I’m Joe. I run Onel Construction out of Boca Raton, and nearly every home I walk into down here has a crack somewhere in the slab. That’s Florida. Concrete shrinks as it cures, our water table sits high, and the ground under your house is sand and limestone, not bedrock. Most cracks are cosmetic. Some are your house telling you something early — while it’s still cheap to hear.
The problem is that homeowners get told two useless things: “all concrete cracks, forget it,” or something designed to scare them into a five-figure repair. The truth is in the crack itself. Here’s how to read it.
Key takeaways
- Hairline cracks, uniform in width, that aren’t changing are almost always normal concrete shrinkage — cosmetic, not structural.
- Width, pattern, and movement are what matter: cracks wider than about 1/8 inch, cracks where one side sits higher than the other, and cracks that keep growing deserve a real look.
- Sticking doors, sloping floors, and tile “tenting” or popping are your house’s other witnesses — they often show up alongside slab movement.
- The vast majority of Florida slab cracks are NOT sinkholes. Plain fact, no drama.
- An engineer’s report turns an unexplained crack from a deal-killer into a documented, answered question — for insurers, buyers, and permits.
First: how concrete behaves in Florida
Your slab was poured over compacted fill on sandy soil, with the water table sometimes just a few feet below. Concrete shrinks a little as it cures, and that shrinkage shows up as thin, wandering hairline cracks — often visible when tile or carpet comes up. That’s been normal since the day the house was built.
Florida adds two things other states don’t have as much of: heavy seasonal rain that changes moisture in the soil under your slab edges, and older homes — a lot of our housing stock in Palm Beach, Broward, and Miami-Dade was built in the ’60s, ’70s, and ’80s — whose fill and drainage have had fifty years to settle and wash.
How to read a crack
Width. Hairline — thin as a pencil line, hard to slide a fingernail into — is usually shrinkage. Once a crack opens wider than roughly 1/8 inch, it moves out of the “ignore it” category.
Height difference. Run your hand across it. If one side of the slab sits noticeably higher or lower than the other, the ground below has moved unevenly. That’s called differential movement, and it’s a real signal.
Pattern. A single straight-ish crack is one thing. Cracks radiating from a corner, stair-stepping up a block wall, or running in long parallel lines are patterns, and patterns point at causes.
Growth. This is the one nobody checks and everybody should. Mark the end of the crack with a pencil, write the date next to it, and measure the width. A crack doing nothing for six months is an old story. A crack still moving is a current event.
Company it keeps. Doors that suddenly stick. Windows that won’t latch. Floors you can feel sloping when you walk. Gaps opening where walls meet ceilings. And a Florida classic: tile tenting — tiles pushing up into a ridge, or popping loose with a hollow sound — which happens when the floor system is under pressure that’s got nowhere to go. One symptom alone is a curiosity. Two or three together is your house asking for a professional opinion.
The sinkhole question — answered plainly
People in Florida hear “slab crack” and think “sinkhole.” Most slab cracks are not sinkholes. Not close. Sinkholes are a real Florida geology fact in some regions, but the everyday causes of slab cracks down here are shrinkage, fill settlement, drainage washing soil out slowly over decades, and tree roots. If you want certainty instead of worry, that’s what an engineer’s visit is for — but nobody should sell you fear as a diagnosis.
Three buckets: cosmetic, monitor, engineer
Cosmetic. Hairlines, uniform width, both sides level, no other symptoms. Seal them if you like, cover them with your new floor, and move on.
Monitor it. A wider crack, but level across, no symptoms, not sure if it’s moving. Pencil-mark it, date it, photograph it with a coin in the frame for scale, and re-check it in three months. Movement makes the decision for you.
Get an engineer. Any crack with height difference across it, cracks in patterns, anything still growing, or cracks traveling with sticking doors, sloping floors, or wall gaps. Also: any crack you need a documented answer for — before listing the house, after an insurer raises a question, or before you spend real money on new flooring over it.
What an engineer’s visit actually gets you
Our licensed structural engineer examines the crack and the house around it, and produces a written report: what the crack is, what’s causing it, whether it’s active, and what — if anything — needs to be done. That letter works. Insurers accept it as an answer instead of treating the crack as an open risk. Buyers and their lenders see documentation instead of a red flag. And if a repair or a permit is needed, the report is the starting document.
The repair paths, at the level you need them
At the simple end: monitoring costs nothing but a pencil. Sealing and epoxy work handles cosmetic and dormant cracks before a floor goes down. The serious end — where there’s real, ongoing settlement — is underpinning: steel piers driven or screwed down to stable soil to support and stabilize the slab. Nobody should be quoting you piers off a photo, and nobody honest will price any of it before an engineer has said what’s actually moving and why. Get the diagnosis first. Then the repair is a decision, not a scare tactic.
Why this matters even if you’re not worried: insurance and resale
An unexplained crack is a story a buyer, an inspector, or an insurance underwriter gets to write — and they never write the kind version. Sellers lose deals over cracks a $0 pencil line and one engineer’s letter would have answered. If you’re refinancing, selling, or putting expensive new flooring over a slab, the documented answer is the cheapest protection in the whole project.
FAQ
Are cracks in a Florida slab normal?
Mostly, yes. Hairline shrinkage cracks appear in nearly every concrete slab as it cures and ages. Width, uneven height across the crack, continued growth, and symptoms like sticking doors are what separate normal from worth an engineer’s visit.
How wide is too wide for a slab crack?
Once a crack passes roughly 1/8 inch wide — or any width where one side sits higher than the other — it deserves a professional look. Thinner, level, stable hairlines are generally cosmetic.
Could a crack in my floor mean a sinkhole?
It’s the question everyone asks, and the plain answer is that the vast majority of Florida slab cracks are not sinkholes — shrinkage, settlement, drainage, and roots are far more common causes. If you want certainty, an engineer’s evaluation is how you get it, in writing.
What does an engineer’s slab inspection give me?
A written report identifying what the crack is, its likely cause, whether it’s still active, and the recommended path. That document answers insurers, buyers, lenders, and building departments — it turns a worry into a closed file.
Should I fix a crack before putting down new flooring?
Get it read first. New tile or plank over an active crack telegraphs the movement straight through — and tenting tile is how homeowners find that out the expensive way. A documented all-clear, or the right repair first, protects the money you’re about to spend on the floor.
Got a crack you keep looking at?
Text a photo of it to (561) 462-9293. I’ll tell you which bucket it falls in, and if it needs our licensed structural engineer, you’ll get the visit, the written answer, and a free itemized estimate for anything that actually needs doing. Photos are free. Guessing is what gets expensive.
— Joe, Onel Construction LLC, CGC1511492
