Foundation Drainage Grade Check (IRC R401.3)
Whether the ground falls away from a house fast enough under IRC 2024 R401.3.
Example
You enter
- Surface pervious
- Fall measured away from the foundation (in) 4
- Run it was measured over (ft, the first 10 ft or less from the foundation) 10
- Does a lot line, wall or other barrier stop the 10 ft short? no
You get
- Slope (%) 3.33333
- Required fall (in) 6
- Margin (in) -2
Checked against published worked examples from: ICC.
Details, formula, and sources
ScopeNot fewer than 6 in within the first 10 ft from the foundation, a 5 percent grade, and not less than 2 percent away for a patio, walk or drive within 10 ft. Enter the fall a tape or level gives and the run it was measured over. A 4 in drop over 10 ft is 3.3 percent and 2 in short; an 8 in drop is 6.7 percent and passes. A fall measured over a shorter or longer run is read as a uniform slope and applied to the first 10 ft. Where a lot line, wall or other barrier stops the 10 ft, the code calls for drains or swales instead, and the calculator says so rather than failing the lot. An 8 ft patio needs 1.92 in of fall across it. The same rule stands in IRC 2021. Swale and drain sizing are separate.
Formula or tableslope = fall / (run x 12); ground: fall in the first 10 ft = slope x 120 in, required 6 in; barrier within 10 ft and under 6 in of fall: drains or swales; impervious surface: slope >= 2%, required fall = 0.02 x run x 12.
Source and editionIRC 2024 (International Residential Code). Section R401.3 (the same text in IRC 2021), read at codes.iccsafe.org on 2026-10-19.
Free accessFree to read at codes.iccsafe.org.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namessurface, fall_in, run_ft, barrier, slope_pct, required_fall_in, margin_in
- Uniform slope over the run measured a hump or low spot inside the run is not seenstated assumption
- Barrier exception for ground only a patio's 2% applies regardlessIRC R401.3