How to Repair a Shower Pan That Does Not Drain

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Shower pan repair usually means working out why water is not reaching the drain or is getting past the pan. The pan is the waterproof floor under the tile, often a mortar bed over a membrane. Common signs of trouble are standing water, a slow drain, a ceiling stain below, or a soft spot on a basement floor. The sources here treat a slow drain and a leak as different problems. Shut off the water to the shower before you touch the pan, drain, or valve.

Quick answer

Sort the problem first. A pan that drains slowly may have high spots or uneven mortar. A pan that leaks often needs the tile and base mortar removed to reach the membrane.
Home Addition Plus says a slow-draining pan can sometimes be fixed by sanding high spots and adding mortar, while a failed pan usually means ripping out the old pan and starting over.
Fine Homebuilding describes a pitch of about 1/4 inch per foot from the pan edge to the drain, under a vinyl membrane.
Test a repaired or new pan before tile goes back on. Fine Homebuilding fills the pan 2 to 3 inches deep and leaves it overnight.
Shut off the water first. Call a licensed plumber or contractor for waterproofing, drain, and subfloor work.

How we chose these sources

This guide uses three independent publishers. Home Addition Plus covers repair of a mortar shower pan that does not drain. Fine Homebuilding is a trade publication that covers building a leakproof pan and testing it. Bob Vila covers installing a shower pan, including a prefab pan, and its leak check. The three were chosen because each one describes the pan itself, not a product listing. Pages that returned a blocked or error response were not counted. None of the three is a government or standards body, so the guidance below is trade practice, not code. Where only one source speaks to a point, the text says so.

First, confirm the problem is the pan

Water on the floor can come from more than one place. Home Addition Plus notes that a stain on a lower-level ceiling below a tiled shower can mean a failed mortar pan, saying “chances are you have a failed mortar shower pan.” That is a useful starting point, but the same stain could also come from a drain connection or a supply line, so it is a reason to look closer, not a diagnosis.

Fine Homebuilding describes a case where leaks traced to the drain assembly, not the membrane. The author writes that “faulty drain-assembly fittings were responsible for the leaks” in two pans he rebuilt. The lesson for a repair is that the drain connection should be checked before the pan is blamed. Look at the drain flange and its seal, and do not assume the mortar is the cause.

Do this check with the water off. Once the tile is removed, the drain area is visible, and the sources describe the drain as a place where leaks start.

Check the slope before anything else

Slope is the first thing to measure on a pan that does not drain. Home Addition Plus gives a target of about 3/16 to 1/4 inch of drop per 12 inches of run from the walls to the drain. Fine Homebuilding gives a pitch of about 1/4 inch per foot, measured with a 2-foot level. The two numbers are close, and both describe the same goal: water runs to the drain and does not sit.

Home Addition Plus says that if the pan simply does not drain well, the high spots can be sanded down with a brick, checked often with a spirit level, so the surface slopes evenly from the walls to the drain. If low spots remain, more mortar can be added. The source advises marking reference lines on the wall so the finished height of the mortar is consistent as it drops toward the drain.

Sanding and adding mortar are both work on a pan that already holds water. Before doing either, confirm the pan does not leak. A slow drain with no ceiling stain may be a surface problem. A pan that leaks is a different job, covered below.

When the pan leaks: tear out and rebuild

For a leaking pan, the sources describe removal, not patching. Home Addition Plus says that repairing a mortar shower pan “is not a simple task” and that in most cases it means “ripping out the failed mortar shower pan and starting from scratch.” Only in some cases may the pre-slope mortar layer be kept, and only if it was installed properly.

The removal steps in the Home Addition Plus article are as follows:

Remove the shower floor tile and about a foot of wall tile. The source suggests a cold chisel and a hammer, and it advises wearing eye protection and leather gloves because tile pieces fly.
Break up and remove the base layer of mortar.
Pull out the old membrane liner. Inspect the pre-slope mortar layer. If the slope is inadequate, chisel it out too.

Protect the room below, too. If the shower sits above a finished ceiling, the removal can damage it. Dust and debris from chiseling also spread quickly, so cover nearby surfaces before you start.

Check the subfloor and the drain before rebuilding

A rebuilt pan is only as good as the floor under it. Home Addition Plus says the wood subfloor should be at least 1-1/4 inch thick. If it is not, the source says to add another layer of plywood over the existing subfloor and screw it down well to the floor joists. Fine Homebuilding makes the same point from a different angle, saying the first concern is that the subfloor is “structurally sound,” and that a layer of 1/2-inch plywood should be added if it is not, before the plumber sets the drain assembly.

For tile and grout care once the job is finished, see cleaning bathroom grout and tile. Fine Homebuilding also describes a pitched mud base under the membrane, so that any water reaching the membrane drains through the weep holes in the drain assembly. It recommends keeping gravel or pea stone around the drain so those holes stay clear of cement. Those details are specific to a mortar build, and a repair on an existing pan should not change the drain layout without a plumber.

Bob Vila adds a note for the drain. It says some manufacturers recommend silicone rather than plumber’s putty for a drain flange, “as putty may affect a fiberglass shower pan.” If your pan is not mortar, check the manufacturer’s instructions for the flange material before you choose a sealant.

Membrane, mortar, and the backer board

Fine Homebuilding describes a vinyl membrane with the drain cut and sealed to the drain assembly. The author cuts small slits over each bolt head, cuts a circle between the bolts, and bonds the membrane to the drain plate with PVC membrane cement or an elastomeric sealant. Membrane seams at the threshold are folded, glued, and nailed. Those are trade steps, and the source describes them for a new pan. The same principle applies to a repair: the membrane must be sealed to the drain with no gaps.

The mortar bed is then laid over the membrane. Fine Homebuilding uses a mix of one part portland cement to three or four parts sand, wet enough to hold together when squeezed, and the pitch is kept at about 1/4 inch per foot. The source notes that a pitched base lets water that reaches the membrane drain out through the weep holes in the drain assembly.

When the walls go back up, the backer board matters. Fine Homebuilding says to keep the bottom course of backer board about 1-1/2 inch from the pan floor, and says “moisture can wick up into the wall” if it is installed too low. It also says never to nail the board lower than the top of the threshold or the step.

Test before the tile goes back

Fine Homebuilding describes the test in plain terms. The drain is plugged with an expandable rubber drain plug, and enough water is poured in to cover the whole pan 2 to 3 inches deep. The pan is left overnight. The next day the water level is checked, and the ceiling below the stall is checked for leaks. The author adds that the builder or homeowner should witness the test. The source puts the point simply: “Better to find leaks now than after the tile is in.”

Bob Vila describes a shorter check for a prefab pan. After installation, run the shower once, plug the drain for a minute while the water runs, then stop the water and see whether the level holds. Bob Vila is about installing a pan, not repairing one, so the check is useful for a repair only in the sense that it tests the same surface.

Do not step into the pan until the mortar has cured, and do not lay tile until the pan has passed the test. Fine Homebuilding says that once the pan has passed inspection, nobody should step into it until the mud base for the tile has been poured.

Common mistakes the sources point to

Uneven slope or a thin subfloor. Home Addition Plus names installer shortcuts and an uneven pre-slope as the usual causes of a pan that does not drain.
Skipping the water test. Fine Homebuilding describes the overnight test as the step that catches leaks before tile.
Backer board set too low, so moisture wicks into the wall.
Drain fittings. Fine Homebuilding traced two leaks to the drain assembly.
Letting water damage go on. Bob Vila lists mold, mildew, and “wood rot” among the problems that lingering water can cause.

If mold has already started behind the tile or on the grout, read bathroom mold cleaning and prevention for surface cleaning, and treat any wet framing or soft subfloor as a structural problem for a licensed contractor. Surface cleaning does not fix a leaking pan.

When to call a licensed plumber or contractor

Call a licensed plumber or contractor for any of the following: a leak that has reached a ceiling or basement; a pan that needs a new drain or drain relocation; a soft, rotted, or unlevel subfloor or joist; any work on the membrane that sits behind a wall; and any work that touches the water supply or drain lines. Bob Vila says that when the access to the drain is limited, for example on a ground-floor bathroom, it is “usually best to call a pro” for the drain assembly. The same logic applies to a repair.

Ask the contractor to show how the pan will be sloped, how the membrane will be sealed at the drain, and how the pan will be tested before tile. Those are the points the sources focus on, and they are the points most worth checking on a bid.

What we could not verify: what the sources do not say

None of the three sources gives a repair method for a small crack or a single failed seam in a mortar pan. They describe removal and rebuilding.
No source gives a code minimum for slope or subfloor thickness. The figures above are what the sources recommend, not what a local inspector will require.
No source describes how to repair an acrylic, fiberglass, or prefab pan that cracks. Bob Vila covers installing a prefab pan, not repairing one.
No source says whether a permit is needed for a pan repair. Local rules vary.
None of the three sources says to shut off the water before starting. That step is general guidance, not a quote from these sources.
No source opened says whether a sealant or caulk can fix a leaking pan. Do not assume a sealant will do it.

FAQ

Can a shower pan be repaired without removing the tile?

The sources described here do not describe a repair from above the tile. Home Addition Plus says that a failed mortar pan usually means removing the tile and the base mortar. A small surface issue may be different, but none of the sources covers it.

How do I tell if my shower pan is leaking?

Check the ceiling or floor below the shower for a stain or a soft spot. Then plug the drain and fill the pan 2 to 3 inches deep, leaving it overnight, as Fine Homebuilding describes. The sources use this test on new pans. It can also show a leak in an existing pan, but the sources do not say that directly.

What slope should a shower pan have?

Home Addition Plus gives 3/16 to 1/4 inch of drop per 12 inches of run. Fine Homebuilding gives about 1/4 inch per foot. Both describe a mortar pan. Your manufacturer may specify a different slope for a prefab pan, and the manufacturer’s instructions should govern.

Is shower pan repair a DIY job?

Some steps, such as removing tile, are the kind of work the sources describe as hard, dusty, and risky. Changes to the drain, the subfloor, or the water lines should go to a licensed plumber or contractor. Bob Vila recommends a pro for limited-access drain work.

How long should a prefab shower pan last?

Bob Vila says that, when installed properly, a prefab shower pan should last “a decade or more.” That is a general statement from one source, not a warranty.

Will caulking the seams stop a leak?

The sources do not say that caulk repairs a leaking pan. Fine Homebuilding seals the membrane at the drain with cement or sealant during the build, which is different from sealing a finished joint. Do not rely on caulk for a leak without a plumber’s assessment.

Bottom line

If the pan drains slowly, check the slope first, and compare it with the 1/4 inch per foot the two mortar-pan sources describe. If the pan leaks, plan for tile removal, a check of the drain and subfloor, and an overnight water test before tile goes back. Shut off the water first, and bring in a licensed plumber or contractor for waterproofing, drain, and structural work.

Sources

Home Addition Plus, “How to Repair a Mortar Shower Pan that Does Not Drain Properly.” https://homeadditionplus.com/dev/tiling-floors-walls/how-to-repair-shower-pan-does-not-drain/ (opened 2026-10-27)
Fine Homebuilding, “Installing a Leakproof Shower Pan.” https://www.finehomebuilding.com/project-guides/tiling/installing-a-leakproof-shower-pan (opened 2026-10-27)
Bob Vila, “How to Install a Shower Pan in Your Bathroom.” https://www.bobvila.com/articles/how-to-install-a-shower-pan/ (opened 2026-10-27)