How To Install a New Toilet Flange on a Concrete Slab


Installing a new toilet flange on a concrete slab is an important part of replacing or installing a new toilet. The toilet flange, also known as a closet flange, is a pipe fitting that connects the toilet to the drain pipe in the floor. It provides a stable mounting surface for securing the toilet to the floor.

Replacing an old, cracked, or corroded toilet flange with a new one allows you to install the toilet securely and prevent leaks. Installing a toilet flange on a concrete slab requires careful preparation, but can be done by a handy DIYer with proper planning and the right tools.

In this comprehensive guide, we will provide detailed step-by-step instructions on how to install a new toilet flange on a concrete slab.

Step 1: Turn Off Water Supply and Remove Old Toilet

The first step is to turn off the water supply to the toilet. Locate the shutoff valve behind or beside the toilet and turn it clockwise to shut off the water. Flush the toilet to empty the tank and bowl. Use a sponge to soak up any remaining water in the bowl.

Unbolt the old toilet from the floor using a wrench or pliers to loosen the bolts. Carefully lift the toilet straight up and set it aside. Scrape away any old wax, putty, or caulk from the floor flange with a putty knife. Remove all toilet mounting bolts from the flange.

Also disconnect and remove any flexible water supply lines from the toilet. You will install new toilet supply lines later. Cover the open drain pipe temporarily with a damp rag to prevent sewer gases from escaping.

Step 2: Remove Old Flange

Now you can focus on removing the existing toilet flange from the concrete floor. First, use a utility knife to scrape away any remaining wax or putty around the old flange. Wiggle the flange back and forth to break the seal.

You may need to use a hammer and chisel to carefully crack the old flange free from the concrete. Tap gently on the sides of the flange to loosen it. Take care not to damage the surrounding concrete.

Once loosened, the old flange should lift out easily. Use pliers to remove any remaining bolts or screws. Thoroughly clean the area with a wire brush and shop vacuum to remove all residue and debris.

Step 3: Size the New Flange

There are two main types of toilet flanges – standard and offset/extra-thick. Standard flanges sit level with the finished floor and are used with tile or floors without subflooring. Offset flanges raise the toilet connection up to account for new flooring layers.

Determine if you need a standard or offset flange for your toilet installation. Measure the distance from the top of the drain pipe to the surface of the concrete floor. If it is 1/4″ or less, you can use a standard flange. For 1/2″ or thicker finished floors, use an offset flange.

Purchase an appropriately sized 3″ or 4″ inside diameter ABS or PVC toilet flange for your drain pipe size. It should fit snugly into the waste line without any gaps. Measure the bolt circle diameter to ensure your new flange matches the old one.

Step 4: Dry Fit the New Flange

With the old flange removed, you can test fit your new toilet flange. Place the new flange over the drain opening in the concrete floor. Use a pencil to trace a circle along the edge of the flange.

This circle marks the rough area that needs to be cleared for installation. Next, flip the flange over and place it upside down over the drain pipe. Use a marker to trace the outline of the drain onto the new flange.

Set both the flange and drain pipe concrete section aside. Use a chisel and hammer to carefully chip away just enough concrete to create a recess for the new flange to sit flush. Be sure to chip outside your marked circle.

Test fit the new flange into the recessed concrete. Check that it sits level and fits tightly over the drain pipe. If needed, chip away additional concrete evenly until the new flange fits properly. Remove the flange and sweep away all concrete dust.

Step 5: Apply Plumber’s Putty

To help create a watertight seal between the new flange and concrete, you need to apply plumber’s putty. Roll some putty between your hands into a long snake shape.

Run a bead of putty around the bottom of the new flange, about 1/2″ from the outer edge. Then press the flange back into place in the recessed concrete with the putty underneath.

The plumber’s putty will help fill any small gaps between the bottom of the flange and the concrete. Wipe away any excess putty squeezed out around the outer edge with a paper towel or rag.

Step 6: Install Flange Bolts

Your new toilet flange should come with metal bolts and washers. Insert the bolts up through the bolt holes in the flange. Place a washer and nut on the protruding threaded end of each bolt.

Hand tighten the nuts just enough to hold the bolts upright in the flange, but still allow some adjustment. You want the bolt length to match the mounting holes on your new toilet.

Later during toilet installation, you will tighten these flange bolts fully to secure the toilet. For now, just get them started in the flange. Having the bolts pre-installed makes it easier to set the toilet.

Step 7: Seal the Flange

For maximum leak protection, you need to seal the gap between the new flange and the concrete floor. You can use traditional wax toilet bowl rings, silicone adhesive sealant, or a hybrid sealing gasket.

Wax Ring Method

Using wax rings is the most common sealing method. Press a new wax ring over the horn of the flange, molding it into place. The wax ring should fully contact the bottom of the flange to seal it during toilet installation.

Silicone Sealant Method

Caulk around the underside outer edge of the flange with silicone adhesive sealant. Apply a thick, continuous bead and tool it smooth. Allow the caulk to fully cure overnight before installing the toilet.

Hybrid Gasket Method

For superior sealing, use a specialty hybrid gasket instead of regular wax. These provide more flexibility and better adhesion than standard wax. Install as directed by the manufacturer.

Step 8: Set the Toilet and Connect Supplies

Once your new toilet flange is fully installed onto the concrete slab, you are ready to install the replacement toilet:

  1. Turn the toilet upside down and apply your chosen sealing wax, caulk, or gasket method around the toilet horn.
  2. Carefully set the toilet upright over the flange bolts. Press down firmly to set the wax ring and create a tight seal.
  3. Install washers and nuts on the mounting bolts. Alternately tighten the nuts until the toilet is firmly, but not over-tightened, to the floor.
  4. Reconnect the water supply line to the tank fill valve. Turn on the water and test for leaks.
  5. Make any final caulk or sealant application around the base of the toilet.

Frequently Asked Questions About Installing a Toilet Flange on Concrete

Installing your own replacement toilet flange on a concrete slab is doable, but it does take some DIY skills. Here are answers to some frequently asked questions about this toilet repair project:

How do I know if I need a new toilet flange?

Signs that indicate it’s time to replace the flange include rocking or loose toilets, damage or corrosion on the existing flange, leaks between the toilet and flange, and being unable to tighten toilet mounting bolts. Replacing the flange ensures a secure connection.

What tools do I need to install a new toilet flange?

You will need a utility knife, putty knife, hammer, chisel, pliers, pencil, safety glasses, vacuum, rag, plumber’s putty, and a socket wrench. A reciprocating saw to cut bolts may also be useful.

Do I need special flange bolts?

Yes, you should use solid brass or stainless steel bolts at least 3/8” diameter and 2 1⁄2” tall for maximum strength. Avoid zinc bolts. The supplied closet flange bolts are ideal.

How do I cut the drain pipe for the new flange?

Use a reciprocating saw with a metal demolition blade to neatly cut the cast iron or PVC drain pipe. Always wear eye protection when cutting pipe.

What’s the best way to seal the new flange?

Wax rings are standard, but for a more durable seal consider using 100% silicone sealant or a specialty hybrid gasket with built-in wax ring.

How soon can I use the toilet after installing the new flange?

Allow silicone caulk to cure overnight before using the toilet. Other sealing methods only require waiting about one hour for the sealant to set before the toilet can be used.

What if my floor is uneven around the flange?

Use an extra-thick wax ring or a wax-free hybrid gasket which can flex to account for uneven floors. Grinding down any high spots can also help.

Can I install a flange on a wood subfloor?

Yes, use an extra-thick wax ring and longer bolts to span the subfloor and reach down to the flange beneath. Offset flanges also work well on wood subfloors.


Replacing a deteriorating or damaged toilet flange with a new one provides a leak-free, stable connection for the toilet. With proper tools and materials, you can tackle a DIY flange replacement on a concrete slab to save on plumber costs. Carefully remove old flange, prepare the concrete beneath, and install an appropriately sized new flange. Proper sealing and bolt tightening helps ensure a watertight seal. Follow the steps outlined here, work carefully, and you can successfully install a replacement closet flange on your concrete bathroom floor.