Driveways are not just a site for parking a car. Poured concrete driveways provide a stable surface for your vehicle. Compared to gravel or mud driveways, concrete drives are less susceptible to pitting and washing away in heavy rains, and the smooth surface makes parking easier. The depth of the concrete in the driveway is important for properly supporting the weight of your vehicle and allowing drainage of the driveway.
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Generally, most residential driveways are four inches thick, according to Charles G. Wing in "Ortho's Home Improvement Encyclopedia Problem Solving A to Z." This includes only the concrete of the driveway and not sublayers. Check the local code in your area. You will likely need reinforcement inside the concrete. This might include reinforced steel bars - also called rebar - or a wire mesh structure.
Christine Beall in "Masonry and Concrete," notes some prefer to create a sand bed below the concrete in a driveway to help the concrete cure faster. This sand layer, if used, should slope down 1/8 to 1/4-inch per foot of distance. The shallowest the sand should be is 1-1/2 inches. To determine the how deep the sand should be, measure the length of the driveway from the street to the sidewalk where the slope will begin. Multiply this distance by 1/4. This will give you the number of inches to add to 1-1/2 inches to find the depth of the deepest sand layer. For instance, if the distance between the street and the sidewalk where you will begin the slope is four feet, you will add one inch to the 1-1/2 inch depth of the initial sand level for the deepest sand portion. Reducing the amount of sand over the sloping portion of the driveway allows you to keep the concrete a uniform depth. For the sand level from the garage to the sidewalk, slope it down 1/8-inch per foot of length.
Sloping the driveway does not begin at the garage. Slope the driveway in the sand layer by 1/8-inch per foot of distance until you reach the sidewalk. There you can increase the slope to 1/4-inch per foot of distance, if needed to reach the street. Sloping should be done in the sand layer and not in the concrete. You must maintain the same depth of concrete throughout the driveway.
Some driveways will require a deeper concrete layer than others. If you own several cars or large vehicles such as trucks or SUVs, add an extra inch of depth to the concrete layer, and use reinforced bars or a wire mesh inside the concrete, even if your code does not require it. These measures will provide the additional strength needed in the concrete to support the weight of your vehicles.