Why Your Garage Slab Is Different
Rick poked around my garage for a few minutes, then sat on an overturned bucket and gave me the rundown.
Thinner Concrete
My house foundation walls are 8 inches thick, poured on footings that go 4.5 feet down to below the frost line. My garage slab? About 3.5 inches of concrete sitting on maybe 4 inches of gravel. That's it.
Rick said most residential garage slabs are 3.5 to 4 inches thick. Some builders pour them at 4 inches and call it good. According to the Portland Cement Association, residential slabs should be a minimum of 4 inches thick, but that's often the maximum builders use for garages. My house basement floor is 4 inches too, but it has the protection of being underground and inside a heated envelope. The garage slab is basically exposed.
No Deep Footings
This was the big one. My house sits on footings that extend below the frost line. The ground at that depth doesn't freeze. Doesn't heave. Stays relatively stable year-round.
My garage has what Rick called a thickened-edge slab. The concrete gets a little thicker at the edges, maybe 8 inches, and that's the footing. It's maybe 12 inches below grade. The frost line where I live is around 4 feet. So the garage "footing" is sitting in soil that freezes solid every winter.
"It's like building on a trampoline," Rick said. "The ground under your garage moves. The ground under your house doesn't."
No Climate Control
My house stays between 65 and 75 degrees year-round. The concrete stays at a relatively stable temperature. Thermal expansion and contraction are minimal.
My garage hit 95 degrees last August and negative 10 in January. That's a 105-degree swing. Concrete expands and contracts with temperature. A 20-foot slab can move around 1/8 inch across that temperature range. That doesn't sound like much, but it adds stress at every joint, every crack, every connection point. Year after year.
The Vehicle Factor
My car weighs about 3,800 pounds. My wife's SUV is closer to 4,500. Both rolling over a 3.5-inch slab twice a day, every day, concentrating all that weight on four tire contact patches.
Rick pointed out that my basement floor supports a couch and some bookshelves. My garage floor supports vehicles. The loads are completely different. And every time I drive in, the impact loading as the tires cross the apron and transition to the slab creates stress that a living room floor never experiences.
What Actually Cracked My Garage
Rick walked the crack carefully, then looked at the grade outside.
The Wet Spring Problem
Three weeks of rain saturated the soil under and around my garage slab. The gravel base provides some drainage, but it's not infinite. With constant rain, the water table rose and the soil beneath the slab stayed wet.
Wet soil is soft soil. My garage sits on a slight slope. The back corner of the garage was lower, so that area got the most water accumulation. The soil there softened more than the rest. When I parked my car, the extra weight on softened soil caused uneven settlement. One area sank just enough to crack the slab.
Rick measured maybe 3/16 of an inch of vertical displacement at the crack. One side slightly higher than the other. Not much, but enough to snap a 3.5-inch slab.
The Stress Concentrator
The crack started at the corner of the garage door opening. Rick wasn't surprised. He said garage door openings are the number one stress point on a garage slab.
Think about it. The slab has a big rectangular hole cut in the front of it. All the forces that want to move through the slab have to detour around that opening. The corners become stress concentrators. When the soil settled unevenly, the stress found the weakest point and cracked right from the corner.
"If I had a dollar for every garage with a crack starting at the door opening corner, I'd be retired twice," Rick said. He was already retired once, so I guess that would make three times.
Years of Freeze-Thaw
Rick said the wet spring was the final straw, not the root cause. My garage slab had been weakening for years. Each winter, moisture in the concrete and the soil beneath it went through freeze-thaw cycles. Each cycle caused micro-damage. Tiny cracks inside the concrete. Slight heaving and settling of the subgrade.
Forty-plus winters of this. Then one really wet spring pushed it past the breaking point.
"The crack was coming eventually," Rick said. "The rain just accelerated it."
Gary's Garage Was Worse
My neighbor Gary had garage problems that same spring. Except his were worse, and he made them worse by ignoring them.
The Cracks He Didn't Fix
Gary noticed cracks in his garage slab two years before I got mine. Small stuff. He figured it was cosmetic. He wasn't wrong at first. But he didn't seal the cracks, and water got in. Two winters of freeze-thaw inside open cracks turned hairlines into gaps. The concrete around the cracks started spalling.
The Heave
By the time the wet spring hit, water had been getting under Gary's slab through those open cracks for two years. The gravel base was compromised. Soil had migrated up through the gravel. The drainage under his slab was basically gone.
His slab heaved that spring. One section lifted about 3/4 of an inch. Not the whole slab. Just one section. Made it impossible to park straight without the car sitting crooked. And the crack where the sections met was wide enough to lose a quarter in.
Gary's $2,800 Repair
Gary called a concrete contractor. They ended up removing the heaved section, re-compacting the base, and pouring a new section. Labor, removal, disposal, new concrete. $2,800.
Gary was not happy. Especially after Rick told him that sealing those original cracks when they first appeared would have cost maybe $50 in materials and an hour of work.
I sealed my crack within two weeks of finding it. Lesson learned from watching Gary.
How I Fixed My Garage Crack
Rick helped me figure out a plan. The crack wasn't structural in the sense that my garage was going to collapse. But it needed attention before winter turned it into Gary's situation.
The Diagnosis
Rick categorized my crack as settlement-related, non-structural, but worth sealing and monitoring. The 3/16-inch displacement meant the subgrade had settled unevenly but wasn't actively moving. He recommended I wait two months to see if it stabilized, then repair.
I checked it every two weeks that summer. No change after the initial crack. Good sign. The soil dried out, firmed up, and the movement stopped.
The Repair
I used a two-part epoxy crack filler for the main crack. Cleaned it out, filled it, smoothed the surface. Cost $35 for the materials. Then I applied a concrete sealer over the entire garage floor. $90 for two gallons. Took a Saturday afternoon.
For the slight displacement, I used a self-leveling compound to feather the transition between the two sides. Another $40.
Total cost: about $165 in materials. Compare that to Gary's $2,800.
Drainage Correction
Rick pointed out that the grade outside my garage was directing water toward the back corner. Same corner where the settlement happened. He said I needed to fix that or I'd be back here in a few years.
I regraded along the side of the garage, building up soil so it sloped away instead of toward the slab. Added a splash block to the downspout on that corner. Extended the downspout another 4 feet.
Materials for the drainage work: about $310 including topsoil, the splash block, and downspout extension. So $475 total for everything. And two weekends of work.
Garage Cracks vs House Foundation Cracks
After going through this, I asked Rick how to tell the difference between a garage crack that's no big deal and one that signals something serious.
Cracks You Can Mostly Ignore
Hairline cracks that appear within the first year or two of construction. Those are shrinkage cracks. The concrete cured and shrank slightly. Normal. Control joint cracks where the builder cut lines in the slab to control where cracking happens. Also normal. That's literally what those joints are for.
Small random cracks with no displacement that don't change over time. My house has plenty of these in the basement floor. The garage equivalent is the same thing. Seal them so water doesn't get in, and move on.
Cracks That Need Attention
Cracks with vertical displacement, where one side is higher than the other. That means the subgrade has moved. Cracks wider than 1/4 inch, which indicate significant movement. Cracks that are actively growing, getting longer or wider over months.
And the one Rick emphasized most: cracks in the garage that align with cracks in the house foundation. If the garage and house share a wall, and you see cracking on both sides, that could mean the movement isn't limited to the garage slab. That's when you call an engineer.
When a Garage Crack Signals a Bigger Problem
Rick told me about an inspection he did years ago. Homeowner called about garage cracks. Turned out the soil under the whole property was settling due to a broken water main two houses down. The garage showed it first because it was the weakest structure on the lot, but the house foundation was moving too. Just slower.
"The garage is like a canary in a coal mine," Rick said. "It fails first because it's built lighter. Sometimes that's a gift. It warns you before the house gets serious damage."
So I monitor my garage cracks along with my house cracks now. Same schedule. Same notebook. If the garage starts moving again, I pay attention to the house too.
Keeping Your Garage Slab Healthy
Things I do now that I didn't do before Rick's education session on my garage floor.
Seal the Floor
I reapply concrete sealer every two to three years. Keeps water from penetrating the surface. Reduces freeze-thaw damage to the concrete itself. A gallon costs about $45 and covers my two-car garage floor. Takes an hour to apply.
Manage Water Around the Garage
Gutters clean. Downspouts extended. Grade sloping away. I check these every spring, same as the house. The garage doesn't get the same obsessive attention most homeowners give the house, and that's part of why garage problems develop.
Watch the Door Opening Corners
Those are where cracks start. I check the corners of my garage door opening every time I check my foundation. If a new crack appears there, I want to catch it early. Not two years later like Gary.
Think Twice About Heavy Storage
My buddy wanted to put a car lift in his garage. Two-post lift, rated for 9,000 pounds. On a 4-inch residential slab. Rick said that would be asking for trouble. The point loads from the lift posts would concentrate thousands of pounds on small areas of a slab that was designed for distributed vehicle loads, not concentrated ones.
If you're storing heavy equipment, large amounts of lumber, or anything that creates concentrated loads, consider whether your 3.5-inch slab was designed for that. It probably wasn't.