The Minimum Pitch & Proper Drainage for an Interior Basement Concrete Floor

Slab Slope

• If you are building a concrete slab for an interior space, the rules are the same for exterior slabs in the sense that sloping, or pitch, is required so that the slab will drain any water or excess moisture. The minimum requirements are one-quarter inch of slope per linear foot, which is the 2 percent minimum required by building code. While you can go more, it is generally not recommended for interior spaces.

Gravel Base

• The base of compacted gravel underneath the concrete slab helps with airing and breathing, which keeps down on the moisture content of a concrete slab. Instead, the extra air from beneath the slab helps it breathe properly. In addition, the gravel helps the water in the soil and from rain drain away rather than pool in the soil to form a moisture bed. Soil retains moisture far easier than gravel, which is why all slabs need a minimum base of 6 to 8 inches of gravel.

Interior Drains

• For interior slabs that have slope, drain tiles or some type of drainage system must be installed to help the slab drain. These can be in the form of floor drains that the slab slopes to, or they can be trenches that run to drains. The type of drainage system included with your interior slab depends on the design structure, which is completed with the help of a structural engineer for proper building code adherence.

Ideal Conditions

• Not all concrete slabs require sloping, but unless you are a licensed professional, you cannot know the ideal conditions for such a slab. Because there is a delicate balance of gravel base, soil conditions, moisture testing and other requirements, a professional’s guidance is always required if you want to build things to code. Dry soil with plenty of gravel that is built above grade (above the ground surface) generally doesn’t require drainage, but there are exceptions to the rule. Always talk with a licensed professional to know the requirements for your area and structure.