Step 1: Run a drainpipe under the concrete floor
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Shower stall on concrete
A shower usually requires a 2-in. drain, with the P-trap located under the concrete slab.
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
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
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Drain assembly close-up
The assembly locks tightly to the pipe and clamps to the edge of the shower base to make leakproof seals.
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
- 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).