The slope code ladder — drainage minimums and accessibility maximums from 0.2% to 15%

Codes write slopes both directions: drainage slopes are minimums (gutters ≥ 0.2%, french drains ≥ 1%, patios ≥ 2%, foundation grading ≥ 5% per IRC R401.3) while accessibility and driveways are maximums (walkways ≤ 5%, ramps ≤ 1:12 = 8.33% per ADA 405.2, driveways ≤ ~15%). The calculator checks your grade against these presets.

What this diagram shows

A bar chart of the slopes building codes actually specify, from nearly flat to steep. Drainage slopes are minimums, shown with a greater-than-or-equal sign: gutter runs at least 0.2 percent, a quarter inch per 10 feet; french drains and 3-to-6-inch drain pipe at least 1 percent, an eighth inch per foot per IPC Table 704.1; patios and paving within 10 feet of the house at least 2 percent per the IRC R401.3 exception and ICPI guidance; drain pipe two-and-a-half inches and smaller at least 2.08 percent, a quarter inch per foot; and the ground away from a foundation at least 5 percent, 6 inches of fall in the first 10 feet, per IRC R401.3. Accessibility and driveway slopes are maximums, shown with a less-than-or-equal sign: accessible walkways at most 5 percent, 1 in 20, per ADA 403.3; accessible ramps at most 8.33 percent, 1 in 12, per ADA 405.2; and residential driveways at most about 15 percent by typical municipal standards. Flatter than a minimum and water stands; steeper than a maximum and the surface fails code.