How Much of a Slope Is Needed for a Daylight Basement?

Daylight Basements

• Daylight basements -- or walkout basements -- take advantage of slopes and hillsides to create a basement that's partially at or above grade level. Usually the slope is to the rear, allowing a door to exit to the back yard. The configuration adds habitable space to the house -- an asset for the real estate value. When the slope is appropriately sized and the soil is suitable, building a daylight basement is cost-effective and visually pleasing.

Slope for Daylight Basements

• The best slopes for daylight basements are rated as steep, or moderate. Along with adequate slope, the soil should be well drained, and ideally the soil composition has good bearing capacity for construction. Without adequate slope, excavation and other construction activities add to the cost of the house, and importing granular material to replace poor soil also causes costs to escalate. Grading has to be expertly handled, and attention to erosion control and drainage is critical.

Surface Drainage for Daylight Basements

• Habitable space is regulated strictly by building codes. Waterproofing and drainage issues are a large part of the construction program for basements. Surface drainage is prescribed in many building regulations as an apron around the perimeter of the foundation, sloped away from the structure. Since daylight basements are usually located at or near the bottom of a slope, the gradient may be difficult to achieve, and subsurface drainage may be necessary.

Additional Considerations

• In regions with sensitive environments, such as wetlands, expansive soils or flood-prone conditions, daylight basements may not be encouraged, even for sloped lots. Local codes may specify a height above grade for the lowest floor that can't be met with the conditions. Where the slope is gradual, other options may be possible. A low slope may provide enough depth for a lookout basement, where windows can be set above grade or have stairs that lead up to a ground-level exit.