What Is Chert Tile?

Chert tile is tile cut from chert, a natural rock made mostly of very fine-grained quartz. It is not a standardized performance grade, so the name alone cannot tell you how strong, slip-resistant, or moisture-sensitive a tile will be. Check the specific stone’s thickness, finish, absorption, and use rating, then install it as natural stone over a suitable, stable floor.

A chert sample can feel hard and dense in your hand, yet those qualities do not answer the questions that matter on a floor: Will it absorb water? Is its surface suitable for a wet room? Can the floor beneath it carry stone without too much movement? Those answers come from the actual tile and floor assembly, not from the word “chert.”

I approach a chert product as a natural-stone tile with a trade name that needs checking. The goal is to confirm what the stone is, understand its limits, and match it to a sound installation system.

What chert tile is, and what the name tells you

Chert tile is cut from chert, a sedimentary rock composed mainly of microcrystalline quartz. “Microcrystalline” means its quartz crystals are too small to see with the naked eye. Some chert is sold as flint, but neither name gives a reliable performance rating for a finished tile.

Chert can vary in color, texture, porosity, and finish. It may be sawn or polished, and its appearance can vary from piece to piece. Even within one product line, ask whether the tile comes from a consistent batch and whether the supplier can provide technical information for that specific material.

Most important, “chert tile” is not a tile-performance class. Do not assume that a tile is suitable for showers, outdoor use, or floors simply because it is stone or quartz-rich. Request the product’s stated applications and test data, where available.

How chert compares with other flooring

The table compares material types, not specific products. Actual performance depends on the tile, its finish, the substrate, and how the floor is installed.

For chert, a sample board can help you judge color and texture, but it does not prove floor suitability. Ask for written specifications rather than relying on a sales description such as “hard,” “waterproof,” or “commercial grade.”

How to read a chert tile specification

A chert specification should help you identify the product and decide where it can be used. Look for measurable information, test methods, and clear limits. If a key value is missing, ask the supplier or manufacturer rather than treating the stone’s name as a substitute.

Check these items before ordering:

• Thickness and size: Measure the actual tile. Thickness affects transitions, mortar selection, and finished floor height.
• Absorption: This is the amount of water the tile takes in under a test. It can help guide use and care, but it does not make a tile waterproof.
• Finish: A polished, honed, brushed, or textured surface can behave differently underfoot and may need different care.
• Use rating: Confirm whether the specific product is approved for floors, wet areas, or exterior exposure.
• Slip data: For a level interior wet floor where the test applies, request the tile’s ANSI A326.3 DCOF result and use classification. DCOF means dynamic coefficient of friction, a measure of how much a surface resists slipping under specified test conditions. A value of 0.42 is not a universal guarantee of safety.
• Batch consistency: Ask whether the tiles may vary in color, thickness, or surface. View several pieces, not just one.

A specification that says only “chert, natural stone” is incomplete for installation planning. Obtain the tile’s technical sheet, installation instructions, and care guidance, then compare them with the requirements of the mortar and other system products.

Chert Tile Substrate Inspection, Deflection & Subfloor Preparation

The substrate is the prepared surface beneath the tile. It must be clean, flat, and stable, and a stone floor needs enough stiffness to limit bending. A membrane or backer board cannot correct a floor frame that moves too much under load.

Start by identifying the substrate: wood framing, concrete, or an existing floor. For a wood-framed floor, check joist size, spacing, span, condition, subfloor layers, and fastening. Natural-stone floor assemblies must meet a deflection limit of L/720. In simple terms, deflection is how far a floor bends; L/720 limits that movement in relation to the span. The full floor assembly matters, not just one layer.

An uncoupling membrane may help manage certain stresses between layers, depending on the approved system. It does not make an insufficiently stiff floor meet L/720. Excess movement can crack or debond stone tile and grout even when the membrane is installed correctly.

Flatness is different from level. A floor can slope and still be flat; flatness describes whether it has dips or bumps. Under the applicable ANSI A108.02 requirement, the usual maximum variation is:

• Tile with an edge 15 inches or longer: 1/8 inch in 10 feet.
• Smaller tile: 1/4 inch in 10 feet.

Confirm the current requirement for the tile format and installation. Check with a long straightedge, and correct high and low areas before setting tile. Do not try to make up major floor defects with uneven mortar thickness.

For concrete, inspect for cracks, contamination, and moisture. When required by the selected system, test moisture using ASTM F2170 (in-situ relative humidity) and/or ASTM F1869 (moisture vapor emission rate, or MVER). There is no one allowable RH percentage or MVER that works for every chert tile installation. Follow the limits for the specified mortar, membrane, and any moisture-control product.

Before proceeding, confirm the tile thickness and finish, examine the whole floor, and make a small layout mock-up. Correct structural or flatness problems first.

Chert Tile Installation: Mortar Selection, Coverage & Precision Setting

Mortar bonds the tile to the prepared substrate. Select a mortar that the manufacturer explicitly approves for natural stone and the substrate, often a product meeting ANSI A118.4 or A118.15 when required by the system. The label and installation instructions matter more than a general claim that a mortar is “premium.”

Plan the installation in this order:

1. Check compatibility. Confirm that the mortar, backer or membrane, grout, and any moisture-control products can be used together. Follow each product’s written instructions.
2. Set the layout. Dry-lay pieces to see color and size variation. Choose joint width and starting lines before mixing mortar. Keep the tiles in the same orientation if the supplier recommends it.
3. Apply mortar evenly. Select a trowel notch based on tile size and the product instructions. Comb the mortar in a consistent direction. Back-butter a tile, meaning spread a thin coat on its back, when needed to improve contact.
4. Check the bond. Lift tiles from time to time while the mortar is fresh. This shows whether the ridges are collapsing and whether the tile back is making enough contact. Adjust the trowel or method if it is not.
5. Keep joints clear. Remove mortar from grout joints before it hardens. Avoid spreading more mortar than can be covered before it skins over.

Aim for at least 80% mortar contact in dry interior areas and 95% in wet areas and exterior applications. These are minimum coverage targets, not a reason to accept large hollow spots. Do not spot-bond, or “dot mount,” tiles with separate blobs of mortar. Voids can leave the stone less supported and increase the risk of breakage or debonding.

Keep tile faces aligned, manage lippage (uneven height at adjoining edges), and protect the installation for the required cure time. Do not assume that a thicker mortar bed will fix an uneven substrate.

Transitions, Movement Joints & Slip-Safety Considerations

Movement joints are flexible gaps that allow a tiled floor to expand, contract, or shift without forcing the tile and grout to carry all that stress. Tile installations need the right joints at edges and other required locations, with placement based on the room and exposure.

Follow TCNA EJ171 guidance for movement joints, including perimeter joints and transitions. Their spacing and size depend on factors such as location, sunlight, and exposure. Use the project design and product instructions to determine exact placement. Do not fill a required movement joint with grout; use a compatible flexible sealant or joint system.

Plan transitions to adjoining floors before setting chert. The required height depends on the tile’s thickness and the full installation build-up, including mortar and underlayment. A mock-up can reveal a height difference before it becomes difficult to correct.

For level interior wet floors, check applicable ANSI A326.3 DCOF test data and use classification for the actual tile finish. DCOF is one factor, not a complete prediction of slip safety. Water, soap, wear, cleaning, footwear, and the way a space is used also matter. Do not rely on a single number or a “slip-resistant” label without knowing the test and intended application.

Chert Tile Maintenance, Sealing & Long-Term Protection

Chert care depends on the specific stone, finish, grout, and sealer. A sealer may reduce staining, but it does not turn tile into a waterproof surface or replace sound grout, movement joints, or moisture control below the tile.

First, ask the supplier whether the stone needs sealing and which product is compatible. If unsure, test a spare piece: follow the sealer instructions, then compare the treated and untreated surfaces. Do not seal by habit, since some stones and finishes may not need it, and a product can change appearance.

Use the cleaning method recommended for the stone and grout. Avoid harsh products unless the tile supplier confirms they are suitable. Follow sealer cure and reapplication instructions, and remove spills promptly when the care guidance calls for it.

Practical installation scenarios

These examples are diagnostic scenarios, not reports of named projects. They show how to apply the requirements before a problem is covered by tile.

• Wood floor with a membrane: The floor feels firm, but the joist and subfloor assembly has not been checked for L/720. The correct next step is to assess the full framing and subfloor assembly. Adding an uncoupling membrane alone does not resolve inadequate stiffness.
• Concrete floor with dark patches: The slab looks dry, but that does not establish acceptable moisture levels. Check whether the chosen system requires ASTM F2170 or F1869 testing, then meet that system’s stated limits before installation.
• Large-format chert with hollow spots: A lifted tile shows that mortar ridges have not made enough contact. Stop and adjust the trowel or setting method to reach the required coverage; do not leave voids or use isolated mortar dots.

Chert Tile FAQ

Is chert tile a type of quartz tile?
Chert is a natural rock composed mainly of microcrystalline quartz. It is not the same as engineered quartz surfacing, and the name alone does not provide a tile performance rating.

Is chert tile suitable for a shower?
Only if the specific tile is approved for that use and its finish, absorption, and care needs are suitable. The entire shower assembly also needs a compatible waterproofing and tile system.

Is chert tile waterproof?
Do not assume so. Chert and grout can have different absorption characteristics, and a sealer is not a substitute for a properly designed waterproofing system.

Can I install chert tile over a wood floor?
Yes, if the full floor assembly is suitable for natural stone, including the required L/720 stiffness, proper subfloor construction, and a compatible backer or membrane system.

Does an uncoupling membrane make a floor stiff enough for chert?
No. It may serve a role allowed by its manufacturer, but it does not correct excessive joist or subfloor movement.

How flat should the floor be for chert tile?
For tile with an edge 15 inches or longer, the usual maximum variation is 1/8 inch in 10 feet. For smaller tile, it is 1/4 inch in 10 feet. Confirm the applicable ANSI A108.02 requirement.

What mortar should I use?
Use a mortar explicitly approved for natural stone and the substrate. The selected installation system may specify a mortar meeting ANSI A118.4 or A118.15.

How much mortar coverage does chert tile need?
Target at least 80% contact in dry interior areas and 95% in wet or exterior applications. Lift tiles during setting to check coverage.

Does a DCOF of 0.42 mean chert is slip-proof?
No. That value is not a universal safety guarantee. Check applicable ANSI A326.3 test data and use classification, as well as the finish and conditions where the tile will be used.

Should chert tile be sealed?
Not automatically. Follow guidance for the specific stone and finish, and test a spare piece if the need or effect of sealer is unclear.

The central decision is straightforward: treat chert as variable natural stone, not as a guaranteed performance grade. Verify the product, meet the stone-floor stiffness and flatness requirements, and use compatible materials with proper coverage and movement joints.

(This article was written by one of our staff writers, Marcus Vance. Visit our Meet the Team page.)