A.Rocky Geans, president of L.L. Geans Construction Co. in South Bend, Ind., and member of the board of directors for the American Society of Concrete Contractors, responds: A garage floor does require a slope for drainage so water won't pool on the floor, and 1/4 inch in a foot is typical. But if you do a good tight screed and finish job, 3/16 inch per foot will do. And yes, the gravel sub-base should be graded to the same slope as you intend for the slab surface.
The bigger question is, where do you drain the water to? If you are located in a freeze-thaw climate such as we have here in South Bend, drainage toward the overhead door could be a problem.
When snowmelt or water from washing the car seeps under the door and meets 25°F or colder temperatures, it freezes. Frequent seepage can repeatedly melt and refreeze the existing ice, setting up a continual freeze-thaw cycle that can cause the surface to scale off early in the driveway's service life. One solution is to pitch the slab to the center of the garage and provide drainage into a dry well or, if the elevation allows, to an exterior drain.
But even with drainage through the overhead doorway, many concrete driveways and aprons hold up well under freeze-thaw conditions. The keys are to make sure you have air entrainment in the mix, seal the slab and driveway when new, then instruct the homeowner to maintain it properly.
We seal new driveways and slabs with Kure-N-Seal 30 (editor's note: now called MasterKure) from BASF. The frequency of resealing depends on wear patterns for a particular driveway. Basically, as soon as the sealer has worn off, it's time to apply a new coat. A good test is simply to wait until the drive is perfectly dry and apply a few separate drops of water on the drive using an eyedropper. If the drops soak in, you need to seal the surface; if they bead up, you're okay.
If you do install the drain in the center of the garage, be sure to check your local and state building codes. Some may allow a dry well, while others may require connecting to an exterior drain.