A tile floor fails from the bottom up. By the time you see a crack running across three tiles in a straight line, or a grout joint that keeps reopening, the mistake is buried under the finished floor.
The short answer is this. A bathroom tile floor over a slab is installed in a fixed order: find the slab's joints, make the slab flat enough for the tile size, add the right membrane where one is needed, set the tile with full mortar contact, and finish with flexible sealant, not grout, at every joint and inside corner. Most of what decides whether the floor lasts sits under the tile and around its edges.
A Tile Floor Is a Stack, and Every Layer Has a Job
From the bottom up, a tile floor is a slab or a wood subfloor, then whatever was done to make it flat, then a membrane where one is needed, then the mortar that bonds the tile, then the tile, then the grout between it. This post covers tile over a concrete slab, which is what a ground-floor bathroom in a slab-on-grade house sits on, and it is where our flooring and bathroom remodeling work overlap. A second-story bathroom over wood framing is a different assembly with its own rules, and it is not covered here.
You see the top two layers, and the ones you never see decide whether the floor lasts. That is why two quotes for the same tile can look alike while only one of them has priced the prep underneath it.
What the Code Says About the Slab
The Florida Building Code, Residential, 8th Edition (2023), Chapter 5 covers floors. Section R506.1 says: "Concrete slab-on-ground floors shall be designed and constructed in accordance with the provisions of ACI 332 and this section."
The building department for your address enforces the Florida Building Code, and it can add local amendments and its own procedures, so it has the final word on how any of this applies to your project.
The vapor retarder
Section R506.2.3 reads: "A 6-mil ... polyethylene or approved vapor retarder with joints lapped not less than 6 inches ... shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists." Its exceptions are unheated structures and small unheated rooms, carports, flatwork such as driveways and patios, and cases the building official approves for local site conditions. A conditioned bathroom is not among them.
On a remodel, you usually cannot see whether one went in decades ago, so ask your installer how they will treat a slab whose history nobody can confirm.
The joint your slab was designed to crack along
Section R506.2.4.2 requires that slabs on ground be provided with contraction joints "in accordance with ACI 332 or other approved industry standards". It then allows crack containment instead: synthetic fiber reinforcement complying with ASTM C1116, or welded wire fabric located in the middle to the upper third of the slab. It closes with an exception for cases "where random cracking is acceptable to the building owner".
Read that closely. A contraction joint is not a defect. It is a line the slab was given on purpose so it would crack there instead of somewhere random, which means the saw cut crossing your bathroom floor is expected to move. The only question is what gets bonded across it.
If your bathroom is in a condo
A bathroom on an upper floor of a condo building sits on an elevated structural slab, not a slab on ground, so Section R506 does not describe it. The tile standards below still apply, with tighter joint spacing. The association also has its own say first, which is covered in our guide to condo remodeling in Southwest Florida.
The Standards Your Tile Setter Is Working To
Above the slab, the rules of the trade are the ANSI A108 and A118 tile standards. The Tile Council of North America keeps the current list of ANSI tile standards. They are industry standards, not a Florida code section, and you can ask for a quote that follows them.
Writing in TileLetter, the magazine of the National Tile Contractors Association, Scott Carothers of the Ceramic Tile Education Foundation, in a May 2023 article, quotes ANSI A108.02 on the point that matters most for a floor over a slab. Section 4.4.1 says: "Movement joints are required over all construction, control, and expansion joints in the backing and where backing materials change or change direction including terminations of tilework where it abuts restraining or dissimilar surfaces." Section 4.4.2 is a single sentence: "Movement joints are a requirement for tilework."
The same article lists the spacing. Interior tile gets a joint at a maximum of 25 feet in each direction. Interior tile exposed to direct sunlight or moisture, and tile on an above-ground concrete slab, gets one at a maximum of 12 feet in each direction.
In most bathrooms, the joints that decide everything are the ones the room already has. One goes directly over any joint in the slab. The other runs around the perimeter, where the tile meets the walls, the shower curb and a different floor at the doorway.
Carothers notes that the perimeter joint at the walls should be at least 1/4 inch wide and is "commonly covered by tile trim, cove base, or shoe molding." Because neither joint shows in a finished room, both are easy to leave out, so ask about them by name.
Flatness Is a Line Item
Large tile does not bend. The bigger the tile, the more an uneven slab shows up as lippage, one tile edge sitting higher than its neighbor, which you feel with a bare foot before you see it.
In a 2020 article, the Ceramic Tile Education Foundation quotes ANSI A108.02 Section 4.1.4.3.1: for tiles with at least one edge 15 inches or longer, the surface may vary by "no more than 1/8” in 10’" and "no more than 1/16” in 2’" from the required plane, measured from the high points. The same article says to flatten the floor with "the appropriate patch or self-leveling compound (with the same manufacturer’s primer), but never thin set mortar."
That makes flattening a real line item: grinding high spots, patching or self-leveling low ones, and letting it cure. If a quote for large tile has no allowance for it, ask where the flatness will come from.
Mortar Coverage Is a Number, Not a Feel
The standard also says how much of each tile has to touch the mortar. The Ceramic Tile Education Foundation quotes ANSI A108.5 Section 3.3.2, the section on setting floor tile, from the 2017 edition: "The average uniform contact area shall be not less than 80% except on exterior or shower installations where the contact area shall be 95% when no less than three tiles or tile assemblies are removed for inspection." It adds that the coverage must give full support "with particular attention to this support under all corners of the tile."
A dab of mortar at each corner and one in the middle cannot meet that. Ask how the installer checks coverage. Lifting a freshly set tile to look at the back is the test the standard itself describes.
Waterproofing and Crack Isolation Are Two Different Jobs
Section R307.2 of the Florida Building Code, Residential, Chapter 3 is short: "Bathtub and shower floors and walls above bathtubs with installed shower heads and in shower compartments shall be finished with a nonabsorbent surface. Such wall surfaces shall extend to a height of not less than 6 feet (1829 mm) above the floor."
That is a rule about the finish surface in the shower and at a tub with a shower head. It does not address the open floor between the vanity and the door. That is not an argument against a membrane out there, only for knowing which problem you are paying to solve. ANSI publishes separate standards for the two jobs, both on the TCNA list linked above. A118.10 covers "Load Bearing, Bonded, Waterproof Membranes", which keep water from getting through. A118.12 covers "Crack Isolation Membranes", which keep movement in the slab from cracking the tile above it.
Both are sold as a "membrane." Ask which standard the product on your quote meets, and where in the room it starts and stops. For more on what holds up in a wet Florida room, see our guide to materials that last in Florida's humidity.
The Installation Sequence, Step by Step
Here is the order, with the standard behind each step:
- Find and mark the slab's joints and cracks. Every one of them needs a movement joint directly above it in the tile, under A108.02 Section 4.4.1.
- Make the slab flat enough for the tile size. For tile 15 inches or longer on a side, that means within 1/8 inch in 10 feet and 1/16 inch in 2 feet, using patch or self-leveler, not thinset.
- Install the membrane the room actually needs. That means a waterproof membrane to A118.10, a crack isolation membrane to A118.12, or both, placed where the plan says.
- Lay out the tile around the joints. Plan the pattern so the movement joints over the slab and at the walls and curb fall where they belong.
- Set the tile with full mortar contact. That means at least 80 percent coverage on a dry floor and 95 percent in the shower, with the corners supported, under A108.5.
- Grout the field, then seal the joints. Grout fills the joints between tiles. The movement joints and every inside corner get a flexible sealant, which Carothers lists as silicone, urethane or polysulfide, installed after the tile and grout are dry.
That last step is where grout goes wrong. The TileLetter article states that interior movement joints are required at all inside corners, and it shows "the inside corner failure when grout is substituted for the suitable flexible sealant." Where the floor meets the wall, the tub or the curb, the joint has to be able to move, and grout cannot. Grout color and joint width are yours to choose. Whether the corners get grouted is not.
Where the Shortcuts Show Up Later
| The shortcut | What it leads to |
|---|---|
| Tile bonded straight across the slab's contraction joint | A straight crack in the tile directly over the joint once the slab moves |
| No perimeter joint at the walls and the curb | Tile with nowhere to expand, which can tent up or loosen at the edges |
| Corners grouted instead of sealed | Hairline cracks in the inside corners, refilled and reopened |
| Slab never flattened before large tile | Lippage you feel with your feet, and hollow spots under the tiles |
| Mortar coverage below the A108.5 minimum | Unsupported corners that crack and, in a shower, voids that hold water |
| Membrane priced but its standard never named | Nobody can tell you afterward whether the floor was waterproofed, crack-isolated, or neither |
Every row on that table is cheaper to prevent than to fix. That is the honest reason the prep costs what it costs. For how the whole room comes together on price, see what a bathroom remodel costs in Southwest Florida.
Frequently Asked Questions
Can you tile a bathroom floor directly over a concrete slab?
Yes. The slab has to be clean, sound and flat enough for the tile size. Any joint in the slab has to be carried up through the tile as a movement joint rather than bonded over, which is what the industry standard ANSI A108.02, Section 4.4.1, calls for.
Does a bathroom floor need a waterproof membrane?
The Florida Building Code, Residential, Section R307.2, requires a nonabsorbent finish surface on tub and shower floors and on the walls of showers and of tubs with a shower head. It does not address the open bathroom floor. Movement in the slab is a separate problem, and ANSI A118.12 covers the crack isolation membranes made for it.
Why did my bathroom tile crack in a straight line?
A straight crack across several tiles is the pattern you would expect when tile was bonded rigidly across a joint or crack in the slab, which is what A108.02 says to avoid. A single tile cracked with no line to it can point to a void in the mortar under it, which is what the coverage minimum in A108.5 is meant to prevent.
Before You Sign for a Tile Floor
On a tile floor, the decisions that matter are made before the first tile goes down, and that is where we put the attention. HomeWorks is owned by Chad and Lyndee, and Chad is personally on every project, so the person who walks the slab with you is the person accountable for the floor. Our estimate comes from a recorded site visit, and the price and a 15 percent contingency are on the table before demolition starts.
We are a Florida Certified Building Contractor, license CBC-1261775, working across Lee and Collier County. We run 81 percent on schedule, and we carry 4.8 stars across 60 Google reviews.
Slip resistance and step-free entries have their own considerations, and those are covered in our guide to aging-in-place bathroom design.
Have a look at a bathroom remodeled by HomeWorks with custom tile and flooring, or request a quote and we will start with what is under the floor.
