Q: What is a good rule of thumb for the fall (or slope) on exterior flatwork? I've heard both 1/8 inch per foot and 1/4 inch per foot.
A: It depends on the application. You have to consider the surrounding grades and any nearby structures, of course. But the reason for including any fall, rather than making the surface level, is to allow the concrete surface to drain. Generally, using 1/8-inch fall—which is 1-1/4 inch per 10 feet—works well on larger surfaces like driveways and patios, but it requires good screed, bullfloat, and finishing operations to minimize water puddling (birdbaths). For smaller surfaces, such as the treads on steps, use 1/4 inch/foot fall.