3. 03

   Step 03 of 03

   Fit a new supply line if it is old or cracked

   • Time: 20 min
   • Adjustable wrench
   • 5-gallon bucket

   Close the shutoff valve and flush to empty the tank first. Fluidmaster says to replace a supply line older than 5 years.⁠[5] Oatey's steps: disconnect the old line from the shutoff valve and the fill valve with a wrench, fit the new line and tighten it by hand, then turn the water on and check for leaks.⁠[2]

   You'll know it worked when: A new line is in and both ends are dry after several flushes.

Oatey suggests replacing the supply line every 3 to 5 years or whenever the toilet is replaced.⁠[2]

Still not fixed? Next likely cause: Worn tank-to-bowl gasket or bolt washers →

Leak at the tank bolts

On a two-piece toilet, a gasket seals the tank to the bowl, and two or three bolts with rubber washers hold it down. Fluidmaster says water running down the back of the base points to failed tank-to-bowl seals, and the fix is a new gasket and new bolts with washers.⁠[5]

1. 01

   Step 01 of 03

   Confirm the bolts are wet

   • Time: 5 min
   • LED work flashlight
   • Microfiber towels

   Dry the underside of the tank, flush, and look at each bolt and the joint between tank and bowl.

   You'll know it worked when: Water forms at a bolt head or under the tank after a flush.

2. 02

   Step 02 of 03

   Snug the bolts a little at a time

   • Time: 10 min
   • Adjustable wrench

   Korky says tightening may fix the leak, but not to overtighten or let the tank touch the bowl, because the porcelain can crack.⁠[3]

   You'll know it worked when: The drip stops and the tank does not touch the bowl.

3. 03

   Step 03 of 03

   Swap the gasket and hardware if it still drips

   • Time: 1–2 hr

   Korky's method: drain the tank, take it off and set it on the closed seat, fit the new gasket and bolts, then tighten the nuts alternating sides while a partner holds the tank level.⁠[3] The full steps are in reseal the tank to the bowl.

   You'll know it worked when: A new gasket and bolts are in and the joint stays dry.

Korky says the gasket wears from the weight of the tank and from people leaning back on it, so a tank that rocks is a warning sign.⁠[3]

Still not fixed? Next likely cause: Failed wax ring or flange →

Failed wax ring or flange

If water seeps from under the bowl while the tank, bolts and supply line all stay dry, the seal between the toilet and the floor drain is the likely leak. Oatey says leaks around the base usually mean the wax ring or the closet flange is failing.⁠[2] Rocking the toilet during installation is a common reason: wax has no memory, so it does not reseal evenly.⁠[2]

1. 01

   Step 01 of 04

   Stop using the toilet

   • Time: 1 min

   Oatey says water escaping a failed seal leaks onto the floor and can damage the structure below.⁠[2]

   You'll know it worked when: No more water reaches the seal.

2. 02

   Step 02 of 04

   Close the water and empty the tank

   • Time: 2 min

   Turn the valve behind the toilet clockwise, then flush once.

   You'll know it worked when: The tank is empty and cannot refill.

3. 03

   Step 03 of 04

   Check whether the bowl rocks

   • Time: 2 min

   Korky says a wobbling base can mean a problem with the floor, the closet flange or the wax ring.⁠[8]

   You'll know it worked when: You can tell the plumber whether the toilet moves.

4. 04

   Step 04 of 04

   Book a plumber

   • Time: 5 min

   Oatey rates flange and wax ring work as expert-level and recommends a professional plumber.⁠[2]

   You'll know it worked when: A plumber is booked to reset the toilet on a new seal.

Korky adds that a cracked or damaged flange must be replaced, and it does not recommend sealing over a crack.⁠[8] More on this repair is in how to fix a toilet wax ring.

Still not fixed? Next likely cause: Cracked bowl or tank →

Cracked bowl or tank

Fargo's water department lists a cracked bowl among the causes of water on the floor around a toilet.⁠[1] A hairline crack can be hard to see and may only weep when the toilet is full.

1. 01

   Step 01 of 03

   Inspect the porcelain with a light

   • Time: 5 min
   • LED work flashlight

   Look closely at the base of the bowl, the foot and the tank, especially near bolts.

   You'll know it worked when: You find or rule out a crack.

2. 02

   Step 02 of 03

   Close the water if you find a crack

   • Time: 1 min

   Turn the valve behind the toilet clockwise and flush to empty the tank.

   You'll know it worked when: The cracked part cannot refill.

3. 03

   Step 03 of 03

   Call a plumber

   • Time: 5 min

   Fluidmaster lists cracks or chips in the tank among the leak points of a toilet.⁠[9] None of the manufacturer guides used here patches cracked porcelain. Kohler's pressure-assist leak chart, for example, ends with a new bowl when the bowl is warped or not level.⁠[10]

   You'll know it worked when: A plumber is booked to assess the toilet.

Go easy on bolts during any toilet repair. Korky warns that overtightening the bolts can cause the porcelain to crack.⁠[8]

Still not fixed? See when to call a pro →

When to call a pro

Stop and call a plumber when the leak is under the bowl, in the porcelain, or past the shutoff valve.

Tell the plumber where the first water showed up and whether the toilet moves. It helps them bring a wax ring, a flange repair kit or a new supply line.

Frequently asked questions