Q.: We're going to be paving a parking lot that is 90 by 220 feet. The plans call for a 1/2 percent slope in both the east-west and north-south directions. I told the owner's representative that to avoid birdbaths the minimum slope should be 1 percent (1/8 inch in 1 foot). The representative said a 1/2 percent slope in each direction is equivalent to a 1 percent slope. How do I deal with this kind of logic?
A.: You're right about the 1 percent minimum slope needed to prevent ponding or birdbaths. The Portland Cement Association (PCA) publication Building Quality Concrete Parking Areas recommends sloping the pavement 1 percent to drainage. The American Concrete Institute's (ACI) Guide for Concrete Floor and Slab Construction recommends a slope of about 2 percent (1 inch in 4 feet) to provide positive drainage.
Here's the logic we'd use to show that 1/2 plus 1/2 doesn't equal 1. Take a one-foot-square section of the lot sloped 1/2 percent in each direction. The greatest fall from the high corner to the low corner will be 0.01 foot (0.005 foot in the east-west direction plus 0.005 foot in the north-south direction). Water will drain on a 45 angle across the section from the high corner to the low corner. The diagonal has a length of 1.414 feet so the slope is (0.01/1.414) x 100 = 0.71 percent.
We'd recommend using a minimum 1 percent slope in both directions even though the resulting slope in the direction of water movement will then be slightly greater than 1 percent.