Does Injection Repair Meet the ANSI Coverage Standard?

ANSI A108.02 section 4.3.1 sets minimum thinset mortar coverage at 80% for interior floor installations and 95% for exterior tile and anything set in a wet area such as a shower pan or curb. The number describes how much of the tile's back has mortar transferred onto it, not how the finished floor sounds or looks.

The test only works during installation: the setter presses a tile into fresh mortar, then lifts it back off within the mortar's open time and measures the transfer pattern. It's a construction-time check meant to confirm the tile is sitting on a near-continuous bed rather than isolated ridges left by an undersized trowel, so the tile doesn't flex, crack, or come loose over unsupported gaps once foot traffic and moisture cycles start working on it.

Injection repair runs in the opposite direction from that test. A technician drills small holes into a tile that already sounds hollow, pumps epoxy or polyurethane resin into the void underneath, and caps the holes. Nothing gets lifted, so nobody measures what fraction of the original void the resin actually reached — the resin is relying on gravity and pressure to find whatever gaps, ridges, and low spots were left by the original spread, none of which were ever mapped or quantified.

ANSI's 80%/95% figures describe a mortar-bed condition confirmed at the moment of setting, before grout and years of use. Once a tile has been sealed and walked on, that specific test can't be run again without pulling the tile — which defeats the purpose of a repair. Injection repair isn't a failing grade against the standard; it's simply outside the scope the coverage clause was written to check.

In practice, injection can stop the hollow sound and stabilize a tile solidly enough that it never moves again, but the result still can't be documented as A108.02-compliant the way a freshly set tile can. That distinction matters most for warranty claims and liability disputes, less for whether the floor physically holds up.

Injection repair earns its place when the hollow sounding is limited to one tile or a small cluster, the tile itself isn't cracked, and there's no sign of lippage or movement around it. Stabilizing a small, isolated void with resin is faster and cheaper than demolition, and it holds up fine even without an ANSI certification attached to it.

It stops making sense when hollow sounding shows up across a large section of the floor, when it's paired with cracking, or when it's happening under large-format tile, since bigger tiles have more unsupported center area if coverage was thin to begin with. Any of those patterns points to a systemic coverage failure in the original install — likely never anywhere near 80%, let alone 95% in a wet area — and injecting a few tiles won't fix a pattern that runs under the whole floor. That's the point to pull tile and reset it to spec rather than keep drilling and patching.

If a full reset is where this lands, treat the affected area like a new install: verify the substrate is flat, choose a trowel notch sized for the tile format, and re-spread mortar to hit 80% (or 95% for wet or exterior tile) before setting anything back down. Use the /floor-tile-calculator to size mortar and tile needs for the section being redone, and check /tile-estimator if the repair footprint doesn't line up cleanly with full boxes.

On floors where nobody checked coverage at install, hollow spots tend to show up in expanding clusters rather than staying isolated. Injecting the first few tiles buys time, but if the trowel technique behind the whole floor never hit the mortar minimum, more tiles will start sounding hollow later — the repair treats the symptom on one tile while the same undersized spread sits under everything else.

Manufacturer and NTCA-referenced tile warranties typically require the original installation to follow ANSI A108 methods, coverage included. A repair performed after the fact — even a good one that fully silences the hollow sound — doesn't retroactively bring an out-of-spec installation into compliance, so a warranty claim built around inadequate mortar coverage can still get denied even after injection work is complete. Anyone counting on the repair to preserve warranty standing should confirm that with the manufacturer before paying for it.