Plumbing
Shower pan and wet-area waterproofing field guide for plumbers and tile setters
Tile is not the barrier. Build the waterproof pan right, slope it to the drain, detail the corners, and flood test it before the tile goes on.
Direct answer
A shower pan is the waterproof barrier under and around a tiled shower that catches the water tile and grout let through and routes it to the drain. Tile and grout are not waterproof. The membrane is the barrier, built either as a traditional liner over a pre-slope or a modern bonded surface membrane.
Key takeaways
- Tile and grout are not waterproof; the membrane is the barrier that catches water and routes it to the drain.
- Shower floors fall commonly 1/4 in per foot to the drain; confirm with the drain maker and adopted code.
- Match the drain to the method: clamping drain with weep holes for a traditional liner, bonded-flange drain for a surface membrane.
- Flood test every pan before tile: plug, fill below the curb, hold at least 24 hours, and check below for leaks.
- Corners and changes of plane are the number-one leak point; use fabric or preformed corners and no fasteners below the flood line.
The shower pan, and why the membrane does the work
A shower pan is the waterproof barrier built under and around a tiled shower or wet area. Its job is to catch the water that tile and grout let through and route it to the drain before it reaches the wood, the slab, or the room below. The tile you see is the wear surface and the looks. The pan is what keeps the building dry.
Most people think the tile keeps the water out. It does not. Water moves through the grout joints and through the tile body itself, soaks into the setting bed, and keeps going until it hits something that stops it. That something is the membrane. Get the membrane right and a cracked grout line is a cosmetic problem. Get it wrong and the same crack feeds a slow leak you will not see until the framing is soft or the ceiling below is stained.
The failure is never dramatic at first. It is rot you smell before you find it, mold behind the wall, and a leak that shows up a floor down. By then the tile has to come out. The waterproofing is the one part of a shower you cannot inspect after it is covered, which is why every method below is built around getting it right once. The drain and the rough-in feed this work; see the floor and trench drain installation guide and the fixture rough-in guide for how the plumbing ties in.
Is tile and grout waterproof?
Tile and grout are not waterproof, and building a shower as if they were is the root of most tile-shower failures. Ceramic and porcelain tile are dense, but the grout between them is a cement product full of tiny voids, and water passes through it steadily under the constant wetting a shower sees. Even the tile body absorbs some water. The water does not pool on top and run to the drain. It goes through.
This is the one idea the whole trade is built around: the waterproofing does the work, not the tile. The membrane, the liner, or the liquid coating is the barrier. Cement backer board feeds the confusion, because it is sold as a tile substrate and it does not rot, so people assume it stops water. It does not. Cement board is water-resistant, not waterproof. Water runs straight through it and through its seams to whatever is behind.
So when you plan a shower, plan for the water that gets past the tile, not the water the tile sheds. Every method below answers the same question in a different way. Once the water is through the tile, where does it go, and what stops it from reaching the structure.
The two ways to build a waterproof shower
There are two families of shower waterproofing, and they put the barrier in different places. The traditional method buries the membrane deep, under a thick mortar bed. The modern method puts the membrane at the surface, right under the tile. Both work when built correctly. They fail in different ways and use different drains, so the first decision on any shower is which system you are building, because it drives the drain, the slope, and the sequence.
In the traditional method, a sheet pan liner sits on a sloped mortar bed, the pre-slope, a second mortar bed goes on top, and the tile bonds to that. Water that gets through the tile travels down through the top mortar bed to the liner and out through weep holes in a two-piece clamping drain. It is a water-in, water-out system. The mortar bed stays damp by design.
In the modern method, a bonded sheet or a liquid coating goes directly on the substrate or on a pre-sloped foam tray, and the tile bonds on top of the membrane. Water that gets through the tile hits the membrane within a fraction of an inch and runs to a bonded-flange drain. The assembly stays mostly dry because the water never gets deep. Mixing the two, a clamping drain with a surface membrane or the reverse, is one of the more common ways to build a leak.
The traditional method: liner over a pre-slope, mortar bed on top
The traditional shower floor is five layers from the bottom: the subfloor or slab, the pre-slope mortar bed, the sheet pan liner, the top mortar bed, and the tile. The liner is the waterproofing. It is a flexible sheet, usually PVC or CPE today, sometimes lead or copper on older and specialty work, laid in one piece across the floor and up the walls above the curb.
The liner runs up all the walls a few inches above the finished curb height, folds at the corners without being cut, and laps into the drain where it is clamped. Above the curb the liner gets fastened only on the top outside face, never on the inside or the top edge where water sits, because every fastener through the liner below the flood line is a hole in your waterproofing.
The top mortar bed, the deck mud, goes over the liner to carry the tile and spread the load, since tile cannot bond to the liner directly. This is the proven system, decades old, and it still works. Its weakness is that it depends on two details people skip: the pre-slope under the liner and the weep holes in the drain. Miss either and the system traps water instead of draining it.
What is a pre-slope and why does it matter?
A pre-slope is the sloped mortar bed built under a traditional pan liner so the liner itself pitches to the drain. It is the single most skipped step in traditional shower building, and skipping it is the number-one traditional-method failure. Without a pre-slope the liner sits dead flat on the subfloor, and the water that reaches it has nowhere to go.
Here is the mechanism. Water gets through the tile and grout, down through the top mortar bed, and onto the liner. If the liner slopes to the drain, that water runs to the weep holes and out. If the liner is flat, the water sits on it, soaks the mortar bed permanently, and you get a shower that smells, grows mold, and weeps water at the curb. The tile looks fine. The bed underneath is a sponge.
Build the pre-slope first, on the bare subfloor, pitched to the drain at the slope the code and the drain maker call for, commonly 1/4 in per foot. Lay the liner on top of that cured slope. Then build the top bed. The liner has to fall to the drain, not lie flat. If you remember one thing about traditional showers, remember that the liner must slope.
The clamping drain and the weep holes
A traditional liner needs a two-piece clamping drain, also called a sub-drain or a weep-hole drain. The drain body sits on the pre-slope, the liner drapes over it, you cut the liner to the drain opening, and a clamping ring bolts down and squeezes the liner against the drain body to make the seal. The bolt holes get cut in the liner over the bolt studs, not over open membrane.
The weep holes are the part that gets forgotten and the part that makes the system work. The clamping drain has small openings, the weep holes, at the level of the liner, below the strainer. Water that drains down to the liner enters through these weeps and leaves. If they clog, the water has no way out and the bed stays saturated, the same failure as no pre-slope.
Protect the weeps when you pack the top mortar bed. The trade standard is to set a layer of coarse gravel or a few pebbles, or pieces of broken tile, around the drain over the weep holes so the mortar cannot pack into them. Keep that drainage path open. A clogged weep is a slow leak waiting on time, and you cannot clear it after the tile is down.
The modern method: bonded surface membrane
The modern method puts the waterproofing at the surface, bonded to the substrate directly under the tile, instead of buried under a mortar bed. The membrane is either a bonded sheet, the kind sold as Schluter Kerdi and similar polyethylene sheets, or a liquid coating rolled and troweled on. Either one bonds to the substrate with thinset and the tile bonds on top of it. Water that gets through the tile hits the membrane within a fraction of an inch and runs to the drain.
This system uses a bonded-flange drain, not a clamping drain. The drain has a wide flange at the surface, and the membrane laps onto and bonds to that flange so the waterproofing is continuous from the field of the floor into the drain. There are no weep holes, because the water never travels deep enough to need them. The assembly stays mostly dry.
The advantages are speed and a drier wall. Fewer trades, fewer steps, less mortar, and the shower can be tiled the same day in many cases. The catch is that the membrane is thin and unforgiving at the corners and seams. A bonded system lives or dies at the changes of plane, so the corners and the drain bond get the attention the mortar bed used to get.
Liquid-applied membrane
A liquid-applied membrane is a paint-on or trowel-on waterproofing that cures to a continuous rubbery film over the substrate. The common products are elastomeric coatings like RedGard and Hydro Ban, sold by the gallon, and they go on by roller, brush, trowel, or sprayer. Two things make or break a liquid membrane: the film thickness and the reinforcement at the corners.
Thickness is measured in wet mils, and the number the manufacturer prints is not optional. Most call for a minimum cured thickness reached in two coats, often around 30 wet mils per coat, roughly the thickness of a credit card, with the second coat run perpendicular to the first so you do not leave a thin spot. Too thin and the film fails under flexing. A wet-film gauge is cheap, and it is how you prove the thickness instead of guessing it.
The corners and changes of plane get a reinforcing fabric or mesh embedded in the first coat, run a few inches onto each surface. That is where the film thins and where movement concentrates, so it is where a plain coating cracks. Let each coat cure to the maker's time before the next, and let the whole thing cure before the flood test. Most warranty failures on liquid membranes trace to thin film or skipped fabric at a corner.
Bonded sheet membrane
A bonded sheet membrane is a factory-made waterproof sheet, a polyethylene core with a fleece on both faces, bonded to the substrate in thinset and overlapped at the seams. Schluter Kerdi is the best known, and the same idea is sold by several makers. Because the sheet is a known thickness from the factory, you do not have the thin-film risk of a liquid. The risk moves to the seams and the bond.
Seams get overlapped, commonly by a few inches, and fully embedded so there is no gap and no air pocket. Voids behind the sheet are the enemy. A bubble is an unbonded spot where water can track. You flatten the sheet into the thinset with a float so the fleece is fully wetted out and there are no hollows. Corners use preformed inside and outside corner pieces, and changes of plane get a band of the same membrane.
The drain is a matching bonded-flange drain, and the sheet laps onto the flange and bonds to it. Use the membrane maker's own thinset spec. The wrong thinset behind a polyethylene sheet will not bond, and the whole point is the bond. A sheet that is bonded everywhere and lapped at every seam is hard to beat. A sheet with hollow spots is a slow leak with good looks.
Pre-sloped foam pans and trays
A pre-sloped foam pan is a lightweight molded tray with the slope already built into it, so you set it in thinset on the subfloor and the slope is done. Schluter Kerdi-Shower trays, Wedi, and similar products are the common ones. They pair with a bonded sheet or come ready for the membrane and a bonded-flange drain, and they take the hand-packed pre-slope off the job.
The appeal is consistency. A hand-floated mortar slope is only as good as the hand that floated it, and a factory tray gives the same 1/4 in per foot every time with no low spots that pond. For a small to mid-size shower on a standard drain location, a foam tray is faster and more repeatable than building two mortar beds.
The limits are size and drain location. Trays come in set sizes and drain positions, and once you are cutting and seaming a tray heavily to fit an odd footprint, you have given back the speed and added seams to fail. For large or oddly shaped showers, a floated bed or a fully bonded membrane on a sloped substrate often makes more sense. Set the tray in full thinset coverage, not dabs, so it is fully supported and does not flex underfoot.
What goes under the waterproofing?
The substrate under a surface membrane has to be solid, flat, and rated for wet-area tile, which in practice means cement backer board, fiber-cement board, or a foam tile backer, not paper-faced gypsum. Greenboard, the moisture-resistant gypsum sold with a green or purple face, is not a shower substrate. The paper and gypsum core break down when they stay wet, and behind a membrane that fails even slightly, they turn to mush.
This trips people up because cement board is water-resistant and does not rot, so it gets treated as the waterproofing. It is not. Water runs through cement board and through the seams. The board is the base for the tile and the membrane. The membrane is the barrier. You still waterproof over cement board, every time, in a shower.
Set the board on framing at the spacing the board maker calls for, tape and fill the seams, and keep fasteners out of the area below the flood line where they would pierce the floor membrane. On a slab, a floated mortar bed or a foam substrate can replace the board. The rule holds regardless of substrate: the substrate carries the tile, the membrane keeps the water out, and greenboard does neither in a shower.
Slope to the drain
The finished floor of a shower has to fall to the drain so water runs off and does not pond, and the common figure is 1/4 in per foot of run from the wall to the drain. In a traditional system you build that slope twice, once in the pre-slope under the liner and again in the top mortar bed, so both the liner and the tile drain. In a surface system the slope lives in the substrate or the foam tray, under the membrane.
Ponding is the tell that the slope is wrong. Water that stands on the floor after the shower is off means a low spot or a flat area, and standing water finds every weakness in the grout and the membrane over time. A level laid across the floor and a few minutes with a hose during the flood test will show you the low spots before the tile hides them.
Hedge the exact number to the drain maker and the adopted plumbing code, because some drains and some codes call for a specific minimum and a maximum, and a curbless or roll-in shower has its own slope rules at the entry. The principle does not change: enough fall everywhere to move the water, no flat spots, no reverse pitch holding water away from the drain.
The curb
The curb, or threshold, is the dam at the shower entry that keeps the water in, and it is one of the easiest places to put a hole in your waterproofing. In a traditional system the liner runs up and over the curb in one piece, with the dam corners folded, never cut. In a surface system the membrane wraps the curb the same way, with preformed or folded corners.
The rule that gets broken is fasteners. Do not drive screws or nails through the pan or the liner where it covers the curb, and do not fasten through the inside face or the top of the curb below the flood line. People tack the liner down at the curb to hold it while they work and put two neat holes right where the water sits. Use the liner maker's adhesive, or fold and clamp it, but keep the metal out of the membrane.
A curbless shower trades the curb for a linear drain and a sloped entry, which moves the waterproofing problem to the transition at the bathroom floor rather than removing it. Either way the curb or the entry is a corner of the waterproofing, not an afterthought, and it gets the same continuous membrane and the same no-fasteners rule as the rest of the pan.
Corners and seams, the number-one leak point
Corners and seams are where tile showers leak, far more than the open field of the floor or wall. The field is one flat plane of membrane and it rarely fails. The inside corner where two walls meet the floor is three planes coming together with movement in every direction, and that is where a membrane tears, thins, or pulls loose.
Every system has a corner answer, and using it is not optional. Traditional liners fold the corners and use preformed dam corners at the curb, folded and bonded, never cut through. Bonded sheet systems use factory preformed inside and outside corners set in thinset and lapped by the field membrane. Liquid systems get a reinforcing fabric embedded in the first coat at every change of plane, run onto both surfaces, because the liquid film thins exactly at the fold where it is needed most.
This is the detail a flood test exposes and the detail an inspector looks at first. If a shower leaks the week after it is built, look at the corners and the drain before you look at anything else. The field almost never fails. The corners do, and they fail because someone treated a change of plane like a flat surface and ran the membrane straight across it with no fabric and no preformed piece.
Which drain goes with which method?
The drain has to match the waterproofing method, and matching it wrong is a built-in leak. A traditional liner needs a two-piece clamping drain with weep holes, because the waterproofing is deep and the water has to drain from the liner level through the weeps. A surface membrane needs a bonded-flange drain, because the waterproofing is at the surface and laps onto the drain flange.
Put a clamping drain under a surface membrane and there is no clean way to bond the thin membrane into the clamp, so the connection at the drain, the lowest and wettest point in the shower, is the weakest. Put a bonded-flange drain under a traditional liner and the liner has no weeps to drain through and no clamp to seal to, so the mortar bed holds water. The drain and the method are one decision, not two.
Linear and trench drains come in both styles and matter for curbless and large showers. The floor and trench drain installation guide covers grate load class, the trap, and the DWV tie in depth, and the same matching rule applies there. Pick the method first, then the drain that bonds or clamps to it. Confirm the drain against the membrane maker, because some drains are listed to work with a specific membrane and not with others.
Do you have to flood test a shower pan?
Yes. The flood test is the one check that proves the waterproofing holds before you cover it with tile, and it is the step that separates a shower you can stand behind from one you are hoping about. Plug the drain, dam the entry, fill the pan with water to just below the curb, mark the water line, and let it sit. The common hold is 24 hours, with some specs and inspectors calling for longer.
While it sits, check below. If there is a floor or a ceiling under the shower, that is where a leak shows. Watch the water line for any drop and the area below for any sign of water. A pan that holds the line for 24 to 72 hours with nothing showing below is a pan that passed. A line that drops or a stain below is a leak you found while you can still fix it cheaply.
Run the test after the waterproofing is complete and cured and before any tile goes down. This is the non-negotiable. Once the tile is on, a leak means a tear-out, so the test that costs you a day before tile saves you a bathroom after. Many jurisdictions require the shower pan test as part of the plumbing inspection, so confirm whether the AHJ wants to witness it before you fill.
Waterproofing the walls and tying them to the pan
The shower walls take as much water as the floor in spray and steam, and the wet area runs past the obvious splash zone, so the waterproofing goes up the walls and ties into the pan with no gap. The common practice is to waterproof the walls to the showerhead height and beyond, and the whole wet wall on a tub or a steam shower, with the membrane lapping down over the top of the floor pan so water always runs onto, never behind, the lower membrane.
The tie between the wall and the floor is the joint that has to be continuous. In a surface system the wall membrane laps down over the floor membrane at the curb and the corners. In a traditional system the liner runs up the wall behind the wall substrate and the wall waterproofing overlaps it. Lap the upper membrane over the lower so gravity keeps the water on the right side of the barrier.
Do not rely on the tile and grout on the walls any more than on the floor. Water gets through wall grout and runs down inside the wall to the pan, and if the wall is not waterproofed and tied in above the pan, that water gets behind the pan and into the framing. The wall and the floor are one waterproof envelope, not two separate jobs.
Steam showers
A steam shower changes the problem from liquid water to vapor, and that raises the bar on the waterproofing. Steam under pressure drives moisture through assemblies that handle splash fine, so a steam shower gets a continuous vapor-rated waterproofing over the whole enclosure, walls and ceiling included, not just the wet zone. The ceiling is part of the envelope and it gets the same membrane.
The standard worth knowing is that the common bonded-membrane standard, ANSI A118.10, qualifies a membrane as a barrier to liquid water, not as a vapor barrier. A membrane that passes for a standard shower may not be rated to stop vapor in a steam room. For steam, confirm the membrane is listed for steam-shower or vapor service with the manufacturer, and slope the ceiling slightly so condensate runs to the wall instead of dripping on the user.
Steam also wants the membrane behind a substrate that handles constant high humidity, and the detailing at the door and the bench gets tighter because vapor finds every gap. Treat a steam shower as a sealed wet box. The same corners-and-seams rule applies, with more at stake, because vapor will exploit a marginal corner that a splash shower would tolerate for years.
Niches and benches
A recessed niche and a built-in bench are both extra leak points, because both add corners, changes of plane, and a horizontal surface that holds water, right in the middle of a wet wall. A niche is a box let into the wall with five inside surfaces and many corners, and every one of them has to be waterproofed and tied into the surrounding wall membrane. A bench is a horizontal ledge that should be sloped slightly to shed water back into the shower, not flat where it ponds.
Build both as part of the waterproof envelope, not as additions to it. The niche membrane laps into the wall membrane so water that gets behind the niche tile runs out onto the wall barrier and down to the pan. The bench top slopes a small amount toward the shower so water runs off, and the front edge and the wall joint get detailed like any other change of plane, with fabric or preformed corners.
Preformed niches and bench forms exist for the bonded systems and they take much of the guesswork out of the corners. Whether you build it or buy it, the niche and the bench get the same membrane, the same corner treatment, and the same continuous tie to the wall as the rest of the shower.
Commercial and data-center wet areas
Commercial wet areas, gang showers, locker rooms, kitchens, and the emergency and equipment-area showers in industrial and data-center facilities, are the same physics at a larger scale and a higher cost of failure. A leak in a single home shower damages one bathroom. A leak in a slab-on-deck gang shower or above a data hall reaches the floor below, and below a data center that floor holds equipment that does not tolerate water.
The methods do not change, but the loads and the area do. Large floors mean more seams, more drains, and often linear or trench drains rather than a single point drain, so the slope and the drain matching matter more, not less. Heavy traffic and rolling loads mean the membrane and the setting bed have to take the abuse without the assembly cracking, which is where crack-isolation comes in alongside the waterproofing.
For these jobs the flood test, the corner detailing, and the documentation carry more weight, because the building owner and the design team will hold the record. Confirm the membrane is rated for the traffic and that it is a load-bearing bonded membrane under ANSI A118.10, and tie the wet-area work to the building drainage the way the floor and trench drain guide lays out. The bigger the floor below, the less a marginal corner is worth gambling on.
How do you fix a leaking tile shower?
A leaking tile shower almost always means a tear-out, not a patch, because the leak is in the membrane and the membrane is buried under the tile. Re-grouting, re-caulking, and surface sealers do not fix a failed pan. They hide the symptom for a while and let the water keep working behind a fresh-looking surface. If the waterproofing has failed, the tile comes out and the pan gets rebuilt.
The signs are consistent. Loose or hollow-sounding tile, especially low on the wall or at the floor, means water has gotten into the setting bed. A musty smell that never clears, grout that stays dark and damp, efflorescence at the curb, and a stain on the ceiling or a soft spot in the floor below all point to a pan that is leaking, not a grout problem. Water at the base of the curb is a classic no-pre-slope or clogged-weep failure on a traditional shower.
There is no reliable band-aid for a failed pan. You can chase a single cracked wall-tile leak in rare cases, but a floor or pan leak is a rebuild. Tear out to the substrate, find why it failed, usually the pre-slope, the weeps, the corners, or a missing flood test, and rebuild it as one of the systems above with a flood test before the new tile.
Grout, caulk, and resealing
Maintenance on a properly built shower is about the surface, not the waterproofing, and it is worth understanding the difference. Grout is not the barrier, and resealing grout does not waterproof a shower. A penetrating grout sealer slows water absorption into the grout and keeps it cleaner, but the water still gets to the membrane, and the membrane is what keeps the building dry. Sealing grout is housekeeping, not waterproofing.
The joints that actually need maintenance are the soft joints, the changes of plane where the wall meets the floor and the inside corners. These move, and they get a flexible sealant, not hard grout, because grout in a moving joint cracks. When that caulk line splits or pulls away, it gets cut out and replaced, since a failed corner caulk lets water sit at the most vulnerable joint even over good waterproofing.
Tell a homeowner the truth about their shower. If it was built with a real membrane and passed a flood test, normal grout cracks are cosmetic and the shower is fine. If they are seeing the leak signs from the retrofit section, no amount of sealing or re-caulking will fix it. Resealing buys looks and a little resistance. It does not rebuild a pan.
Inspection and QC
The waterproofing is the one part of a shower you cannot inspect after it is covered, so the inspection happens before the tile, and it follows a short list. Check the pre-slope on a traditional shower before the liner goes down, because once the liner is in you cannot see whether it falls to the drain. Check the membrane for thickness on a liquid system and for full bonding and lapped seams on a sheet system. Check every corner and change of plane for the fabric or the preformed piece.
Check that the drain matches the method, the weep holes are clear on a clamping drain, and the membrane laps onto the flange on a bonded drain. Then witness the flood test, full and held, before a single tile goes on. A plumbing inspector will often want to see the shower pan test as part of the rough or the waterproofing inspection, so coordinate it rather than filling and draining before they arrive.
The order matters. Each of these is a step you cannot redo without tearing out, so the QC is front-loaded by necessity. A shower that passed a witnessed flood test with documented corners and a matched drain is a shower with a record behind it. One that was tiled fast with no test is a gamble the owner does not know they are taking.
Common mistakes
- Laying a traditional liner flat with no pre-slope, so water sits on the liner and the bed stays saturated.
- Letting mortar pack into the weep holes, or building over a clamping drain with no gravel protecting them.
- Running the membrane straight across inside corners and changes of plane with no fabric and no preformed corner piece.
- Tiling over the waterproofing with no flood test, so the first leak shows up when a tear-out is the only fix.
- Driving fasteners through the pan or liner at the curb or anywhere below the flood line.
- Treating tile, grout, cement board, or greenboard as the waterproofing instead of installing a membrane.
- Mismatching the drain to the method: a clamping drain under a surface membrane, or a bonded-flange drain under a traditional liner.
What to document
The waterproofing disappears under the tile, so the record is the only proof it was built right. Capture the method, the slope, the membrane and its thickness or seams, the drain type, the corner treatment, and the flood-test result with who witnessed it. If the shower ever leaks, this is the document that shows whether the pan was sound and who signed off before the tile went on.
| Element | Requirement | Note |
|---|---|---|
| Method | Traditional liner or surface membrane | Drives the drain and the slope |
| Pre-slope (traditional) | Sloped to the drain before the liner | Cannot be seen after the liner is in |
| Slope to drain | Commonly 1/4 in per foot | Confirm with drain maker and code |
| Membrane | Type, plus wet mils or lapped seams | Liquid thickness or sheet bond |
| Drain type | Clamping with weeps, or bonded flange | Must match the method |
| Corners and curb | Fabric or preformed, no fasteners in the pan | The number-one leak point |
| Flood test | Held, with date and witness | Before any tile goes down |
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The framework lives in a few places, and the membrane standard is the one to know. ANSI A118.10 is the standard for load-bearing, bonded, waterproof membranes for thin-set tile, and it qualifies a product as a barrier to liquid water. A related standard, ANSI A118.12, covers crack-isolation membranes, which matter where the substrate may move. A118.10 addresses liquid water, not vapor, which is why steam showers need a membrane separately listed for vapor service.
The TCNA Handbook for Ceramic, Glass, and Stone Tile Installation gives the assembly methods, the shower and wet-area details, and the flood-test recommendation, commonly a hold of at least 24 hours. Follow the membrane manufacturer's published instructions for thickness, seams, thinset, and the matching drain, because the listing is tied to those instructions, and deviating from them voids both the listing and the warranty.
The plumbing side, the shower pan, the drain, and the required test, falls under the adopted plumbing code, the IPC or the UPC depending on the jurisdiction, with local amendments. Many jurisdictions require a shower pan test the inspector witnesses. Cite the standard that controls the point, hedge the slope and the exact figures to the drain maker and the adopted code, and stress the three details that fail showers: the pre-slope, the corners, and the flood test.
Units, terms, and what they mean
Shower waterproofing carries a vocabulary that shifts between the tile trade, the plumbing trade, and the membrane data sheets, so the same part goes by a few names. Slope is given in inches of fall per foot of run. Liquid membrane thickness is measured in wet mils, where a mil is one thousandth of an inch. Membranes are sold as PVC, CPE, or polyethylene sheets, or as elastomeric liquids by the gallon.
- Shower pan / pan liner
- The waterproof barrier under and around the shower; the sheet liner in the traditional method
- Pre-slope
- The sloped bed built under a traditional liner so the liner drains to the weep holes
- Weep holes
- Openings at liner level in a clamping drain that let water in the mortar bed drain out
- Clamping drain
- Two-piece drain that clamps a traditional liner; pairs with weep holes
- Bonded-flange drain
- Surface drain whose flange the bonded or liquid membrane laps onto and seals to
- Wet mils
- Thickness of a liquid membrane while wet, in thousandths of an inch, set by the maker
- Flood test
- Plugging and filling the pan to prove the waterproofing holds before tile goes down
FAQ
How does a shower pan work?
A shower pan works by putting a waterproof membrane under and around the tile that catches the water tile and grout let through and routes it to the drain. The tile is the wear surface. The membrane is the barrier that keeps water out of the framing and the floor below.
What is a pre-slope?
A pre-slope is the sloped mortar bed built under a traditional pan liner so the liner itself falls to the drain. Without it the liner sits flat, water that reaches it has nowhere to go, and the bed stays saturated. It is the number-one skipped step in traditional showers.
Do you have to flood test a shower pan?
Flood testing is how you prove the waterproofing holds before tile covers it. Plug the drain, fill the pan to just below the curb, and hold it, commonly 24 hours, while you watch the line and check below for leaks. Many jurisdictions require the test as part of inspection.
What is the difference between a traditional and a Kerdi shower?
A traditional shower buries a sheet liner under a mortar bed over a pre-slope, with a clamping drain and weep holes. A Kerdi or bonded-membrane shower puts the waterproofing at the surface under the tile, bonded to the substrate, with a bonded-flange drain. Both work; the drain must match the method.
Is tile and grout waterproof?
Tile and grout are not waterproof. Water passes through grout joints and the tile body steadily under the constant wetting a shower sees, soaks the setting bed, and keeps going. The waterproofing membrane behind the tile is the barrier, not the tile. Cement board behind it is water-resistant, not waterproof either.
What slope does a shower pan need?
A shower floor commonly falls 1/4 in per foot of run to the drain so water sheds and does not pond. In a traditional system you build that slope twice, in the pre-slope under the liner and the bed on top. Confirm the figure against the drain maker and the adopted code.
Why is my tile shower leaking?
A leaking tile shower usually means the buried membrane has failed, not the grout. Loose tile, a musty smell, dark damp grout, water at the curb, or a stain below all point to a failed pan, often from no pre-slope, clogged weeps, or unsealed corners. The fix is a tear-out, not a patch.
Can you waterproof over greenboard in a shower?
Greenboard, the moisture-resistant gypsum board, is not a shower substrate and should not be used behind shower tile. The paper and gypsum core break down when they stay wet. Use cement board, fiber-cement, or a foam tile backer, then waterproof over it with a membrane, since cement board is not waterproof either.
What is a clamping drain versus a bonded-flange drain?
A clamping drain is a two-piece drain that clamps a traditional liner and has weep holes so water at liner level drains out. A bonded-flange drain has a surface flange the bonded or liquid membrane laps onto and seals to. The drain has to match the method, or the connection at the drain leaks.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.