How to Install a Floor Drain in a Garage

Floor drains in garages do one job well: they get water off the concrete before it pools, stains, or freezes. Whether you're dealing with seasonal snowmelt, equipment washdown, or just standing water after heavy rain, a properly installed floor drain makes your garage usable year-round. The real work here isn't complicated, but it demands precision—you're cutting through concrete, installing below-grade plumbing, and tying into your home's drainage system. Done right, it's invisible and permanent. Done wrong, you've got a pooling hazard or a backed-up drain that costs thousands to repair.

1. Find the Right Spot. Choose a spot in your garage that's both low-lying (water naturally flows there) and near your main drain line or sump pit. Check your floor slope by pouring water and watching where it runs. The drain should sit at least 4 feet from your foundation wall and in a spot you won't trip over or park on. Mark the center point with chalk. If you're tying into a sewer line, you need to know where it exits the house—you may need to call your local utility to mark it first.
2. Break Through and Excavate. Use a concrete saw to cut a circle 12 inches in diameter through your garage slab at the marked location. Cut at least 4 inches deep, then use a pry bar and small sledge to break up and remove the concrete. Excavate the hole another 18 to 24 inches deep beneath the slab—this is where your trap will sit. The total pit should be 24 to 28 inches deep and about 14 inches in diameter to give you working room. Remove all broken concrete and soil.
3. Set the P-Trap Below Grade. Place a 4-inch PVC P-trap assembly (complete unit with trap and clean-out fitting) into the pit. The inlet port should face upward and sit about 4 inches below the concrete surface level. Use a level to make sure the trap is true, then backfill around it with gravel, tamping gently as you go. Leave the inlet port clear and accessible. If your trap has a clean-out cap, leave it finger-tight for now.
4. Run Line to Main Drain. From the outlet port of the trap, run 4-inch PVC drain line to your sump pit, main sewer line, or existing floor drain system. Slope the line gently downward toward the exit point—at least 1/8 inch drop per foot of run. Use PVC couplings and elbows as needed. All connections should be glued with PVC cement per the manufacturer's instructions. If you're tying into a sump pit, the line enters above the water level. If you're tying into a sewer line, the connection must be made via a sanitary tee, not a straight coupling.
5. Set Basin Flush and Level. Place a 12-inch cast-iron or plastic drain catch basin (the visible part) into the concrete opening, centered over the trap inlet. The basin should sit flush or slightly recessed below the concrete surface to prevent tripping. Check that it's level. Backfill around it with concrete patching compound, smoothing it flush with the surrounding slab. Once the patch cures (usually 24 hours), set the removable grate into the basin opening. The grate should sit slightly above the concrete surface so water flows in, not over it.
6. Run Water and Watch. Once all concrete has cured (48 hours minimum), pour 5 gallons of water slowly into the drain basin. Watch for pooling around the concrete patch and check the clean-out access below if visible. Water should drain within 10 seconds. If it drains slowly or backs up, the trap or line is blocked or mis-sloped. If you see water leaking around the basin edge, the concrete patch didn't seal properly and you'll need to dig it out and re-patch.
7. Keep Gases Sealed Out. Floor drains in garages collect oil, dirt, and debris that can clog downstream lines. Pour a quart of biodegradable enzyme drain cleaner down the drain once a month during heavy-use seasons. If odor becomes an issue, install a drain trap primer (a small valve above the basin that refills the trap automatically) or pour a quart of mineral oil down the drain quarterly to maintain the water seal. Never pour gasoline or flammable liquids down a floor drain.
8. Vacuum Out Sediment Monthly. Remove the grate monthly and use a shop vacuum to pull out sediment, leaves, and debris that collect in the basin. If you notice water draining slowly, use a plumbing snake or clean-out auger to break up blockages in the line below. Never assume a slow drain will fix itself—it only gets worse.
9. Prevent Winter Freeze Damage. In cold climates, water sitting in the basin freezes and cracks the cast iron. If your garage is unheated and sees winter weather, cover the basin with a removable insulated cap or seal it with a rubber dome cover during winter months. Remove it in spring. Alternatively, pour automotive antifreeze down the drain to lower its freezing point, though this only works if you do it before temperatures drop.
10. Get Approval First. Before you cut concrete, contact your local building department. Most jurisdictions require a permit for any floor drain installation because it ties into your drainage system. The permit ensures the work meets code and your septic or sewer connection is legal. An inspector will want to see the trap depth, slope, and connection point. Get approval before you backfill the trap—once it's covered, changes cost thousands.