Keeping the basement dry is one of the most important tasks for a builder. The finished first floor elevations, as well as the top of the foundation -- or basement -- wall are considered critical construction information for the site drainage plan. Most building codes define the finished first floor as the lowest floor level that is entirely at or above the ground. Since the basement is below grade, basements are generally not included as first floor elevations.
Finished Floor and Foundation Wall
The elevation of the structure above the surrounding ground is key to the structural integrity of the foundation. The finished floor elevation -- indicated as FFE on drawings -- is a significant reference point for construction, especially for site grading and drainage activities that ensure water will flow away from the structure. The finished floor elevation is related to the top of the foundation wall, which is set above the elevation of the street adjacent to the property.
Regulations require the ground to slope away from the foundation walls, usually at 5 percent -- 6 inches drop over 10 horizontal feet -- to keep water away from the foundation. This is referred to as positive drainage. The ground around the structure is lower than the foundation wall, or slab-on-grade, by a specified dimension, and runoff is directed to surface or subsurface drainage systems that convey it to a safe discharge point.
Finished Floor Elevation
The finished floor is constructed on the foundation wall, or the slab-on-grade. Depending on the terrain, the top of the foundation wall and the FFE may be close to the adjacent ground, or set several feet above it, requiring steps or a ramp to reach the entrance to the house. The top of the foundation wall is a visual aid for site grading activities, but the finished floor elevation can be used for calculations.
Sloped land offers the opportunity to create walkout basements at the low end of the slope. Part of the walkout basement meets the ground plane, but depending on local regulations, it may not meet the definition of being a first floor, since a portion is below grade. Special attention has to be paid to walkout basements to ensure positive drainage. Where ground moisture or flooding is a concern, most building officials prefer raising the foundation wall and the finished floor elevation at a safe distance above grade.
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