Plan Ahead for Sufficient Site Grading and Surface Water Drainage

Site grading plans should be developed to direct water away from the building foundation, particularly if the building is located down-slope from a hill or similar land formation that may produce significant rainfall runoff. Figure 3–2 illustrates key elements of a site grading and drainage plan.

Use of grassy swales is a common and cost-effective practice when the potential water volume is not large, wetting is not constant, and the swale is not sloped steeply enough to produce high water velocities. The range of acceptable swale slope depends on many factors, but slope should not be less than about 1% to prevent ponding, nor more than about 15% unless rip-rap (4” to 8” stone) with a filter cloth underlay is used to line the swale.

Model building codes typically require grading of 6” of fall, or drop, out 10’ from the foundation of the building or as far as practical. Care should be taken in the backfill process to avoid settlement of the grade closest to the foundation as this is one of the most likely places a drainage failure can develop. Compaction during the backfill process is a direct solution to this issue. Additional soil may also be added to help offset a moderate amount of backfill settlement, but not so much as to reduce the clearance between grade and the bottom plate to < 6”. Self-compacting backfill such as pea gravel may also be used to avoid settlement which occurs over time and results in insufficient grade away from the foundation.

Concrete flatwork, such as walkways, driveways, and patio slabs, which is adjacent to the building should be sloped ≥ 2% (about ¼” in 12”) away from the building. The soil beneath the flatwork should be properly compacted when installed and is critical to minimize differential settling of the walkway or driveway that can frequently reverse slope and bring water toward the house. Exterior flatwork settlement is a very troublesome durability mistake as it will create messy puddles, icy spots, high step-ups to front porches, etc. that homeowners must endure for years to come (Figure 3–3). In addition, gutters and gutter drains should be used to further remove roof run-off from the foundation area (see Section 4.2.6).