TL;DR
- Customers want 24x48 porcelain tiles. Demand is real, and it's not going away.
- Florida slabs need flatter prep than most installers default to: 1/8" in 10 feet for LFT, not the 1/4" you'd get away with on smaller tile.
- Back-buttering is not optional. 95% mortar coverage is the spec for exterior and wet areas — the inspector will check.
- Crack isolation membrane is cheap insurance on Florida slabs. Skip it on one job, pay for it on the next ten.
- Modified vs. unmodified thinset is the most common spec violation we see. Read your tile manufacturer's TDS before you mix.
Why this matters right now
Homeowners and designers are committing to large-format porcelain on master baths, great rooms, lanais, and pool baths — and they're not interested in going back.
That means more 24x48 installs on your schedule. It also means more callbacks if you treat them like a 12x24 with extra steps. Large-format porcelain (LFT) tile — defined by TCNA as any tile with at least one edge longer than 15 inches — has its own set of requirements that are non-negotiable. Florida slabs add a second layer of considerations on top of that.
This post walks through the full install sequence the way we'd brief a new crew lead. If you're a 20-year veteran, skip to the failure-mode section at the end and check your habits against it.
Step 1: Read the slab before you bid
Most LFT failures in Florida start at the bid walk, not on install day. Before you commit to a price, get on your knees and check the slab.
What you're looking for:
- Flatness. Use a 10-foot straightedge across the floor in multiple directions. For tile with any edge 15" or longer, the spec is 1/8" deviation in 10 feet (TCNA EJ171 reference). For tile with all edges under 15", you have more room — 1/4" in 10 feet. Almost no Florida slab meets the LFT spec out of the box.
- Cracks. Hairline cracks in a residential slab are normal. Cracks wider than 1/8", cracks that have moved (offset top to bottom), or cracks across multiple bays are different problems. Photograph and document everything before you bid.
- Moisture. New slabs in Florida hold moisture longer than the calendar suggests. ASTM F2170 in-situ probe testing is the standard. If the slab is under 60 days old or you can see efflorescence, test before you set.
- Post-tension cables. If you're working in a high-rise or a newer Tampa/Sarasota build, assume the slab is post-tensioned until you confirm otherwise. You're not drilling anything until you've located the cables.
- Substrate behavior. Lanai transitions, garage-to-house transitions, and slab-on-grade-to-suspended-slab transitions are all expansion joint candidates. The tile must respect the joint. If you bury it, the floor will tell on you.
If the slab is out of spec on flatness — and it usually is — you have two real options: grind down high spots, or pour self-leveling underlayment (SLU). For LFT, SLU is almost always the right call.
Step 2: SLU selection and prep
TEC 567 Contractor Grade SLU 50lb (Item #428005) is our most-purchased SLU and the one we recommend for most LFT prep on residential slabs. It's a Portland cement-based, pumpable SLU rated for up to 1.5" pour depth in a single lift, with foot traffic in 4 hours and tile-ready in 16. For deeper pours or commercial-grade jobs, Merkrete Underlayment options are available — ask the desk.
Coverage math for #428005:
- One 50 lb bag mixed at manufacturer spec yields approximately 0.45 cubic feet.
- At 1/4" pour depth, that's roughly 22 square feet per bag.
- At 1/2" pour depth, roughly 11 square feet per bag.
Build your bid with at least 10% waste on top of measured square footage. Florida slabs love to surprise you with a low spot in the corner of the room.
Prep before you pour:
- Clean. Vacuum, scrape, and degrease. SLU bonds to a clean substrate or it doesn't bond at all.
- Prime. Use the manufacturer's matched primer (TEC Multipurpose Primer for the #428005 system). Two coats on porous slabs. Let it dry to clear-tacky before pouring.
- Dam transitions. Doorways, transitions, and anywhere the SLU could escape to a lower elevation need a foam dam or wood lath.
- Mix to spec. Cold water, full bag, 2 minutes mixing. Don't eyeball water — SLU is a chemical reaction, not a recipe.
- Pour from a single point and let it find level. Walk it with a smoother, not a trowel. Spike roller for air release.
After pour, do not set tile for 16 hours minimum on the #428005 system. In Florida humidity, give it 24 if the schedule allows. The slab being "walkable" doesn't mean it's "tile-ready."
Step 3: Crack isolation — when, why, and which membrane
Florida slabs crack. Movement is a function of temperature swing, soil settlement, post-tension creep, and the simple fact that concrete cures and shrinks for years. Your job is to keep that movement from transferring to the tile.
The ANSI standard: A118.12 governs crack isolation membranes. Pick one that meets that standard, not a cheaper "waterproofing" sheet that doesn't.
When you need it:
- Any slab with visible cracking (even hairline)
- Any slab less than 12 months old, regardless of visible cracking
- Lanai installs (expansion + UV + moisture cycling = movement)
- Pool-deck-to-house transitions
- Any time you're setting LFT or stone over a slab you didn't pour yourself
When you can skip it:
- New, controlled-environment commercial pours with documented stable substrate
- Indoor installs over a slab you've installed on before with no callbacks
- Anywhere the GC has explicitly waived it in writing (rare, but it happens)
Common options at TOA:
- Schluter DITRA — Uncoupling membrane, 1/8" profile, also handles minor crack isolation. The default for most residential LFT in Florida. Sets in unmodified thinset above, modified below.
- NAC Strataflex — Liquid-applied crack isolation. Cheaper per square foot, faster on big floors, but requires careful application thickness.
- SubSeal Liquid Waterproofing — When you need waterproofing and crack isolation in one product (wet rooms, curbless showers, pool decks transitioning to lanai).
Pick one and follow its specific install instructions. Mixing systems (DITRA below, Strataflex above) voids warranties on both.
Step 4: Thinset selection — the spec violation we see most
The single most common LFT spec violation we see at the warranty desk is wrong thinset.
The two rules that matter:
- LFT requires a "Large and Heavy Tile" mortar — meeting ANSI A118.4 and A118.11, with non-sag properties. Standard thinset will let the tile slide and lippage will appear before cure.
- Modified vs. unmodified depends on what's underneath. Schluter DITRA, KERDI, and similar uncoupling/waterproofing membranes generally require unmodified thinset above, because the membrane already prevents moisture migration and modified mortar can't cure properly in a sealed sandwich. Read the membrane manufacturer's TDS, every time.
Recommended SKUs for LFT on Florida slabs:
| Application | Thinset | Notes |
|---|---|---|
| LFT directly to prepared slab (no membrane) | TEC 487 Ultimate 6 Plus or Merkrete 855 XXL | Modified, LHT-rated, non-sag |
| LFT over Schluter DITRA / KERDI | Schluter ALL-SET (#413067) | Unmodified per Schluter spec, dual-purpose (below membrane: modified rules apply) |
| LFT over liquid crack isolation membrane (cured) | Merkrete 855 XXL | Check membrane manufacturer; most allow modified above cured liquid CIM |
| Wet area floor with porcelain | TEC PermaFlex 300 (#423035) | Polymer-modified, flexible, premium pricing but the right tool |
When in doubt, the spec sheet beats the habit. If you've been using one thinset for a decade and the manufacturer changed the formulation last year, your habit is out of date.
Step 5: Layout, dry-lay, and shade variation
LFT punishes bad layout decisions more than small tile does. A 1/4" misalignment that disappears on 12x12 will read as a 1" diagonal drift across an 18-foot great room with 24x48.
Before you mix mortar:
- Dry-lay the field. Pull from at least 4 boxes simultaneously. Tile manufacturers shade-grade by run, but variation within a run is normal. Mix it.
- Find the visual center. Not always the geometric center — sight lines from main entrances matter more than centerlines on a plan.
- Run direction. For LFT planks (12x60, 9x60), run parallel to the longest sight line by default. For 24x48 squares, the long edge usually runs perpendicular to the main natural light source.
- Offset spec. TCNA's current guidance for plank-format LFT is a maximum 33% offset (1/3 brick pattern). 50% offset is no longer recommended for plank tiles over 18" because manufacturing warpage tends to peak at midspan. If the homeowner wants 50% offset, get it in writing.
- Cut planning. Avoid cuts under 2" at field edges. Shift the entire field by half a tile if necessary to land a usable cut at both walls.
Step 6: Back-buttering and mortar coverage
This is where most callbacks originate. The spec is 95% mortar coverage for exterior, wet, and large-format installs. 80% is the indoor dry minimum and it's not enough for any LFT install in Florida.
The technique:
- Trowel size. For 24x48, use a 1/2" x 1/2" square-notch trowel minimum. For very flat substrates, a 1/4" x 3/8" U-notch may work, but verify with a pull test.
- Direction. Comb mortar on the substrate in straight, parallel lines — not arcs, not swirls. Parallel lines collapse and release air; arcs trap it.
- Back-butter. Apply a thin, even skim coat to the back of the tile with the flat side of the trowel. This is not optional on LFT. It fills the manufacturer's relief pattern and ensures full transfer.
- Set and beat. Set the tile, then beat it with a rubber float or beating block in line with the trowel ridges. The ridges collapse perpendicular to the comb direction.
- Pull-test. Every 50-100 square feet, pull a freshly set tile and inspect the back. You're looking for full transfer with no voids over 2 square inches and no voids at corners or edges.
If you're getting less than 95% transfer, your options are: larger notch, wetter mortar (within manufacturer slump spec), more back-butter, or all three.
Step 7: Leveling systems and lippage management
LFT will telegraph lippage that you'd never see on small tile. Use a leveling system. The math is simple: one callback on a 600 sq ft floor costs more than leveling clips for the entire job.
At TOA:
- Raimondi VITEXpress (#507519) — Reusable cap, disposable strap. Our most-purchased system for LFT.
- Raimondi RLS Wall Leveling Pliers (#507517) — Hand-tension pliers for the strap-and-cap system. Spec the pliers with the system.
- LASH Tile Leveling Pliers (#507359) — Budget alternative; works for shorter jobs.
Spacing strategy for 24x48:
- One leveling clip every 8-12 inches along all four edges of every tile
- Minimum four clips per tile on a 24x48 (one per corner is the floor, not the ceiling)
- Set, level, beat, then tension the clips — never tension a tile you haven't beaten in first
Remove clips after the manufacturer's stated cure window. Hitting them with a rubber mallet at an angle is faster and cleaner than the kick-out method, especially on polished tile where the kick can chip the edge.
Step 8: Grout and movement joints
For LFT, grout joint minimum is 1/16" for rectified tile and 3/16" for non-rectified. Don't go tighter than spec — the joint exists to absorb dimensional variation and ambient movement.
Movement joints (TCNA EJ171):
- Interior, every 20-25 feet in each direction
- Exterior (lanai, pool deck), every 8-12 feet in each direction
- At every plane change (floor to wall, slab to slab)
- At every transition (room to room, material to material)
- At every restraining element (columns, curbs, drains)
Movement joints are flexible sealant, not grout. Get the color matched and the homeowner won't notice. Skip them and the floor will crack at the worst possible location six months out.
Failure modes: A 30-second self-audit
If you've been doing LFT installs for years, run this list against your last three jobs:
- Did I use a 10-foot straightedge to verify slab flatness, or did I eyeball it?
- Did I read the tile manufacturer's TDS for this specific SKU before specifying thinset?
- Did I pull-test for 95% coverage at least once per room?
- Did I dry-lay from multiple boxes to mix shade variation?
- Did I install movement joints at every plane change and 25 ft on center?
- Did I document the slab condition with photos before pour?
- Did I follow the membrane manufacturer's modified/unmodified rule, or did I default to habit?
Five out of seven is a problem. Three out of seven is a callback waiting to happen.
What to buy at TOA for an LFT install
This is the standard pull list for a residential 600 sq ft LFT install over a typical Florida slab. Adjust for your specific job.
Tile (choose by spec):
- Onyx Reale 24x48 (#113243) — high-traffic interior, dark tone
- Nero Laurent Polished 24x48 (#113339) — statement floor, 64% margin (we have inventory depth)
- Olimpo Polished 24x48 (#113245) — light tone, broad availability
- NX Ferrol Ice 24x48 (#113419) — neutral, popular new intro
Substrate prep:
- TEC 567 Contractor Grade SLU 50lb (#428005)
- TEC Multipurpose Primer
Crack isolation / membrane:
- Schluter DITRA (sized to floor) — default
- Or NAC Strataflex (#408213) for liquid-applied
Thinset:
- Schluter ALL-SET (#413067) if using DITRA/KERDI
- TEC 487 Ultimate 6 Plus or Merkrete 855 XXL if direct-to-slab
- TEC PermaFlex 300 (#423035) for wet areas
Leveling:
- Raimondi VITEXpress system (#507519) + RLS Pliers (#507517)
Grout & finish:
- Match grout to tile manufacturer recommendation
- Color-matched sealant for movement joints
- Floor Mud 60lb (#413061) for any mud-set transitions
Need it delivered to the jobsite?
Don't haul 800 sq ft of porcelain in your truck. We deliver to the jobsite — staged where you need it, on the day you need it. Talk to your store about contractor delivery scheduling and Pro account terms.
Hablamos español. Pregunte por nuestro equipo de servicio en español para revisar especificaciones, hacer pedidos, o coordinar entregas.
What your homeowner client should know
Working with a homeowner on a large-format porcelain install? Send them our companion piece for the Homeowner side: Large-format tile in small bathrooms: Yes or no? It covers the visual and design tradeoffs without getting into install technique, so your conversation with them starts on common ground.
Related Pro Posts
- Crack isolation for Florida slabs: When DITRA, when Strata-Mat, when nothing (coming next month)
- Thinset selection in 60 seconds: TEC vs. Merkrete vs. Schluter ALL-SET (bilingual)
- Pool deck DCOF in 2026: The spec, the inspectors, the SKUs
This post is part of The Job Site, TOA's editorial series for tile installers, contractors, and remodelers working in Florida. Have a topic request or a job that taught you something? Email [editorial contact].
TOA does not install. We equip the professionals who do.