Begin by preparing the work area. Turn off the water supply to the toilet using the shut-off valve on the wall behind it. Flush the toilet to empty the tank and bowl. Use a sponge and bucket to remove any remaining water from both the tank and the bowl. Disconnect the water supply line from the toilet tank. Finally, use a putty knife to pry off the bolt caps at the base of the toilet, and use an adjustable wrench to remove the nuts from the closet bolts. Rock the toilet gently to break the wax seal, then lift it straight up and set it aside on old towels or cardboard.

With the toilet removed, use a putty knife to scrape all the old wax from the top of the cast iron flange. Use a shop vac to clean up the wax and any other debris. Thoroughly inspect the flange and the point where it connects to the pipe. Confirm that it is a lead and oakum joint inside the pipe hub and assess the extent of the damage to the flange ring. This is the final check before beginning demolition.

This step requires force and caution. Put on your safety glasses and heavy-duty gloves. Place the tip of the cold chisel on the top ring of the cast iron flange, angled away from the center drain pipe. Strike the end of the chisel firmly with a hammer or small sledgehammer. Reposition the chisel and repeat, working your way around the flange to create score lines. The brittle cast iron will eventually crack and break apart. Use a pry bar to lift and remove the broken pieces. Be extremely careful not to hit or damage the main cast iron drain pipe.

With the flange ring removed, you must now remove the old seal. You will see a ring of soft, dull gray lead inside the pipe hub. Carefully drill a series of small, shallow holes into this lead ring, being cautious not to drill too deep and hit the cast iron pipe wall. Once the lead is perforated, use a flathead screwdriver or the cold chisel to pry up a section of the lead ring. Use pliers to grab the lead and peel it out of the joint. Below the lead, you'll find a tarred, rope-like material called oakum. Use needle-nose pliers or a screwdriver to pull all of the oakum out of the joint. Vacuum all debris from the area.

Once all the lead and oakum are removed, the inside surface of the cast iron pipe hub must be thoroughly cleaned. Use a wire brush—either a handheld version or a drill attachment—to scrub the inside of the hub. The goal is to remove any remaining residue, corrosion, or debris to create a smooth, clean surface for the new flange's rubber gasket to seal against. Use the shop vac to remove all the dust and particles created by the brushing.

Before final installation, perform a dry fit. Take the new PVC compression flange and insert it into the cleaned cast iron pipe hub. Push it down until the flange base rests on the finished floor. Use a level to check that it sits flat. Confirm that the top surface of the flange is level with or slightly above the finished flooring. If the pipe is too low, you may need a flange extender kit. If it's too high, the project becomes much more complex. This dry fit ensures you won't have height issues after the final installation.

Remove the flange from the dry fit. Ensure the rubber gasket is properly seated on the flange. Insert the flange back into the cast iron pipe, pressing it down firmly until it rests on the floor. Use a wrench or socket set to begin tightening the compression bolts on top of the flange. Alternate between the bolts, tightening each one a little at a time, similar to tightening lug nuts on a wheel. This ensures the rubber gasket expands evenly against the inside of the cast iron pipe, creating a strong, watertight seal. Tighten until snug and the flange feels secure. Do not overtighten.

With the flange sealed to the pipe, it must now be anchored to the subfloor. Position the flange so the closet bolt slots are parallel to the back wall. Use a drill with a small bit to drill pilot holes into the wood subfloor through the screw holes in the flange ring. Then, drive corrosion-resistant screws (stainless steel is best) through the flange and into the subfloor until the flange is held down firmly. This prevents the toilet from rocking and breaking the wax seal over time.

Remove the rag from the drain pipe. Insert new closet bolts into the slots on the flange, with the long threaded end pointing up. Place a new wax ring directly on the flange, centered over the opening, or onto the horn on the underside of the toilet. Carefully lift the toilet, align the holes in its base with the closet bolts, and lower it straight down onto the flange. Press down firmly with a slight twisting motion to compress and seat the wax ring. Place the washers and nuts onto the closet bolts and tighten them, alternating from side to side, until the toilet is stable and does not rock. Do not overtighten.

Reconnect the water supply line to the bottom of the toilet tank, hand-tightening first and then giving it a final quarter-turn with a wrench. Turn the water shut-off valve back on slowly and check for leaks at the connection point. Allow the tank to fill completely. Flush the toilet several times. Use a flashlight and paper towels to carefully inspect the base of the toilet all the way around for any signs of water seepage. If it remains dry after several flushes, the installation is successful. You can then use a hacksaw to cut off the excess length of the closet bolts and snap on the decorative caps.