What drainage problems do concrete garage floors have?

expert EXPERT April 26, 2007

Modern garage floors in most areas slope to drain water out of the garage through the overhead door and down the driveway. You may also find floor drains in garages. These may drain by gravity or by pump.

Below grade garages often have a drain just outside the garage door to collect driveway runoff as well as garage floor drainage. These drains often run the full width of the garage door.

Some attached garages are located at the basement level of a house, and the driveway slopes down toward the garage. In these cases, a special drain is provided. Within the garage, the floor slope is the same as that used in conventional modern construction. Water inside the garage flows out through the overhead door opening. Outside the door, a drain catches the water coming down the driveway and also catches water from the garage. This drain is usually the full width of the garage door.

Drains may be undersized, clogged or not at the low point. Covers may be rusted or exit pipes may be poorly arranged. The walls or floor of the drain may not be sound.

Garage floor drain problems may be the result of

• original construction
• poor maintenance
• settling or heaving
• rot

Drainage problems on garage floors can cause water damage to the garage and house. Authorities in many areas do not permit floor drains in garages because spills of automobile fluids (gas and oil) may find their way into municipal sewer systems. This can be a pollution and life safety problem.

Look at the bottom of the garage door for evidence of water problems. The garage door, jambs and operating tracks may show rot or rust suggesting chronic flooding problems. Look at the inside walls of the garage just above floor level for high watermarks from flooding. Look for stains on the bottom of shelves, or damage to storage in the garage.

Check that the garage floor slopes to a drainage point. In most cases this is out through the overhead door. If the slope is incorrect, it can usually be corrected by pouring additional concrete on top or by mud jacking, as long as the slab is not still moving. If there is a drain in the garage floor, we do not recommend abandoning it, but there are dangers associated with automobile fluids entering the drain.

A sump with a grate in the garage floor is not always a floor drain. It may contain a waste plumbing cleanout. Watch for rotted wood walls in such sumps. Watch also for rusted cleanout covers.

Look in these drains to make sure that they are not filled with debris. You should be able to see the exit pipe. Exit pipe diameters of less than 3 inches may not be large enough to carry the accumulated rain water away.

The end of the pipe in the drain should include an elbow that turns down. Much of the debris that falls into a drain will be floating debris. This includes leaves and twigs. If the drain pipe is simply a hole in the side of the drain, as the water level rises to the drain height, all the floating debris will enter the drain. An elbow only allows water to flow into the drain through a 3-inch-diameter opening that will be below the water surface. Far less debris will enter the drain with this arrangement.

Make sure the walls, floor and cover grille are intact. Check to see that the drain is at a low point. It is very common for the paving around the drain to settle, causing low spots to develop. Often water sits in these areas instead of draining into the grate. We recommend correcting the paving.