What a toilet flange is and why it fails on concrete

A toilet flange is the ring that bolts your toilet to the floor and seals the waste pipe opening. On concrete, the flange sits in a recessed area cut into the slab, and it's held down by bolts that screw into the concrete or sit in slots cast into the floor. When the flange cracks, rusts, or the bolts loosen, water leaks underneath the toilet and into the concrete—a problem that gets worse fast because you can't see it happening.

Concrete flanges fail more often than wood-frame flanges because concrete doesn't flex, so any movement in the toilet or settling of the slab puts all the stress on the flange itself. Rust is also common in older homes where the flange was cast iron. Once a flange leaks, the concrete underneath absorbs water and can soften, which makes the toilet rock and speeds up the leak. Replacing it stops the damage and prevents mold and structural rot.

Key Takeaways

  • You will need to remove the toilet completely, which means shutting off the water, draining the tank, and unbolting the flange bolts from underneath the floor.
  • The old flange is either bolted to concrete anchors, glued with wax or plumber's putty, or cast into the concrete itself—the removal method depends on which type you have.
  • A new flange must sit at the exact height of the finished floor surface so the wax ring seals properly; too high or too low causes leaks.
  • Most homeowners can replace a flange on concrete, but if the concrete is cracked, the waste pipe is damaged, or the flange is cast into the slab, you should call a plumber.

Shut off water and remove the toilet

Turn off the water supply valve behind the toilet—it's usually a small knob on the wall or floor. Turn it clockwise until it stops. Flush the toilet to empty the tank and bowl, then use a sponge or wet/dry vacuum to remove any remaining water from the bowl.

Unscrew the two bolts that hold the toilet to the flange. These bolts come up through the floor on either side of the toilet base. You may need a wrench or socket set; the bolts are usually 7/16 inch. Once the bolts are loose, rock the toilet gently side to side and lift it straight up. It will be heavy—ask someone to help. Set it on a towel or cardboard to protect your floor and to catch any water still in the trap.

Scrape away the old wax ring from the flange opening with a putty knife. Get it as clean as you can; dried wax left behind will prevent a good seal with the new ring.

Identify and remove the old flange

Look at how the old flange is attached. If you see bolts coming up through the concrete on either side, the flange is bolted to concrete anchors—this is the easiest type to replace. If the flange appears to be glued down with no visible bolts underneath, it's held with wax or plumber's putty. If the flange is broken or you see concrete around it that looks like it was poured at the same time, the flange may be cast into the slab, which is much harder to remove.

For a bolted flange, unscrew the bolts from underneath using a wrench. Once the bolts are out, the flange should lift away. If it's stuck, tap it gently with a rubber mallet from underneath or use a pry bar to work it loose. For a glued flange, use a pry bar and hammer to break the seal, working around the edge until it comes free. This takes patience and muscle.

If the flange is cast into the concrete or if you see cracks in the concrete around it, stop here and call a plumber. Removing a cast flange usually means breaking concrete and may require cutting into the waste pipe itself.

Measure the flange height and prepare the opening

The new flange must sit exactly at the height of your finished floor—not below it and not above it. Measure from the top of the concrete to the floor surface. If you have tile, vinyl, or carpet, measure to the top of that material. The flange should sit flush with this height.

If the concrete has settled or the old flange was too low, you may need to build up the opening with a concrete shim or epoxy filler. Some plumbers use a rubber or plastic flange extension ring that sits on top of the concrete and raises the flange height. Check your new flange's instructions—many modern flanges come with adjustable height options.

Clean the opening thoroughly. Remove any old wax, putty, rust, or debris. If the concrete is cracked or crumbling around the opening, chip away the loose material and fill it with concrete patching compound. Let it cure before installing the new flange.

Install the new flange

Most new flanges for concrete are bolted down with lag bolts or concrete screws that you drive directly into the concrete. Place the flange over the waste pipe opening so the pipe goes through the center hole. The flange should sit flat and level on the concrete.

Mark the bolt holes with a pencil, then remove the flange. Drill pilot holes at each mark using a concrete drill bit—usually 3/16 inch for lag bolts. Drill straight down and about 2 inches deep. Blow out the dust with a shop vacuum or compressed air.

Set the flange back in place and insert the lag bolts or concrete screws through the flange holes into the pilot holes. Tighten them firmly with a wrench or socket, but do not over-tighten—you want the flange snug, not crushed into the concrete. The flange should not rock or move when you push on it.

Some flanges come with a rubber gasket or wax ring already attached. If yours doesn't, you'll install a new wax ring after the flange is bolted down, just before you set the toilet back.

Reinstall the toilet and test for leaks

If your new flange doesn't have a wax ring attached, press a new wax ring onto the flange opening. The wax should form a seal all the way around. Carefully lower the toilet back onto the flange, aligning the bolt holes. Push down firmly and evenly so the wax compresses and seals.

Insert the bolts through the toilet base holes and thread them into the flange. Hand-tighten first, then use a wrench to snug them down. Tighten a little on one side, then a little on the other, so the toilet sits level. Do not over-tighten—you can crack the porcelain.

Turn the water supply back on and let the tank fill. Flush a few times and watch underneath the toilet for drips. Check around the base where the toilet meets the floor. If you see water, turn off the water, remove the toilet, and check that the wax ring is seated properly. You may need to reset it.

Once the toilet is find and not leaking, caulk around the base with silicone caulk to prevent water from running underneath. Use a caulk gun and smooth it with a wet finger. This is not a seal—it's a barrier to keep spills from getting under the toilet.

When to call a plumber instead

If the concrete around the flange is cracked, crumbling, or soft, the slab may have water damage underneath. A plumber can assess whether the concrete needs repair before a new flange is installed. If the waste pipe itself is cracked or corroded, that's a bigger job that may involve cutting into the concrete to access the pipe.

If the flange is cast into the concrete (poured as part of the original slab), removing it usually means breaking concrete with a jackhammer and possibly cutting the waste pipe. This is not a DIY job. Call a plumber to evaluate whether the flange can be replaced or if the concrete needs to be cut and patched.

If you're not comfortable working under the toilet or drilling into concrete, or if the bolts won't budge, a plumber can do this work in an hour or two. The cost is usually between $150 and $300 for labor, plus the cost of the flange itself.

Frequently Asked Questions

What's the difference between a wax ring and a rubber gasket?

A wax ring is traditional and works well on level concrete. A rubber gasket is reusable and works on slightly uneven surfaces because it compresses more. Both seal the gap between the toilet and flange. Rubber gaskets are becoming more common because they're easier to remove if you need to service the toilet again.

Can I use a flange extension if the concrete is too low?

Yes. A flange extension ring sits on top of the concrete and raises the flange height by half an inch to an inch. It bolts to the existing flange or concrete. This is a good solution if the concrete has settled but is otherwise solid.

How do I know if the flange is level?

Use a small level placed across the flange opening. It should read level in both directions. If the concrete is uneven, you may need to shim the flange with metal shims before tightening the bolts, or use a rubber gasket instead of a wax ring to absorb the unevenness.

What if water leaks from under the toilet after I install it?

Turn off the water and remove the toilet. Check that the wax ring is fully seated and hasn't torn. If the flange is rocking, the bolts may be loose or the concrete may be uneven. Tighten the bolts evenly and reinstall with a new wax ring. If leaking continues, the flange may be cracked or the concrete may be damaged underneath.

Do I need to replace the bolts when I replace the flange?

If the old bolts are rusty, bent, or corroded, yes. New stainless steel bolts cost a few dollars and prevent future rust. Most new flanges come with bolts, but check before you buy.