How to Install a New Toilet Flange on a Concrete Slab

Toilet room in the modern interior
Artem Ermilov / Getty Images
Overview
  • Working Time: 75 mins
  • Total Time: 75 mins
  • Skill Level: Intermediate
  • Estimated Cost: $5 to $15

When replacing a toilet, it's fairly common to discover that the existing toilet flange will not work for the new toilet. When the subfloor is concrete, as is often the case with basement bathrooms or other slab foundation situations, replacing the flange is no easy matter, since it can be quite hard to drill holes in concrete to secure the new flange.

But using two specialty tools—a multi-tool or similar cutting device, along with a hammer drill—makes it fairly easy to accomplish this task.

Unlike conventional drills that just rotate the bit, hammer drills rotate and also move the bit up and down ever so slightly, which creates a slight pounding action along with the drilling motion. You can purchase or rent an inexpensive hammer drill at a home improvement store or tool rental outlet. If your usage will be used only occasionally, renting a hammer drill is probably your best option. If you expect to drill into masonry more often, purchase a good-quality tool, such as a Bosch, Hilti, DeWalt, or Milwaukee hammer drill.

Because the old bolts will need to be cut off, you'll need some type of power tool to do the job. A manual hacksaw will not work, because you need to move the saw blade against and parallel to the concrete slab. Either an oscillating multi-tool equipped with a metal-cutting blade or an angle grinder will do the trick.

What You'll Need

Equipment / Tools

  • Hammer drill with 1/4-inch masonry bit
  • Multi-tool or angle grinder
  • Putty knife
  • Tape measure
  • Screwdriver

Materials

  • Toilet (closet) flange
  • Latex gloves
  • Trash bag
  • Tapcon screws

Instructions

  1. Cut off the Old Flange and Obstructions

    After you remove the toilet, you will probably find an existing toilet flange. It would be very rare to find a toilet that is attached to the sewer line without a flange. You will need to remove this old flange and any obstructions before you can install the new flange.

    On concrete slabs, the old flange will be attached via bolts embedded in the concrete. If the old bolts are in good condition and perfectly match up with the new flange, you can keep them in place to secure the new flange. But if the bolts are unacceptable (too rusted, mismatched, too short, etc.), they must be cut off before installing the new flange.

    Use an angle grinder or a multi-tool equipped with a metal-cutting blade to hack off the bolts and other protruding obstructions. Make sure that all of these pieces are cut down perfectly flush with the concrete.

    Saw using to cut off the old toilet flange and bolts.
    Lee Wallender
  2. Scrape Off the Old Wax Ring

    After you remove the obstructions, the old wax ring will still be in place around the drain opening. The wax most likely will be thick, gunky, and possibly stained with sewage. Removing the wax ring requires patience, a putty knife, and latex gloves. Methodically remove all traces of the old wax ring and deposit the bits directly in a plastic bag for disposal.

    The old wax ring must be scraped off of the concrete floor.
    Lee Wallender
  3. Place T-Bolts in the New Toilet Flange

    Place the T-bolts upside-down in the new toilet flange so that the threaded shafts are sticking up. Having the bolts in place helps you better visualize how the toilet will be positioned.

    Person placing T-bolts on the flange.
    Lee Wallender
  4. Position the Toilet Flange

    Press the toilet flange down into the drain opening, using the T-bolts as reference points for positioning. Make sure both bolts are at equal distances from the back wall, which will ensure that the toilet tank will be parallel to the wall. You can rotate the flange in either direction until you have reached the preferred position. After the toilet is placed, you will still have the opportunity for some minor rotation, but it is still best to get the position as straight as possible before securing the flange.

    Person using a tape measure to determine the toilet distance to the back wall.
    Lee Wallender
  5. Drill Holes Into the Concrete

    With a hammer drill and 1/4-inch concrete/masonry bit, bore four holes equidistant around the ring directly through the holes in the toilet flange and down into the concrete subfloor. Use the hammer drill's depth gauge to reach the necessary depth. Be careful not to let the drill angle towards the sewer pipe—if you accidentally break the top lip of the sewer pipe, repair or replacement will be costly and difficult.

    Person drilling holes into the concrete with the hammer drill.
    Lee Wallender
  6. Screw the Toilet Flange Onto the Concrete

    Use Tapcon anchors or a similar type of masonry/concrete screws to fasten the toilet flange to the concrete slab. If using a plastic flange ring, be careful not to crack the ring by screwing the anchors in too far.

    With the toilet flange in place, you are ready to install the toilet with a new wax ring.

    Person screwing the toilet flange onto the concrete floor.
    Lee Wallender