Replacing a toilet flange in concrete is harder than replacing one in wood, but you can do it yourself if the flange sits above the slab

A toilet flange is the ring that bolts the toilet to the floor and seals the drain opening. In concrete, the flange can crack, corrode, or fail at the seal. If yours is leaking or cracked, you have two paths: remove and reset the old flange, or cut it out and install a new one. The concrete itself rarely needs replacement — the flange does.

The critical first step is determining whether your flange sits above the concrete surface or is embedded in it. If it sits above (the most common case in older homes), you can unbolt the toilet, remove the flange, and install a new one in an hour or two. If the flange is cast into the concrete, the job becomes much harder and may require a concrete saw or professional help.

Key Takeaways

  • Check whether your flange sits above the concrete or is embedded in it before you start — embedded flanges require concrete cutting and are not a DIY job for most people.
  • You will need a new flange (usually PVC or cast iron, matching your existing drain pipe material), a wax ring, bolts, and basic tools including an adjustable wrench and a hacksaw or reciprocating saw.
  • The toilet must be removed and the old flange cut away or unbolted; the concrete around it may need chiseling if the flange is stuck or corroded.
  • The new flange must sit at the correct height — the top should be level with the finished floor so the wax ring seals properly.
  • If the flange is cast into concrete more than a quarter-inch below the surface, hire a plumber or concrete contractor rather than attempting to cut it out yourself.

Determine whether the flange is removable or embedded

Look at the base of your toilet where it meets the floor. If you can see bolts holding the flange to the concrete, and the flange ring sits visibly above the surface, it is removable. If the flange appears to be part of the concrete itself or sits flush with it, it is likely embedded and will require concrete removal.

Run your finger around the edge where the flange meets the concrete. If there is a gap or a visible seam, the flange was set on top. If the concrete appears to flow around the flange with no clear edge, it was cast in place. Embedded flanges are common in homes built before the 1980s or in slabs poured without a rough-in plumber's input.

Gather tools and materials before you start

You will need a new flange (PVC, cast iron, or ABS — match your drain pipe material), a wax ring, new closet bolts (the bolts that hold the toilet down), an adjustable wrench, a hacksaw or reciprocating saw with a metal-cutting blade, a cold chisel, a hammer, a bucket, a sponge, and rags. If the old flange is corroded or stuck, penetrating oil (like WD-40) helps loosen it.

Before you buy the new flange, measure your drain opening. Standard flanges fit 3-inch or 4-inch drains — measure the inside diameter of the pipe. Bring a photo of your old flange to the hardware store if you are unsure. The flange should match the pipe material: PVC flanges for PVC pipes, cast iron for cast iron, and so on.

Remove the toilet and old flange

Turn off the water supply at the shutoff valve behind the toilet. Flush to empty the tank, then use a sponge to soak up any remaining water in the bowl and tank. Disconnect the water line from the tank base using an adjustable wrench.

Unbolt the toilet from the floor using a wrench on the nuts under the base. You may need to hold the bolt head from underneath with a second wrench. Once the bolts are loose, rock the toilet gently side to side and lift it straight up. Set it on its side on a towel or cardboard — do not set it upright on a hard surface, as the rim can crack.

Once the toilet is out, you can see the flange clearly. If it is bolted to the concrete (bolts visible on top), unbolt it using a wrench. If it is corroded or stuck, spray penetrating oil around the base and wait 15 minutes. Use a cold chisel and hammer to tap around the edge and break the seal, then unbolt it.

If the flange is embedded in concrete, use a reciprocating saw with a metal-cutting blade to cut through the flange ring itself, breaking it into pieces you can remove. Work slowly — metal-cutting blades dull quickly and concrete will damage them. Once the flange is cut, use a cold chisel to pry out the pieces. This step is slow and dusty; wear a dust mask and eye protection.

Clean the concrete surface and check the drain opening

Once the old flange is out, scrape away any old wax, putty, or debris from the concrete surface using a cold chisel and hammer. The surface should be clean and flat. If the concrete is cracked or uneven around the opening, use a concrete grinder or chisel to smooth it — the new flange needs a level seat.

Look into the drain opening. If there is a rubber or plastic sleeve inside the pipe, leave it alone — it is part of the drain system. If there is debris, use a wet/dry vacuum to clear it out. Do not pour water down the drain; you want to keep the interior dry so the new flange seals properly.

Install the new flange at the correct height

The new flange must sit at the right height for the wax ring to seal. The top of the flange should be level with the finished floor surface — not recessed below it, and not sticking up more than a quarter-inch above it. If your concrete is uneven, you may need to shim the flange with stainless steel shims or epoxy to bring it to the correct height.

Set the new flange into the drain opening and check the fit. It should sit snugly without rocking. If the opening is slightly too large, you can use plumber's putty or epoxy around the base to seal gaps, but the flange should not rely on putty alone to stay in place. If the opening is too small, do not force the flange — you may crack the concrete or the flange itself.

Once the flange is positioned correctly, bolt it down using the bolts provided. Tighten them evenly in a cross pattern (like tightening a car wheel) so the flange sits flat. Do not over-tighten — you only need them snug enough that the flange does not move.

Reinstall the toilet and test the seal

Place a new wax ring on top of the flange. The wax ring is a soft, sticky ring that seals the gap between the flange and the toilet base. Press it firmly onto the flange so it does not shift when you lower the toilet.

Lower the toilet straight down onto the flange, aligning the bolt holes with the bolts sticking up through the base. Press down firmly and evenly — you should feel the wax ring compress. Do not rock the toilet side to side once it is in contact with the wax ring, as this can break the seal.

Bolt the toilet down using new closet bolts and washers. Tighten the nuts evenly and snugly, but do not over-tighten — porcelain cracks easily. The toilet should not rock when you sit on it. Reconnect the water line, turn the water back on, and let the tank fill. Flush twice and watch for leaks around the base. If water pools around the toilet, the wax ring did not seal — you will need to remove the toilet and try again with a new wax ring.

When to call a professional instead

If the flange is embedded more than a quarter-inch below the concrete surface, or if cutting it out creates a hole larger than the new flange can cover, hire a plumber or concrete contractor. Removing embedded flanges often requires a concrete saw, which is expensive to rent and dangerous to operate without training. A professional can also assess whether the drain pipe itself is damaged — if it is cracked or misaligned, the flange replacement alone will not stop the leak.

If you cut into the flange and find the drain pipe is also corroded or broken, stop and call a plumber. Replacing both the flange and the pipe is a bigger job and may require breaking up more concrete than you should attempt yourself.

Frequently Asked Questions

Can I use the old flange if it is not cracked?

Only if it is not leaking and sits at the correct height. If water is pooling around the toilet base, the old flange is not sealing — replace it. If the flange is corroded or the bolts are rusted, replace it too. A flange that looks fine but leaks will continue to leak with a new wax ring.

What if the new flange is too high or too low?

If it is too low (recessed below the floor), the wax ring will not seal properly and the toilet will leak. Use shims or epoxy to raise it. If it is too high (sticking up more than a quarter-inch), the toilet will rock and the wax ring will not compress evenly. Grind down the flange or the concrete to bring it level with the floor.

Do I need to use plumber's putty around the flange?

No. The wax ring seals the toilet to the flange. Putty is only needed if there are gaps between the flange and the concrete that are large enough to let sewer gas escape — in that case, use it around the base of the flange, not under the toilet.

How long does a new flange last?

A PVC or cast iron flange in concrete typically lasts 20 to 30 years if installed correctly and the wax ring is replaced when the toilet is reset. Corrosion, water damage, and settling concrete can shorten that lifespan. If you live in an area with aggressive soil chemistry or high water tables, inspect the flange every 10 years.

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

The wax ring did not seal. Turn off the water, flush to empty the tank, and remove the toilet again. Scrape off the old wax ring completely, clean the flange and toilet base with a rag, and install a new wax ring. Reinstall the toilet, pressing down firmly and evenly. If it still leaks, the flange may be uneven — check that it is level with the floor.