Calculate drainage slope percentage from two field elevations and a horizontal run length. Checks slope against your minimum requirement and returns slope %, ft per ft, drainage direction, and pass/fail status. Built for grading verification and site drainage documentation.
Rotary laser levels and GPS rovers for drainage slope verification surveys on grading and site work projects.
Shop Express Tools →Drainage slope is calculated from the elevation difference between two points divided by the horizontal distance between them. The result tells you how quickly water flows across the surface and whether the slope meets minimum requirements for drainage design.
| Application | Min Slope | Code / Standard |
|---|---|---|
| Foundation drainage (first 10 ft) | 5% (6 in/10 ft) | IRC R401.3 |
| Foundation drainage (beyond 10 ft) | 2% | IRC R401.3 |
| General site grading | 2% | IBC / local codes |
| Parking lot (paved) | 1–2% | ADA / IBC |
| Swale / drainage channel | 0.5–1% | Project specifications |
| Solar interrow drainage | 1–2% | Racking engineer spec |
| Sewer pipe (8 in) | 0.40% | ASCE/WEF (self-cleansing velocity) |
| Road surface drainage | 1.5–4% | AASHTO / state DOT |
Verify minimum slope requirements with local codes and project specifications. Requirements vary by jurisdiction.
Drainage slope verification is most effective when done as part of a systematic pattern of shots across the site. Take shots in a grid pattern (every 25–50 ft on large sites) or at specific required locations (lot corners, foundation edges, swale centerlines). The goal is spatial coverage that identifies any flat spots or reverse slopes that field observation might miss.
Pay particular attention to areas adjacent to building foundations, on the downhill side of filled areas, and at transitions between graded surfaces and natural ground. These locations are where drainage problems most commonly occur — fill settles slightly below surrounding ground, creating a bowl that directs water toward the structure instead of away from it.
For pad elevation certifications, PE packages, and building department submittals, drainage slope verification must be documented with actual field elevations — not just a statement of compliance. The documentation should show the measured elevations at the required points, the calculated slope at each location, and a pass/fail determination against the applicable code requirement.
Sitemark captures drainage slope shots in the field and generates the documentation automatically. Calculate slopes from field elevations, flag locations that do not meet minimum slope, note corrective actions, and include drainage slope documentation in the as-built report alongside pad elevation data.
Minimum drainage slope requirements vary by application: general site grading requires 2% minimum slope to drain surface water; pad drainage per IRC R401.3 requires the ground to slope away from the foundation at a minimum of 5% (6 inches in the first 10 feet) to prevent water infiltration into the foundation; parking lots require 1–2% minimum; swales and drainage channels typically require 0.5–1% minimum. Always verify with local codes and project specifications.
Slope percentage = (upstream elevation minus downstream elevation) divided by horizontal run length, multiplied by 100. For example: upstream elevation 452.85 ft, downstream elevation 452.40 ft, run length 22.5 ft. Slope = (452.85 - 452.40) / 22.5 × 100 = 0.45 / 22.5 × 100 = 2.0%. A positive result means the surface drains correctly (downhill from upstream to downstream). A negative result means a reverse slope (water flows toward the high end).
2% slope means the surface drops 2 feet for every 100 feet of horizontal run (0.02 ft per ft). Over 25 feet of run, a 2% slope drops 0.50 ft (6 inches). Over 10 feet, a 2% slope drops 0.20 ft (2.4 inches). The IRC foundation drainage requirement of 5% equals 0.5 feet of drop over 10 feet, which is the 6-inch-in-10-feet rule stated in a different form.
A reverse slope is a drainage problem where the ground surface slopes toward a building, structure, or uphill area instead of away from it. Water pools against the building or at the high end and does not drain. Reverse slopes are a common grading defect on residential pads and commercial sites with complex grading. They are identified when the slope calculation returns a negative percentage — the downstream point is higher than the upstream point.
Per IRC R401.3 (International Residential Code), the ground must slope away from the foundation at a minimum grade of 6 inches in the first 10 horizontal feet (approximately 5% slope). Beyond 10 feet, the slope may be reduced to 2% minimum to a drain or swale. These requirements apply to all sides of the structure. Many engineers require 5% minimum for the first 10 feet and 2% beyond — document drainage slopes at each corner of the building to confirm compliance.
Higher elevation (uphill end)
Lower elevation (downhill end)
Horizontal distance between points
2% typical for graded surfaces; 5% per IRC for foundations