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If your garage floor sounds hollow in one spot, you probably noticed it by accident. You drop a wrench, or walk across the floor in boots, and one spot sounds different, like a drum instead of solid rock. I have one in my own garage, about 2 feet across near the middle of the floor, so I dug into what it means and what to do about it.
This guide covers what a hollow spot means, what causes it, how to map it, when to worry, and 5 ways to fix it, from free to call-a-pro.
WHAT’S IN THIS GUIDE
What Does a Hollow Spot Mean?
When your garage floor sounds hollow, it means there’s a gap, or void, between the bottom of the concrete and the soil underneath it. The concrete is still there and still solid. It just isn’t resting on anything in that spot, so it rings when you tap it.
The good news: in most homes, the garage floor isn’t holding up the house. InspectAPedia, a home inspection reference, explains that floor slabs poured inside a separate foundation wall are usually not connected to the foundation and are not supporting any structure. The exception is a slab that has a support column or wall sitting on it. Source: InspectAPedia — Concrete Slab Settlement Cracks
So if your garage floor sounds hollow in a small spot, it’s usually not an emergency. But a slab with nothing under it can crack when you park a heavy vehicle on it, and a void under concrete can grow if water keeps getting in.
What Causes a Void Under Concrete?
InspectAPedia points to two main causes:
- Poorly compacted fill. When the dirt under the slab wasn’t packed down well before the concrete was poured, it settles over the years and leaves a gap.
- Water. Settlement gets worse when water runs under the slab, and loose soil and gravel combined with poor drainage can lead to serious slab settlement.
In a garage, water can get under the slab a few ways: through cracks and control joints, along the edges of the floor, under the garage door, or from a downspout dumping water right next to the slab outside. Snow melting off your car all winter adds even more.
How to Find and Map the Hollow Spot
Before you fix anything, find out exactly how much of your garage floor sounds hollow:
- Tap the floor in a grid with a rubber mallet , or drag a chain or the end of a broom handle across it. Solid concrete makes a dull thud. A hollow spot rings or sounds like a drum.
- Mark the edge of the hollow area with chalk as you go, until you have an outline.
- Write the date inside the outline and take a photo from the same spot every few months. If the outline grows, the void is growing.
- Look for cracks running through or near the spot, and lay a level across it to check if it’s sinking.
When to Worry
InspectAPedia says these signs are worth getting checked by an experienced contractor, foundation repair company, or structural engineer:
- Cracks in the floor that line up with cracks in the foundation wall.
- Cracks that are new, getting wider, or wider at the top.
- A floor that’s tipping or sinking, even if it isn’t cracked.
- Settlement around a support column, or sagging in the floor above it.
If your garage floor sounds hollow in just one small spot, the floor is flat, and there are no cracks spreading from it, you have time to watch it and pick the right fix.
5 Ways to Fix a Hollow Spot Under a Garage Floor
Fix 1: Monitor it and keep water out (free)
If your garage floor sounds hollow in a small spot with no cracks or sinking, the first step is to stop it from getting worse. Map it, date it, and keep an eye on it. Then work on the water. Squeegee snow melt out of the garage instead of letting it sit, or put down a garage floor containment mat. Here’s how to make room to park your car in the garage this winter with a mat that catches the melt.
Fix 2: Seal the cracks and joints (low cost)
Every open crack and control joint is a path for water to reach the soil under the slab. Fill them with a flexible self-leveling concrete sealant so water stays on top. For wide or deep cracks, push in a foam backer rod first. See our guides on how to repair cracks in a concrete driveway and how to seal a garage floor.
Fix 3: Fix the drainage outside (low cost)
Walk around the outside of the garage after a rain. If a downspout dumps water right next to the slab, or the ground slopes toward the garage, that water is soaking in under the floor. A downspout extension that carries water several feet away from the garage is one of the cheapest fixes there is. Here’s how to extend downspouts away from your house.
Fix 4: Fill it with polyurethane foam (pro or specialty kit)
When your garage floor sounds hollow over a small area like mine, the fix the pros use is polyurethane foam injection. They drill small holes through the slab into the void, inject a two-part foam made specifically for supporting concrete, and patch the holes. The foam is lightweight and cures fast.
One warning: don’t grab a can of spray foam from the hardware store. Gap-filling and insulation foams are made for sealing cracks around windows and pipes. They’re too soft to hold up a concrete slab and will compress under the weight of a car. Specialty slab foam kits do exist, but they’re hard to find and easy to overfill, which can lift or crack the slab. For most homeowners, this is the point to call a pro.
Fix 5: Hire a pro for foam jacking or mudjacking
For a bigger void, a slab that has already sunk, or anything on the “when to worry” list, call a concrete leveling company. They’ll either inject polyurethane foam (often called foam jacking or polyjacking) or pump in a cement-based slurry (mudjacking) to fill the void and, if needed, lift the slab back to level.
Polyurethane foam is used on highways too. The New Hampshire Department of Transportation is testing injected polyurethane foam to stabilize the base under concrete slabs, with the goal of a lower cost than tearing the concrete out and starting over. Source: Transportation Research Board — Concrete Slab Jacking (New Hampshire DOT)
Get at least two quotes, and ask how many holes they’ll drill, how big they’ll be, and whether they guarantee the work. If the slab is badly cracked and broken up, replacing that section may be the better choice. Our concrete calculator can help you estimate how much concrete a replacement section would take.
What I’m Doing About Mine
My garage floor sounds hollow in a spot about 2 feet across, near the middle of the floor, right up against a corner where two control joints meet. There are no cracks spreading from it and no sinking I can see. I don’t know for sure what caused it, but that open joint is a likely path for water to get under the slab. So I’m starting with the free and cheap stuff. I’m mapping it with chalk and dating it, sealing the control joints around it first, and catching snow melt off the truck this winter. If the outline grows or a crack shows up, I’ll move to a foam fill. I’ll update this article with what I find.
Dealing with stains, pitting, or other floor issues too? See our guide to garage floor problems.
Frequently Asked Questions
Why does my garage floor sound hollow in one spot?
A hollow sound means there’s a gap between the bottom of the concrete and the soil underneath it. The soil settled or washed away, and the concrete is bridging over the gap.
Is it dangerous if my garage floor sounds hollow?
Usually not right away, since most garage floors aren’t holding up the house. But the unsupported concrete can crack under a heavy vehicle, and the void can grow if water keeps getting under the slab. Get it checked if you see new or widening cracks or the floor starts to sink.
Can I fill a void under concrete myself?
It’s possible with a specialty slab foam kit, but it’s not a good DIY job for most people. The kits are hard to find, easy to overfill, and the foam can’t be removed once it cures. Never use canned spray foam from the hardware store. It’s too soft to support concrete.
What’s the difference between foam jacking and mudjacking?
Both fill the void under a slab. Foam jacking uses expanding polyurethane foam injected through small holes. Mudjacking pumps in a heavier cement-based slurry. A concrete leveling company can tell you which fits your floor.
How do I keep a hollow spot from getting bigger?
Keep water out, since water is what makes a void grow. Seal the cracks and joints, move downspouts away from the garage, and don’t let snow melt sit on the floor all winter.