How to Set a Shower Drain and Flood Test the Pan

Why this matters

A leaking shower pan does not show up for months. Water that gets past the liner soaks the subfloor and the plate, and the first sign is a stained ceiling below or a soft spot by the curb, long after the tile is grouted and paid for. Almost every one of those failures traces to three places the plumber controls: a drain body set at the wrong height, a liner clamped badly or punctured at the bolts, and weep holes buried in mortar. The flood test is the one moment the whole assembly is visible and cheap to fix. Skip it or rush it and the next time anyone sees the liner is during a demolition.

This card covers the traditional mortar-bed pan: a clamping-ring drain, a sheet liner over a sloped pre-slope, then a mortar bed and tile. Bonded-membrane systems with their own flanged drain follow that membrane maker's instructions, which are part of the listing, and the tile setter usually owns them end to end.

Who owns which layer

Settle this on the phone before rough-in, because the pan is a two-trade assembly and each trade assumes the other did the part in between.

• The plumber sets the drain body, glues it to the trap, installs and tests the liner where the contract says so, and runs the flood test the inspector signs.
• The tile setter owns the final mortar bed, the tile and any waterproofing above the liner or on the walls.
• The pre-slope and the liner are the parts that drift. In many shops the plumber or a framer floats the pre-slope and the plumber installs the liner; in others the tile setter does both and calls the plumber only to tighten the clamping ring and witness the test.

Write the split on the work order. When the ceiling below stains, the record of who set the liner is what decides the callback.

The section you are building

Read the drawing from the subfloor up. Every layer has one job, and the drain body is the only part touching all of them.

          strainer (threaded, set to tile height)
                     |   |
  tile + mortar bed  |   |  tile + mortar bed
  (tile setter)      |   |  (tile setter)
 ====================+   +====================
  pea gravel -> o weep   weep o <- pea gravel
 ---liner------[clamp ring]------liner--------
  pre-slope    [ bolts   ]       pre-slope
  1/4 in/ft    [flange   ]       1/4 in/ft
 ==============| body    |====================
   subfloor    | to trap |      subfloor

• Drain body: the lower half, glued or no-hub connected to the trap, flange resting on top of the pre-slope.
• Pre-slope: mortar floated under the liner so water that gets through the tile and setting bed runs to the drain instead of sitting.
• Liner: the actual waterproofing, a PVC or CPE sheet pinched between the drain flange and the clamping ring.
• Clamping ring: the upper half, bolted down through the liner, with the weep holes around its edge.
• Weep holes: the only exit for water that reaches the liner. Mortar in them turns the pan into a bathtub under the tile.
• Strainer: threaded into the clamping ring and adjusted later to the finished tile.

Start state and what to check first

You need the trap and drain arm installed (the trap arm length and venting are covered in Drain Pipe Slope + Venting Quick Reference), the subfloor solid with no deflection under your weight at the drain, blocking at the curb, and the shower footprint and curb height from the tile setter.

Check the curb height before anything else. The flood test needs a dam at least as high as the water you will hold. If the finished curb will be low or there is no curb (a curbless entry), you will build a temporary dam for the test, and you need to know that now, not when the hose is running.

Check the liner material against the drain. Solvent-welded PVC liners and CPE liners use different corner and seam methods, and some liners are not compatible with the drain's adhesive or with solvent cement on the flange. The liner maker's sheet says which; follow it.

Steps

1. Set the drain body to height. The flange top sits at the top of the pre-slope at the drain, so the liner lies flat from the slope onto the flange with no step and no dip. Work out the pre-slope height at the drain from the farthest point of the floor (the worked example does this), then cut the riser so the flange lands there, level in both directions. Glue it with the right cement for the pipe (PVC or ABS, not interchangeable), primer where the cement maker requires it, and ventilate: solvent cement vapor is flammable and a respiratory irritant in a closed bathroom, so open a window or run a fan and keep torches and pilots away until it flashes off. A flange set high puts a hump in the liner the tile setter cannot float over; set low, the liner dishes at the drain and holds water at the weeps.

2. Float or confirm the pre-slope. Commonly 1/4 in per foot toward the drain, at the slope the adopted code and the liner maker call for. A pre-slope is not optional just because the final bed is sloped: without it, the liner is flat, and water that gets through the tile sits on it permanently. Wear gloves and eye protection with mortar, because wet portland cement burns skin on prolonged contact; wash it off with clean water, not a solvent.

3. Install the liner. It turns up the walls and over the curb, folded at the inside corners (never cut there), fastened only high on the wall above the water line, never in the floor or low on the curb face. Under the IPC the liner turns up at least 2 in above the finished threshold (IPC section 417, as adopted); the UPC has its own figure in its shower section, so read the adopted edition rather than carrying one number between jurisdictions. Every nail or staple below the top of the flood level is a hole in the test.

4. Clamp the liner at the drain. Cut the liner opening slightly smaller than the drain outlet, find the bolt holes by feel, and cut a small slit for each bolt, not a round hole. Put the sealant the drain maker specifies between flange and liner if they call for it, set the clamping ring, and tighten the bolts evenly in a crossing pattern. Uneven bolts leave a gap on one side the flood test may not find, because a slow seep through a pinched-but-loose edge can take hours.

5. Protect the weeps. A handful of pea gravel or a weep protector around the ring keeps the tile setter's mortar out of the weep holes. This is the step most often skipped, and its failure shows up as a pan that tests perfect and then smells, because water trapped in the mortar bed has nowhere to go.

6. Plug and flood. Plug the drain below the weeps (a test plug in the drain body or a test cap in the riser) so water fills the pan and enters the body through the weeps. Fill to the depth the adopted code and your inspector require. Under the IPC that is not less than 2 in measured at the threshold, held not less than 15 minutes with no leakage (IPC 312.9 in the 2021 edition, as adopted). Under the UPC the shower pan test sits in its own section with its own wording; and many inspectors and tile setters ask for an overnight or 24-hour hold whatever the code minimum says. Ask the inspector which they want before you fill.

7. Mark the waterline. A pencil line or tape edge at the water surface on two walls and the curb. A single mark can hide a slow drop; two marks at opposite ends of the pan make a tilted mark error obvious.

Worked example: a 36 by 60 in pan with a 2 in curb

A 36 in deep by 60 in wide alcove, curb on the 60 in side, center drain. The drain center sits 18 in from the curb and 18 in from the back wall, 30 in from each end wall.

Pre-slope height at the drain. The farthest point is a back corner: the straight line from drain to corner is the square root of 18 squared plus 30 squared, which is the square root of 324 plus 900, or the square root of 1,224, about 35 in. At 1/4 in per foot, 35 in is 35 divided by 12, about 2.92 ft, times 1/4 in, about 0.73 in. Check from the other direction: 3/4 in of fall would need 36 in of run, and 35 in is just under that, so 0.73 in is right. The pre-slope starts about 3/4 in above the drain flange at the corners, which means the flange goes at the pre-slope's low point and the floor rises 3/4 in to the corners.

The curb. The framed curb with the liner over it stands only about 2 in above the liner at the threshold, so water 2 in deep there is right at the curb top and a slosh spills it. You set a 2x4 laid flat (1-1/2 in thick) along the curb top, bedded on a bead of removable putty and weighted, never screwed through the liner, which raises the dam to about 3-1/2 in and gives about 1-1/2 in of freeboard.

Depth across the pan. Water finds level, and the pre-slope does not. At the threshold, 18 in from the drain, the pre-slope rises 18 divided by 12 times 1/4 in, which is 0.375 in. Water 2 in deep there means the surface sits 2.375 in above the flange, so it stands 2.375 in deep at the drain. At the back corners the slope rises 0.73 in, so the water there is 2.375 minus 0.73, about 1.65 in deep. Every part of the liner is under water, which is the point: the corners are where a folded liner leaks, and a test too shallow to reach them proves nothing.

The run. Filled at 9:10 pm, marks on the back wall and the curb dam. The inspector asked for an overnight hold. At 7:30 the next morning the back wall mark was dry by about 1/16 in and the curb mark matched. A real drop shows equally at both marks, because the water surface is level, so a drop at one mark only is not a change in the water; you still check the ceiling below (dry), the plate at the curb (dry) and the liner corners (dry). What the back wall showed was the film splashed above the line during the fill drying back to the true surface. The inspector passed it with both marks in view.

Draining it. Pulling the test plug, the pan emptied in under a minute, most of it straight down the open ring, so the drain-down says nothing about the weeps. Look at each weep instead before the setter floats the bed: open, with gravel or a protector around it. A weep packed with debris now is cheaper to clear than one found under tile.

Finish test and what a drop means

Pass: the waterline is unchanged at every mark, with the ceiling or crawlspace below dry. A measurable drop with water below is a liner leak; find it before you drain, because the wet spot points at it.

• Wet below the drain: the clamping ring or a slit cut too large at a bolt. Drain, loosen, reseat the liner, retighten evenly, retest.
• Wet at the curb: a fastener low on the curb or a fold that was cut. Patch with the liner maker's patch kit and solvent where their instructions allow; a cut at the corner fold is often a liner replacement.
• Wet at a wall plate: a nail below the flood level. Patch it and move the fastener up.
• Level dropped, nothing wet below: a leak into the wall cavity behind the liner turn-up. Pull the baseboard or look from the next room before you call it evaporation.

After any repair, the test starts over with a fresh fill and fresh marks. A re-test that resumes the old clock is not a test.

Handing the pan to the tile setter

Once it passes, the pan is the tile setter's to protect. Leave the test plug out so the drain is open, the weep protection in place, and a note on the subfloor or work order saying the date, depth, duration and inspector. Tell the tile setter in writing: no fasteners through the liner below the curb top, no mortar in the weeps, strainer to be set to finished tile height. When the strainer height comes up wrong after tile, it is adjusted at the threaded strainer, not by cutting the drain body.

References

• IPC 2021 Section 312.9 (shower liner test) and Section 417 (showers), as adopted by your jurisdiction; check the current edition.
• UPC 2021, shower pan and receptor test provisions, as adopted by your jurisdiction; check the current edition.
• Drain and liner manufacturers' installation instructions, which are part of the listing.
• See related: Drain Pipe Slope + Venting Quick Reference
• See related: Rough-In Inspection Readiness Walkthrough