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How to Install a Fiberglass Base Over Concrete

Install a fiberglass shower stall over a concrete floor using a special drain assembly that eliminates leaks. You run the drain line under the concrete.

By the DIY experts of The Family Handyman Magazine

Step 1: Run a drainpipe under the concrete floor

If you want to install a shower over a concrete floor, a fiberglass shower stall will work just fine. Ideally, you’ll already have a drain roughed in, with the pipe sticking a few inches above the floor. Otherwise you have to break through the concrete to run a new line.

If you have a pipe stubbed up, call the plumber or contractor who did the work to confirm it. The 2-in. pipe is typically the shower or tub drain. Line it up exactly with the drain hole in the bottom of your shower bay. This position determines the location of the stud walls around the shower. The supplier may be able to provide a dimensioned drawing of the shower bay, so you don’t actually have to put it in first.

If the existing 2-in. drainpipe isn’t where you want it, you’ll have to break up the concrete and replumb the drain.

Make sure the drainpipe extends at least an inch above the concrete. If the pipe is too short, you’ll have to break up the concrete and extend it. Also, you’ll need at least a 1-1/2-in. space between the pipe and the concrete to accommodate the drain assembly (see photo below). If there isn’t, carefully chisel out the concrete around the pipe.

Tip: If you’re considering a single-piece shower bay, make sure you can get it into the new bathroom space. You may have to bring the unit into the space before framing the doorway or walls. If access is a problem, check out multiple-piece units that will fit through doorways more easily.

Step 2: Mount the leakproof drain assembly

After you frame the shower walls to fit the shower bay, lay the bay on its back and install the drain. Use a special leakproof drain assembly called the No-Caulk Shower Drain. It’s available at home centers and plumbing supply houses. Although these drain baskets are available in plastic, I’d opt for the brass one. Follow these steps to attach the shower to the drain:

  • Roll out a coil (about the diameter of a pencil) of plumber’s putty (available at all hardware stores) and wrap it under the drain basket flange.
  • Hold the drain basket in position in the shower bay drain hole, slide on the washers, and then thread on and hand-tighten the exterior setting ring. Tighten the ring with a large slip-joint pliers until it’s firmly in place. You shouldn’t be able to move the shower basket by hand. The plumber’s putty will squeeze out.
  • Lower the shower bay and basket assembly over the drainpipe. Mark the pipe about 3/4 in. below the drain basket flange.
  • Lift the shower bay out of the way and cut the drainpipe to length. A hacksaw works well. Make sure to cut the pipe square, not angled.
  • Set the shower bay and basket assembly back over the pipe. Slide the caulking gasket over the pipe and push it all the way down so it sits against the lip at the bottom of the drain basket. You may have to tap this gasket into place with a hammer and a thin piece of wood.
  • Thread on the caulking nut and firmly tighten to compress the caulking gasket and seal the pipe. Use the slotted bar tool that comes with the drain assembly. Stick the flat blade of a screwdriver into the bar’s slot and firmly hand-tighten.
  • Snap on the perforated drain cover (not shown).

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • Hammer
    • Hacksaw
    • Slip joint pliers

Flat-blade screwdriver

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Drain assembly
    • Plumber's putty

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How to Install a Fiberglass Base Over Concrete

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