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Hollow Spots and Flex Under a Reglazed Tub: What to Do

By Tub Reglazing Directory Editorial Team · Updated 2026-07-29

That soft thud when you step onto your tub floor, the one that sounds like you’re walking on a drum, is not nothing. Neither is the slight give you feel when you shift your weight. Both are signs that the substrate beneath your tub is unsupported, and if you’re considering reglazing, they matter a great deal. Skip the structural fix and you’re not getting a reglaze. You’re getting a coating that will peel, crack, and delaminate within months, no matter how skilled the technician or how expensive the product.

This is one of the more persistent misconceptions in tub refinishing: that a fresh coating will stabilize a failing surface, seal everything in, and buy you years of use. It won’t. Flex stress shears a bonded coating at the flex point. That’s physics, and no two-component epoxy changes it. What a good contractor does before touching a spray gun is figure out whether your tub can actually hold a coating, then fix it if it can’t.

Here’s what’s actually going on under your tub, how it gets repaired, and what you should be asking any contractor before you agree to work.

Why Hollow Spots Form in the First Place

Most bathtubs installed over the past 50 years, especially fiberglass and acrylic units, were never designed to be freestanding structural elements. They’re shells. They rely on continuous support from below to distribute your weight across their surface without flexing.

The traditional solution is a mortar bed: a mix of portland cement and sand packed to a damp consistency and spread beneath the tub basin before it’s set. When it cures, it becomes a rigid, load-distributing substrate. The Portland Cement Association documents this clearly. A properly installed mortar bed spreads occupant load continuously across the tub floor, preventing point loading and fatigue. Without it, every step concentrates force at a small area of the shell, which flexes, fatigues, and eventually cracks.

Here’s where the hollow spots come from. Mortar beds fail for a few reasons: poor original installation (the mix was too wet, the coverage was incomplete), water infiltration from a leak that eroded the bed over years, or settlement that opened voids between the cured mortar and the tub floor. In some installations, particularly quick residential renovations from the 1980s and 1990s, the mortar bed was skipped entirely. The tub was dropped in, caulked, and called done. It works until it doesn’t.

Acrylic and fiberglass shells are thin enough that even small voids produce the hollow sound you hear. Cast iron is too heavy to flex the same way, but steel tubs absolutely can. If you hear it, there’s a void.

What Flex Does to a Reglazed Coating

This is the part homeowners consistently underestimate, so let’s be direct about it.

When a tub floor flexes under load, the substrate moves. A reglazed coating is chemically bonded to that surface. When the surface moves and the coating can’t move with it (because it’s a rigid film adhered to a substrate that has just deflected), the bond shears. It starts as a microcrack, invisible at first. Then the coating bubbles. Then it peels.

The timeline depends on the magnitude of the flex and the frequency of use, but we’re typically talking months, not years. The ANSI A137.1 deflection limits for tile setting beds exist precisely because engineers have quantified how much substrate movement a bonded surface layer can tolerate before bond failure. Reglazing technicians cite those same limits when they assess fiberglass and acrylic tubs, because the failure mechanism is identical.

There’s a safety angle here too. ASTM F462, reaffirmed in 2020, sets slip-resistance thresholds for recoated bathing surfaces. When a coating microcracks from flex stress, its surface texture changes in unpredictable ways. The coating may no longer meet the friction thresholds it was tested to deliver. A delaminating or cracked coating in a wet tub is a slip hazard, full stop.

The Mortar Bed: What It Is and How It Gets Restored

A mortar bed isn’t complicated, but restoring one in an existing installation is more involved than installing one during new construction, because you’re working around a tub that’s already plumbed.

In new installations, the mortar bed goes in before the tub is set: a 1-to-1.5-inch layer of damp-pack mortar (portland cement, sand, sometimes a small amount of lime) spread across the subfloor, screeded roughly flat, and then the tub is pressed into it while it’s still workable. The mortar conforms to the underside of the tub and cures around it.

In a retrofit situation, say your tub has been in place for 20 years and the mortar bed has failed, the options are more limited. Some contractors inject material through access panels or drilled ports. Others work through a partial removal of the tub surround to reach the underside of the tub basin. The right approach depends on the tub material, the installation type (drop-in, alcove, freestanding), and how much of the mortar bed has failed.

IRC Section R307 requires adequate structural support beneath bathtub installations to prevent movement under occupant load. That’s a baseline code expectation, not a nice-to-have. Your local jurisdiction may have adopted the IRC with amendments, or may still reference an older UPC or SBC edition with different substrate language. Worth checking what code cycle your municipality is on before assuming the current IRC applies verbatim.

Expanding Foam: The Shortcut That Isn’t Equivalent

Polyurethane expanding foam injection has become a common field alternative to mortar bed restoration. It’s faster, it requires no removal of the surround, and contractors can often do it in a single visit.

The problem is that foam is not a mortar bed. Foam compresses under sustained load (compression creep) and gets worse over time with repeated thermal cycling. A mortar bed stays rigid. Foam softens slowly under the weight and heat of use. Some manufacturers explicitly prohibit foam as the sole support method in their installation guides. Check the TDS document for whatever product your contractor plans to use before accepting foam as the solution.

No current model code addresses expanding foam as an accepted mortar bed alternative. Its use is entirely contractor-driven field practice, which means there’s no standardized specification and no established performance benchmark. Some contractors have had good results with it in low-flex situations. Others have seen it fail within a few years.

If a contractor proposes foam injection, ask them specifically: does the coating manufacturer they’re using permit foam as the sole support method in their TDS? If they can’t answer that from the product documentation, or if the answer is no, insist on a more durable fix.

How a Good Contractor Assesses Substrate Before Quoting

Reputable contractors don’t skip this step, and it takes less than five minutes.

Napco’s technical documentation requires technicians to press firmly on the tub floor and listen for hollow sounds, then observe whether visible deflection occurs under load. Ekopel 2K’s TDS states the same requirement differently: the substrate must show no signs of delamination, cracking, or deflection under load, or the product’s performance claims are void. These aren’t suggestions. They’re conditions of the product warranty.

The tap test is straightforward: rap your knuckles across the tub floor in a grid pattern. A solid mortar bed returns a flat, dense sound. A void returns a hollow knock, noticeably different. Most experienced technicians can map the voids in a few minutes and give you a rough sense of how much of the bed has failed.

Flex testing is equally simple. Stand on the tub floor, shift your weight toward the center, and watch for visible movement. Press down hard with your hands. If the surface deflects more than about 1/8 inch under moderate pressure, you have a problem.

PRG industry guidance is explicit: coatings applied over unstable, hollow, or flexing substrates will fail prematurely regardless of coating quality, and contractor warranties are typically voided when substrate defects aren’t corrected before reglazing. A contractor who doesn’t perform this assessment before quoting is either inexperienced or hoping you won’t ask questions.

If you’re searching for tub reglazing pros in New York, ask specifically during the phone call whether substrate assessment is part of their pre-job process. That question alone filters out the operators who skip it.

When the Tub Can’t Be Saved

Sometimes a structural assessment leads to a different conclusion: the tub isn’t worth reglazing.

Severe cracking in a fiberglass shell, not surface crazing but structural cracks that go through the laminate, can’t be reliably repaired before coating. The repaired crack will flex differently than the surrounding shell, and the coating will fail at that boundary. The same holds for acrylic tubs that have been stressed to the point of delamination within the shell itself (the inner and outer layers of an acrylic composite can separate).

Warped or badly deformed shells are similarly disqualifying. If the tub floor has taken a permanent set from years of unsupported load, the surface geometry has changed enough that coating adhesion will be uneven.

Water damage to the subfloor beneath a failed mortar bed is another issue, or at least a sequencing problem. You can’t restore substrate support on a rotted subfloor. The structural repair has to precede the mortar bed work, which has to precede the reglazing. At that point, replacement is often the more economical path.

A straight conversation with a contractor who does this work daily should get you to the right answer. Refinishers in Brooklyn and other markets see this exact situation regularly. They know when a tub is worth saving and when replacement makes more sense.

The Repair Sequence When Reglazing Can Proceed

When the substrate issues are addressable, the repair follows a defined sequence.

First, any existing failed coating gets stripped. If the previous reglaze delaminated because of substrate flex, the coating has to come off before anything else can be assessed or repaired. This is where chemical hazards become relevant. Contractors using methylene chloride-based strippers work under OSHA 29 CFR 1910.1052, which sets a permissible exposure limit of 25 ppm as an 8-hour time-weighted average and a STEL of 125 ppm over 15 minutes. Bathroom stripping jobs push those limits fast in confined spaces without proper ventilation. The EPA classifies methylene chloride as a probable human carcinogen and has pursued TSCA Section 6 restrictions on consumer-accessible strippers containing the compound.

Second, the substrate gets repaired: mortar bed restoration, void filling, subfloor repair if needed. This step needs to cure fully before anything goes over it.

Third, the reglazing proceeds. For two-component coatings (the durable ones, using isocyanate hardeners), EPA guidance requires ventilation controls and respiratory protection during application, with a curing exclusion period of 24 to 48 hours. Legitimate contractors working with these products operate under OSHA 29 CFR 1910.134, which mandates a written respiratory protection program, fit-tested respirators, and medical evaluation for workers spraying isocyanate-containing coatings in enclosed spaces. If your contractor can’t describe their respiratory protection program, that’s a problem worth taking seriously.

Questions to Put to Your Contractor Before Signing Anything

These aren’t trick questions. They’re how you separate the operators who know what they’re doing from those who don’t.

That last point matters legally. Under FTC Magnuson-Moss Warranty Act rules, contractors offering written warranties must disclose all material limitations, including substrate exclusions, before you agree to the service. If a contractor is offering a warranty but hasn’t told you that a hollow tub floor voids it, that’s a disclosure problem, not just a bad business practice.

Get the assessment findings in writing. Get the warranty language in writing. If either request creates friction, take that as information about how the job will go.

The structural fix isn’t an upsell. It’s the job. Any contractor who treats it as optional isn’t one you want working in your bathroom.

Frequently Asked Questions

Can I reglaze a tub that has a hollow spot under it?

Not safely, and not with any warranty protection. Both Ekopel 2K and Napco’s technical documentation explicitly void performance claims when their coatings are applied over a flexing or hollow substrate. The flex stress will shear the coating bond within months.

What causes hollow spots under a bathtub?

The most common cause is a missing or deteriorated mortar bed. Without it, the tub shell bears load unevenly, and voids form between the tub floor and the subfloor below. In fiberglass and acrylic tubs, the shells are thin enough that even minor voids produce audible hollow sounds under foot pressure.

Is expanding foam a good substitute for a mortar bed?

It’s a common field shortcut, not a code-recognized method. Polyurethane foam compresses over time under repeated load, a property called compression creep, and no current model code addresses it as an accepted mortar bed alternative. Some manufacturers explicitly prohibit it as the sole support method in their installation guides.

How do I know if my tub has been properly assessed before reglazing?

Ask the contractor whether they performed a tap test and flex check before quoting the job. Napco’s technical guidance requires technicians to press firmly on the tub floor and listen for hollow sounds. If a contractor skips this step or can’t describe their substrate assessment process, walk away.

What happens if I reglaze over a flexing surface?

The coating delaminates at the flex point, typically within months of normal use. Flex stress shears the bond between the coating and the substrate regardless of coating quality, the same deflection principle that ANSI A137.1 uses to set substrate movement limits for tile. Once the coating fails, you’ll likely need a full strip, substrate repair, and re-coat, which costs more than fixing it correctly the first time.

Does a warranty cover coating failure caused by a hollow substrate?

Almost never. PRG industry guidance states that reputable contractors void warranties when substrate defects are not corrected before reglazing. Under the FTC’s Magnuson-Moss Warranty Act rules, contractors offering written warranties must disclose those limitations before you agree to the work, so ask for it in writing.

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