Tiling Over a Concrete Slab: Prep, Membranes, and the Right Mortar

New concrete needs to cure a full 28 days before tile goes down, per the TCNA Handbook. That's not a superstition — a slab is still shedding mix water and shrinking during that window, and thinset bonded to a slab that's still moving loses adhesion at the tile-mortar interface long before you notice a visible crack.

Flatness comes next. Lay a straightedge across the floor in multiple directions and measure the gap underneath: the ANSI A108.02 tolerance is 1/4 inch (0.25 in) of variation per 10 feet for standard tile, and tightens to 1/8 inch (0.125 in) per 10 feet once you're setting anything with an edge of 15 inches (15 in) or longer. Grind the high spots and skim the low spots with a cementitious patch or self-leveler before you buy a single tile — mortar is not a leveling compound, and troweling extra thinset into a low spot just creates a soft, hollow-sounding pocket that debonds under foot traffic.

Moisture matters even though thinset is cementitious and more forgiving than glue-down flooring. If the slab is on or below grade, or you're installing a waterproofing or uncoupling membrane that specifies a moisture ceiling, run a calcium chloride test per ASTM F1869 (the common threshold is 3 lb per 1,000 sq ft per 24 hr) or an in-situ relative humidity probe per ASTM F2170, where readings above 75% RH call for a vapor mitigation coating before anything else touches the slab. Skipping this step doesn't usually crack the tile — it shows up as efflorescence bleeding through grout lines or a membrane that never fully cures.

Walk the whole slab and mark every visible crack with its width. Anything under 1/8 inch (0.125 in) is a candidate for a standard crack-isolation membrane. Wider cracks, cracks with vertical offset, or ones that open and close seasonally point to a structural or slab-on-grade movement issue that a membrane alone won't solve — that needs a control joint honored through the tile field or a structural evaluation before you tile over it.

A crack-isolation membrane — sheet or liquid-applied — is the standard answer for any slab crack up to 1/8 inch (0.125 in), and it's cheap insurance to run it over the slab's existing control and construction joints too, cracked or not, since those are exactly where reflective cracking starts.

An uncoupling membrane does a different job. A typical sheet product runs about 3 mm (3 mm) thick and creates an air gap that lets the tile layer move independently of the slab, which matters most where you expect ongoing seasonal movement, over a slab with several hairline cracks rather than one clean line, or when you want a small amount of built-in cushioning under a large-format floor.

In showers, laundry rooms, and anywhere water reaches the floor regularly, the membrane needs to waterproof as well as isolate — check /guides/choosing-tile-for-wet-areas before assuming a crack-isolation product alone covers you.

Cement backer board isn't part of this conversation on a slab — it exists to give tile a stable substrate over wood framing, not over concrete. If you're used to backer board detailing, the comparable prep sequence for a wood subfloor is covered separately in /guides/tiling-over-cement-backer-board-prep-membranes-and-mortar and /guides/tiling-over-osb-subfloor-prep-membranes-and-mortar, worth a look if the same job has both a slab area and a framed addition.

Skip the membrane over a known crack and the failure mode is predictable and cosmetic before it's structural: the crack reflects straight up through the tile and grout joint above it, usually within one to two heating-cooling cycles, and once it shows there's no membrane retrofit that doesn't involve pulling tile.

For porcelain or ceramic set directly on bare concrete, a polymer-modified thinset is the default — the added polymers give it the bond strength to hold a dense-bodied tile to a rigid, non-absorbent substrate. If you're setting over an uncoupling membrane, check the membrane manufacturer's mortar requirement specifically: some fleece-backed membranes are designed for unmodified thinset because the membrane needs contact moisture to cure the mortar correctly, and modified thinset can starve of moisture against it.

Any tile with an edge of 15 inches (15 in) or longer needs a large-format tile (LFT) mortar with enough polymer content to resist slump and hold full coverage under the tile's weight. Standard thinset spread too thin under an LFT piece is a common cause of hollow spots that only show up on the sound test after the floor is finished.

For coverage math, a 50 lb (50 lb) bag of modified thinset typically covers around 90 sq ft (90 sq ft) at a 1/2 in x 1/2 in notch trowel, though actual coverage swings with trowel notch size, tile flatness, and how much of that coverage gets consumed leveling minor slab irregularities. Run your exact tile size and trowel notch through /guides/thinset-mortar-calculator before ordering — it's the step that keeps you from a mid-job run to the supply house or a stack of unopened bags going back.

Once the substrate prep and mortar are settled, the rest of the job is standard layout and material math — use /floor-tile-calculator to turn your room dimensions into tile counts and waste allowance, and /tile-estimator if you're pricing multiple rooms or comparing tile formats before you commit to one.

Each prep step maps to a specific, recognizable failure — useful for diagnosing a bad tile job after the fact, or for knowing exactly what you're risking if you're tempted to shortcut one now.