The Flashlight Test: How to Spot Uneven Tile Before You Grout

I spent three days grinding concrete on a job last month just so the floor wouldn’t click like a castanet. My hands were numb from the vibration of the planetary grinder, and the silica dust was trying its best to bypass my respirator, but the results were finally flat. Most guys skip the leveling compound. They think the underlayment will hide the dip. It won’t. I have walked into too many high end kitchens where the homeowner is crying because their twelve dollar a foot Italian porcelain feels like a series of speed bumps under their socks. You cannot hide a bad subfloor with a expensive finish. The physics of gravity and the chemistry of thin-set are indifferent to your budget. If that substrate is not within one eighth of an inch over ten feet, your tile is going to fail. It is not a matter of if, but when. I have been doing this for twenty five years, and the most powerful tool in my bucket is not the wet saw or the laser level. It is a simple, high-lumen flashlight held parallel to the ground. This is the grazing light test, and it is the only way to catch lippage before it is set in stone.

The grazing light reveals the truth

The flashlight test identifies lippage and substrate variations by casting long shadows across the tile surface. This method exposes uneven tile edges, bowed planks, and high spots that standard overhead lighting hides. By placing a high-intensity LED light directly on the floor, you see installation errors and deflection issues before grouting makes them permanent. If you see a shadow, you have a ridge that will catch a toe or a vacuum cleaner head. It is the definitive diagnostic tool for any serious floor inspector or installer who values structural integrity over a fast paycheck.

When you place a light source at a low angle, you are essentially creating an artificial sunset across the horizon of your floor. Any tile that sits even a fraction of a millimeter higher than its neighbor will cast a dark, elongated shadow. In the industry, we call this lippage. According to the Tile Council of North America, or TCNA, lippage is the vertical displacement between the edges of two adjacent tiles. For grout joints wider than one sixteenth of an inch, the allowable lippage is often only one thirty-second of an inch. That is roughly the thickness of a credit card. If you are using large format tiles, which are any tiles with one side longer than fifteen inches, the tolerance for error drops to almost zero. These tiles often have a natural crown or bow from the kiln firing process. If you do not offset them correctly or use a sophisticated leveling system, that bow becomes a trip hazard. The flashlight does not lie. It highlights the failure of the installer to properly back-butter the tile or to address the undulations in the plywood or concrete below.

“A floor is only as good as the subfloor beneath it; deflection is the enemy of every joint.” – Master Flooring Axiom

Why your subfloor is lying to you

A subfloor is lying to you when it appears flat to the naked eye but contains microscopic undulations and dips that compromise tile bonding. These imperfections in concrete or plywood create voids in the thin-set mortar bed, leading to hollow sounds and eventual crack formation in the grout or the tile itself. You must use straightedges and moisture meters to verify the structural readiness of the substrate before a single drop of adhesive is applied. If the subfloor is not dead flat, the tile will follow the curve of the dip, creating a cumulative error that ruins the entire layout.

We need to talk about the molecular reality of concrete. A slab is a living, breathing sponge. It moves. It shrinks. It expands. If you pour a slab and try to tile over it in fourteen days, you are asking for a disaster. The concrete is still off-gassing moisture and undergoing chemical hydration. This movement will shear the bond between the thin-set and the substrate. I always tell my clients that we are not just laying tile, we are engineering a sandwich. The bottom layer is the subfloor, then the moisture barrier or uncoupling membrane, then the mortar, and finally the tile. Every layer must be compatible. If you are working over a crawlspace, you have to check the humidity. If the wood joists are at fourteen percent moisture and you seal them under a tile floor, that wood is going to dry out, shrink, and pull the floor apart. I have seen solid hardwood floors buckle so hard they popped the baseboards off the wall because the installer didn’t understand the physics of acclimation. Tile is even less forgiving. It has zero elasticity. If the subfloor moves, the tile breaks.

The chemistry of the bond

Tile adhesive chemistry relies on polymer-modified Portland cement to create a mechanical and chemical bond between the tile and the substrate. Modern thin-set mortars contain latex and resins that allow for slight flexural movements without losing adhesion. Choosing the correct mortar type, such as large and heavy tile (LHT) mortar, prevents the tile from slumping or sinking into the bed, which is a primary cause of lippage and uneven surfaces. You cannot use standard thin-set for a three foot porcelain plank and expect it to stay level while it cures.

Let us zoom into the microscopic level of the mortar. When you mix a bag of modified thin-set, you are initiating a complex chemical reaction. The water hydrates the cement particles, growing crystals that knit into the pores of the concrete and the back of the tile. If you use too much water, you weaken those crystals. If you use too little, the chemical reaction never finishes. This is why I get so angry when I see helpers just throwing a hose into a bucket and guessing. You use a measuring cup. You want the consistency of creamy peanut butter. If it is too thin, the tile will sink overnight as the water evaporates. You go to bed with a flat floor and wake up with lippage. This is known as shrinkage, and it is the bane of the lazy installer. For large format tiles, you need a medium bed mortar that is specifically designed to hold the weight of the tile without collapsing. This is where the physics of suction comes in. A properly notched trowel creates ridges that collapse when the tile is pressed down, ensuring one hundred percent coverage. If you have air pockets, you have weak spots. If you have weak spots, the flashlight test will show the slight tilt where the tile failed to seat.

“Standardization of the substrate is the first step toward a failure-free installation; shortcuts are just long-term liabilities.” – TCNA Handbook Principles

The 1/8 inch that ruins everything

The one eighth inch rule dictates the maximum allowable variation in a subfloor over a ten foot radius for ceramic and porcelain installations. Exceeding this tolerance leads to lippage and uneven grout joints that are highlighted by directional lighting. To achieve a professional grade finish, you must use self-leveling underlayment or mechanical grinding to bring the high and low spots into compliance. Ignoring this mathematical requirement ensures that the finished floor will feel unprofessional and structurally unsound underfoot.

I have a checklist for every job. If these steps aren’t followed, I don’t sign off. Most homeowners want to talk about grout colors or decorative borders. I want to talk about the L/360 deflection rating of their floor joists. If you jump in the middle of the room and the china in the cabinet rattles, your floor is too bouncy for tile. You need to sister the joists or add a layer of exterior grade plywood. We are building a rock, not a trampoline. When I pull out the flashlight and run it across the raw subfloor, I am looking for the shadows of the past. I am looking for where the previous builder got lazy. Every hump in the floor is a pivot point. If a tile sits on a hump, it will teeter. When you step on one end, the other end lifts. That is how you get lippage. People think grout is glue. It is not. Grout is a filler. It has no structural strength. If your tiles are moving because the subfloor is uneven, the grout will turn to powder and fall out within six months. Then the moisture gets in. Then the mold starts. It is a cascading failure that starts with an eighth of an inch.

• Check subfloor flatness with a ten foot straightedge.
• Inspect for moisture using a pin-type or pinless meter.
• Grind down high spots in concrete using a diamond cup wheel.
• Fill low spots with a high-compressive strength leveling compound.
• Vacuum all dust to ensure a clean mechanical bond for the primer.
• Verify that the tile back-buttering covers ninety percent of the surface.
• Perform the grazing light test before the mortar reaches final set.

The ritual of the grazing light

The grazing light test should be performed during the installation process and again before the grout is applied to the tile joints. By casting light horizontally, you can adjust leveling clips and seat tiles deeper into the mortar while it is still pliable and responsive. This proactive approach prevents costly tear-outs and ensures the aesthetic and structural quality of the floor. It is the final defense against mechanical errors that only become visible once the room is fully lit by natural sunlight or expensive light fixtures.

If you wait until the grout is in, you are too late. Grout creates a visual bridge between tiles, but it also locks everything in a rigid matrix. If you find a bad tile after grouting, you are looking at a hammer and a chisel. You risk damaging the surrounding tiles and the waterproofing membrane. I have seen guys try to sand down the edge of a porcelain tile to fix lippage. It is a joke. You ruin the glaze and leave a dull spot that sticks out like a sore thumb. You do the work right the first time. You use your eyes, you use your hands, and you use that flashlight. I have spent thousands of hours on my knees. My joints hurt when it rains, and my skin is permanently stained with thin-set gray. But when I turn on that flashlight and see a perfectly flat plane of tile with no shadows, I know that floor will last a hundred years. It is about pride in the craft. It is about knowing the difference between a floor that looks good in a brochure and a floor that is engineered to survive the reality of a busy household. Don’t let a lazy installer tell you that the grout will hide it. It won’t. The shadows are always there, waiting for the sun to hit the room at the right angle to reveal the truth of a sloppy job.