Water Behind Tile: Signs, Tests and When to Tear Out

Why tile and grout are not the waterproofing

The TCNA ceramic tile FAQ puts it in one line: grout joints are not waterproof, so in a shower you rely on the method and substrate to protect adjoining areas. The Schluter Shower System Installation Handbook goes further: contrary to popular belief, the tile covering itself is not waterproof, and moisture will get into the mortar bed and backing. Grout absorption limits under ANSI A118.6 are covered on our mold in shower page.

The waterproof layer is one of two things. A bonded membrane meeting ANSI A118.10, which the TCNA list of ANSI tile standards lists as the standard for load bearing, bonded, waterproof membranes for thin-set tile (2023 edition), sits directly under the tile. A traditional pan liner sits under a mortar bed and drains through weep holes. Either way, water behind shower tile is normal. Water past the waterproof layer is the failure.

What you are looking at: signs and look-alikes

Most signs have an innocent twin. Ask whether it returns, spreads or lines up with the shower.

What you see	Look-alike	How to tell	Points to
White crust or fuzz in grout lines	Soap scum, hard water film	Efflorescence grows out of the joint and returns after cleaning	Water moving through the bed
Grout that stays dark hours after a shower	Wet grout after use	Normal grout lightens as it dries	Saturated bed behind it
Hollow sound when tapped	Large tile over a thin bed	Spreading, or tiles that move	Lost bond, often from water
Black line at corners or the tray joint	Surface mold on caulk	Gap or soft sealant under it	Failed change-of-plane joint
Stain, swelling or paint blisters on the far side	Condensation	Lines up with the shower valve or tray	Water in the cavity
Ceiling stain under the bathroom	Toilet or supply leak	Worsens after showers, not flushes	Pan or floor failure

The Schluter Shower System Installation Handbook explains the white crust: in a traditional pan, the mortar bed can stay saturated if the pre-slope was skipped or the weep holes clog, raising the potential for efflorescence and mold. Hollow tiles matter because, according to the same handbook, the TCNA Handbook requires mounted tile to allow 95% contact with the setting material in wet areas.

Five failure points, and how each shows

Failed shower waterproofing is rarely a tile problem. It is a joint, a slope or a missing layer, and each one has its own test and scope.

Failure point	Sign	Test	Scope
No membrane	Far-side damage, soft board, loose tiles in the spray zone	Meter on the far side; inspection opening	Tear out to studs, rebuild with a membrane
Failed pan liner	Floor grout dark or crusted, stain below	Flood test the floor	Floor tile and bed out; pan rebuilt
Cracked grout	Visible cracks, dark lines	Meter compared with dry tile nearby	Regrout if tiles are sound and the bed dries
Failed change-of-plane joint	Gap or black line at corners, tray or tub	Spray the joint; meter below it	Cut out, reseal with sealant
Flat niche or bench	Water standing on the sill or seat; crust below	Watch it drain after a shower	Rebuild the niche or bench with slope

No membrane at all

Tile on bare board relies on the board drying between showers. There is no repair from the tile side.

A failed shower pan liner

A traditional liner fails at the drain clamp, at the curb or where the weep holes clog. Pan repair is a plumbing job, covered by The Plumbing Brief; the water damage around it is covered here.

Grout leak

Over a working membrane, water through a cracked joint still reaches the drain. A grout leak that wets the far side means the layer behind has failed too.

The change of plane

The Custom Building Products grout sheet TDS-178 says grout is not for movement joints or changes of plane, where caulk or sealant is used, and the Laticrete Hydro Ban data sheet DS-663 requires expansion at coves, corners and other changes in plane.

Niches and benches

The Schluter Shower System Installation Handbook says all horizontal surfaces, including benches, curbs, window sills and shelves, must slope toward the drain. A level niche sill holds water against its joints after every shower.

Testing without demolition, and the limits

Meters read through tile, but shallowly. The Tramex Moisture Encounter listing gives about 1 inch of signal penetration through coverings including ceramic tile, on a 0 to 100 comparative scale for non-wood materials. Compare a suspect area with dry tile on the same wall; an absolute number means little. The Teledyne FLIR leak detection guide says a thermal camera cannot see moisture in walls, only temperature differences from evaporation, capacitance or conduction, and that a meter should always confirm it.

Test	What it reads	Limit	Source
Pinless meter on the tile face	About 1 inch deep, comparative 0 to 100 scale	Not the cavity; relative only	Tramex listing
Meter on the far side of the wall	The drywall and cavity face	Only where the far side is accessible	Tramex listing
Thermal camera	Surface temperature	Cannot see moisture; needs confirming	FLIR
Flood test of the floor	The floor assembly and drain	Does not test the walls	Schluter FAQ
Spray test of one joint	That joint only	One suspect at a time	Method
Inspection opening	The cavity directly	Needs a patch	Definitive

The flood test

The Schluter water test FAQ describes it: plug the waste line, fill the base to just below the drain’s bonding flange, mark the level and wait, with 24 hours typical. On a new shower it proves the membrane before tile. On an existing shower it tests the floor assembly only. New membranes need cure time first, and the published waits differ.

Three membranes compared, from their own data sheets

When a shower is opened, the membrane in it, or its absence, decides the rebuild. These three products are common in US showers; the figures are from each maker’s current sheet.

Item	Custom RedGard	Laticrete Hydro Ban	Schluter KERDI-DS
Standard	ANSI A118.10 and A118.12	Exceeds A118.10 and A118.12	ANSI A118.10-SV
Form	Liquid, two coats	Liquid, two coats	Sheet, 20 mil
Cured thickness	0.015 to 0.038 in	20 to 30 mils	20 mil
Vapor permeance	0.35 perms at 30 mils	1.247 perms	0.19 perms
Before flood test	Both coats cured; 1 to 1.5 h a coat, up to 12	2 h at 70F; 24 h at 50 to 69F	24 h
Plain drywall	Interior dry areas only	Gypsum wallboard listed	Handbook lists gypsum wallboard for KERDI

Permeance matters for steam showers, not for ordinary leaks. The Laticrete Hydro Ban data sheet DS-663 says not to use it as a vapor barrier, especially in steam rooms, while the KERDI-DS sheet presents its 0.19 perms for continuous-use steam rooms.

Tile on cement board, drywall or greenboard

The Philadelphia Code Bulletin B-0001 R1 says paper-faced green board is not a waterproof material, will deteriorate if subjected to extended or repeated water, and must not be used in continuous high humidity. It quotes the city’s IBC-based section B-2509.2 requiring glass mat gypsum, fiber-cement or fiber-mat reinforced cement sheets as the base for wall tile in tub and shower areas. For houses, Walls and Ceilings magazine reported in August 2004 that from the 2006 IRC, paper-faced green board no longer met the standard as a wet-area tile backer, while ASTM C1178 glass mat board remained approved.

Substrate	What the sources say	Behind wet tile
Cement or fiber-cement backer	A tile base for tub and shower areas under Philadelphia code B-2509.2	Not waterproof; water passes to the cavity without a membrane
Glass mat gypsum (ASTM C1178)	Also listed in B-2509.2; stayed approved in the 2006 IRC, per Walls and Ceilings	Same: a backer, not a barrier
Green board, water-resistant gypsum	Not waterproof; deteriorates with extended or repeated water, per Philadelphia	No longer an approved wet-area tile backer from the 2006 IRC, per Walls and Ceilings
Plain drywall under a membrane	Manufacturer by manufacturer, per TCNA	RedGard: dry areas only; Hydro Ban and Schluter list it

The TCNA showers FAQ says makers disagree on drywall under a membrane, calls the IBC somewhat ambiguous, and says to check with a local code official.

The repair scope: regrout or tear out to the studs

Work down the list and stop at the first yes.

1. Backer soft, or tiles loose across a wall? Tear that wall out to the studs. A wet backer cannot be dried reliably behind tile.
2. Floor grout crusted or dark, stain below? Flood test. A failure means floor tile, bed and pan; the pan is a plumbing job.
3. Wet only at a corner, the tray or the tub joint? Cut out and reseal the joint with sealant, let it dry, re-read.
4. Water standing in a niche or on a bench? Rebuild that element with slope.
5. Cracked grout, sound tiles, readings fall back to dry? Regrout. It restores the joint; it does not add a missing membrane.
6. Growth on the backer or framing once opened? It is now a mold scope as well.

According to This Old House, if the tiles themselves are damaged, retiling is the better option, and removing old tile adds a few hundred dollars. Two other cost guides refused our fetch, so this is a single published source, shown as printed.

Job	Published range	According to
Regrout a shower	$600 to $2,500, about $1,500 average	This Old House, updated 5 June 2026
Tile a shower	$1,800 to $5,000	This Old House
Remove old tile, per sq ft	$2 to $5	This Old House
Epoxy grout, per sq ft	$8 to $12	This Old House

Mold behind tile

The EPA mold and moisture guide lists the back side of drywall and the inside of walls around leaking pipes among places mold hides, and says to suspect hidden growth when a building smells moldy but nothing shows. A leaking shower wall fits both. Growth on the grout face is a cleaning question, on mold in shower; growth on the backer or studs is a removal question, and the size threshold is on DIY mold remediation. Drying an opened cavity is on water damage in walls, and the board on the far side on wet drywall.

Escalation triggers

• Damage on the far side of the wall or a ceiling stain below; see early ceiling signs.
• More than a few tiles hollow or moving.
• Efflorescence that returns after cleaning.
• A musty smell with nothing visible.
• Readings that stay high days after the shower was last used.

For contractors: scoping water behind tile

The scope turns on what is behind the tile, so record it when the first tile comes off. Duration is the coverage question. The Eighth Circuit, Croxdale v. Travelers (2026) quotes a policy form excluding constant or repeated seepage or leakage over 14 days or more, and a shower leak is often slow by nature. Describe what you find; coverage is for the policy, the insurer and the state insurance department. See homeowners insurance and water damage.

Record	Why it matters
Meter readings, wet and dry reference	Shows extent beyond the visible joint
Flood test: fill level, duration, result	Separates floor from wall failure
What is behind the tile once opened	Membrane present or not drives the rebuild
Grout, sealant and slope condition	Cause, and whether it was long term
Far side and ceiling below	Collateral scope

Questions people actually ask

How do I know if there is water behind my shower tile?

Look for grout that stays dark long after a shower, white crust in the joints, tiles that sound hollow or move, a gap at corners, and damage on the far side of the wall or the ceiling below. Confirm with a moisture meter compared against dry tile, then a flood test of the floor.

Can water leak through grout?

Yes. The Tile Council of North America says grout joints are not waterproof, and Schluter says the tile covering itself is not waterproof. Over a working membrane or pan liner that water drains away. A grout leak that wets the wall behind means the waterproof layer has failed too.

Will regrouting stop a shower leak?

Only if the leak was the grout and the layers behind are sound and drying. Regrouting restores the joint; it does not add a missing membrane or fix a failed pan. If the backer is soft or tiles are loose across a wall, the scope is a tear-out.

Can water behind tile dry out on its own?

Some can, over a membrane that drains to the weep holes or the drain. A saturated bed over a clogged pan or a board with no membrane does not dry between showers, and Schluter links that saturation to efflorescence and mold. Readings that stay high days after use mean it is not drying.

Is green board OK behind shower tile?

Not as the wet-area backer. Philadelphia’s code bulletin says it is not waterproof and deteriorates with repeated wetting, and Walls and Ceilings reported it lost wet-area tile backer approval from the 2006 IRC. Check the code edition your jurisdiction adopted.

How do you flood test a shower pan?

Plug the drain, fill the base to just below the drain flange, mark the water level and leave it, typically 24 hours according to Schluter. A falling level or water below means a leak. New membranes need their published cure time first: Laticrete gives 2 hours at 70F for Hydro Ban.