Yard Grading & Drainage Slope Calculator (Fall Away from the Foundation, Fill & Swales)

How it works

target fall = 6″ over the first 10 ft (5%) + 2% beyond · raise at wall = target − current · fill yd³ = (raise ÷ 12) × band width ÷ 2 × perimeter ÷ 27 × 1.15 · runoff Q (cfs) = (0.95 × roof + C × yard) × rain (in/h) ÷ 43,560 · grass swale capacity by Manning (n = 0.035, 4:1 sides, 2%)

Most wet basements are grading problems: the soil next to the wall has settled or was never sloped, and roof water lands within reach of the foundation. The rule is a six-inch fall in the first ten feet and a gentler two percent beyond, so regrading is a wedge of compacted, clay-rich fill whose volume follows from the perimeter, the band width and the inches to add — with a watchful eye on keeping the siding clear of the new grade. Where the lot itself slopes toward the house the fix becomes interception, a shallow grass swale or a French drain sized to the roof and yard runoff of a design storm, and always after the downspouts have been sent well away from the wall.

Worked example

A 120 ft perimeter with only 1 inch of fall in the first 10 ft needs 5 more inches at the wall over a 10 ft band — about 10.6 yd³ of fill with compaction, roughly $375 delivered. A 2 in/h storm on 1,200 ft² of roof and 2,000 ft² of loam yard produces about 38 gal/min; a minimal grass swale 6 inches deep and 4 ft wide at 2% carries it easily, and the downspouts should discharge at least 6 ft from the wall.

Frequently asked questions