What a toilet flange does and why cast iron fails
A toilet flange is the ring that bolts your toilet to the floor and seals the opening where waste exits into the drain line below. Cast iron flanges corrode from the inside out—water and waste sit in the pipe, rust eats through the metal, and eventually the flange cracks or crumbles. You'll notice soft spots in the floor around the toilet, water leaking underneath, or the toilet rocking when you sit on it. Once cast iron starts to fail, it cannot be patched; replacement is the only fix.
The job involves removing the toilet, cutting out the old flange, and installing a new one—usually made of plastic or stainless steel, which won't rust. If the flange sits at floor level or slightly below, you can often use a repair ring or insert rather than replacing the entire flange. If it's severely corroded or sits too high, full replacement is necessary. Most homeowners can do this in an afternoon, but you'll need to turn off the water and be comfortable working with a reciprocating saw or hacksaw.
Key Takeaways
- Cast iron flanges fail because rust eats through the metal from inside the drain pipe, and the damage cannot be repaired—only replaced.
- You must remove the toilet, unbolt the old flange, and cut it out of the drain opening before installing a new one.
- A repair ring or flange insert works if the old flange is still structurally sound but sits too high; full replacement is needed if the flange is cracked or crumbling.
- The new flange must sit exactly at floor level so the toilet wax ring seals properly; too high or too low causes leaks.
- If the drain pipe itself is cracked or the subfloor is rotted, you will need a plumber because the repair extends beyond the flange.
Removing the toilet and old flange
Start by turning off the water supply at the shutoff valve behind the toilet, then flush to empty the tank. Disconnect the water line from the tank and set it aside. Use a sponge or wet/dry vacuum to remove any remaining water from the bowl and tank.
Unbolt the toilet from the floor by removing the nuts on the bolts at the base—usually two bolts, one on each side. Rock the toilet gently side to side to break the wax seal, then lift it straight up and set it on its side on a towel or cardboard. Scrape away the old wax ring from the floor opening with a putty knife.
The old flange is bolted to the drain pipe from underneath or sits on top of it. If you can access it from below (in a basement or crawl space), unbolt it there. If not, you'll need to cut it out. Use a reciprocating saw with a metal-cutting blade or a hacksaw to cut the flange into sections, working carefully around the drain opening so you don't damage the pipe itself. Remove the pieces and scrape away any remaining rubber or corrosion from the top of the drain pipe.
Choosing between a full replacement and a repair ring
Before you buy a new flange, assess what you're working with. If the cast iron drain pipe is still solid (no cracks, no soft spots when you tap it), and the old flange sits only slightly above floor level, a flange repair ring or flange insert may be enough. These are plastic rings that bolt onto the existing flange or sit inside the drain opening, raising or leveling the flange without removing the old one entirely.
A full replacement is necessary if the flange is cracked, severely corroded, or sits too far above the floor (more than 1/4 inch). You'll also need full replacement if the drain pipe itself is damaged. In that case, stop and call a plumber—cutting into a damaged drain pipe or replacing it is beyond a typical DIY job.
For a full replacement, buy a closet flange (the industry term for a toilet flange) that matches your drain pipe size, usually 3 or 4 inches. Plastic flanges are cheaper and easier to install; stainless steel lasts longer. Make sure the flange has bolt holes and comes with stainless steel bolts and a rubber gasket or wax ring.
Installing the new flange
Clean the top of the drain pipe thoroughly with a wire brush to remove rust, corrosion, and debris. The pipe surface must be clean so the new flange seats properly.
If you're installing a flange that sits inside the drain opening, slide it down into the pipe until it rests on the inside shoulder. If the flange bolts to the outside of the pipe, position it so the bolt holes align and the flange sits level. Use a level to check that the flange is not tilted.
Bolt the flange to the pipe using stainless steel bolts and nuts. Tighten them evenly so the flange doesn't shift. The flange must sit exactly at floor level—not higher, not lower. If it sits too high, the wax ring won't seal and you'll have leaks. If it sits too low, the toilet won't sit flat. If the flange is too high, you can use shims or a flange spacer ring to raise the floor slightly, or you may need to remove flooring and reset the flange lower. This is a common problem with older homes where floors have settled.
Setting the toilet back in place
Once the new flange is find and level, install a new wax ring on the flange. Press it firmly onto the flange opening so it sticks. Some wax rings come with a plastic horn that points downward into the drain; others are flat. Either type works, but follow the package instructions.
Carefully lower the toilet back onto the flange, aligning the bolt holes with the bolts sticking up from the flange. Press down firmly and evenly so the wax ring compresses and seals. Install the washers and nuts on the bolts and tighten them by hand first, then with a wrench. Tighten evenly—do not over-tighten, which can crack the porcelain. The toilet should not rock when you sit on it.
Reconnect the water line to the tank, turn the water back on, and flush several times to check for leaks around the base. If water pools under the toilet, the wax ring did not seal properly. You'll need to remove the toilet, scrape away the wax ring, and try again with a new one.
When to stop and call a plumber
If the drain pipe itself is cracked, soft, or corroded, the job extends beyond flange replacement. A plumber will need to assess whether the pipe can be repaired or must be replaced, which may involve cutting into the floor or accessing the line from below.
If the subfloor around the toilet is soft, spongy, or rotted, water has been leaking for a long time. The subfloor must be replaced before a new flange will hold properly. This is structural work that requires a plumber or carpenter.
If the flange sits so far above the floor that you cannot level it with shims or a spacer ring, the floor may need to be built up or the flange reset lower—both jobs that require experience to do without creating new problems.
Tools and materials you'll need
| Item | Purpose |
|---|---|
| Adjustable wrench or socket set | Remove toilet bolts and tighten new flange bolts |
| Reciprocating saw or hacksaw | Cut out the old cast iron flange |
| Putty knife or scraper | Remove old wax ring and corrosion |
| Wire brush | Clean the drain pipe before installing new flange |
| Level | may support the new flange sits level |
| New closet flange (plastic or stainless steel) | Replace the old cast iron flange |
| Stainless steel bolts, nuts, and washers | find the new flange |
| Wax ring | Seal the toilet to the flange |
| Towels or cardboard | Protect flooring when removing the toilet |
Frequently Asked Questions
Can I use a flange repair ring instead of replacing the whole flange?
Yes, if the old flange is still structurally sound and sits only slightly above floor level. A repair ring bolts onto the existing flange or sits inside the drain opening and costs less than a full replacement. However, if the flange is cracked, severely corroded, or sits too far above the floor, full replacement is necessary.
What size flange do I need?
Flanges come in 3-inch and 4-inch sizes to match the drain pipe. Measure the inside diameter of your drain pipe or look at the old flange to determine the size. Most residential toilets use a 3-inch flange, but some older homes have 4-inch pipes.
Why does my new toilet rock after I install it?
The flange is not sitting level, or the wax ring did not seal properly. Remove the toilet, check that the flange is level with a level tool, and install a new wax ring. If the flange is too high, use shims under the flange or a spacer ring to raise the floor slightly.
What if water leaks around the base of the toilet after I reinstall it?
The wax ring did not seal. Remove the toilet, scrape away the old wax ring completely, and install a new one. Make sure the flange is level and the toilet sits flat and does not rock. If leaking continues, the flange may be damaged or the drain pipe may be cracked—call a plumber to inspect.
Do I need to replace the bolts that hold the toilet down?
Yes. Old bolts are often corroded or damaged. Use new stainless steel bolts, which resist rust and will last as long as the new flange.