When it comes to planning your home or any additions to it, thickness and the overall height of structures is important to determining the finished results. With concrete in particular, there is more to it than just the thickness of the slab, and you have to factor in the extra elements in order to know how to properly install the concrete.
The minimum slab thickness you can use for a residential home is 4 inches. As a general rule this is the thickness for driveways, sidewalks, patios and porches. The more weight that the slab is expected to hold, the thicker the slab needs to be, and the only way to calculate the thickness properly is to contact a licensed structural engineer to help determine the weight of the home or other structure that will be on top of the slab.
For patios, sidewalks and retaining walls that don’t have a major load to bear, part of the overall thickness of the concrete installation is the layer of sand and gravel beneath the slab that works as part of the overall structure. Concrete cannot be poured directly on top of the ground or it will settle according to the contours of the dirt, leading to shearing and cracking where the slab settles at different heights. As such, at least 2 to 4 inches of compacted sand is required as part of the overall excavation thickness for digging the area out for the slab.
Compacted gravel is also part of the thickness of the overall slab preparation area and has to be taken into consideration when planning to pour any type of concrete structure. For regular walls that don’t have a lot of weight to worry about, 6 to 8 inches of compacted gravel is generally used. The gravel is primarily to help the soil drain water away so it never has a chance to freeze and thus expand and buckle the concrete above, but is also helps cushion the concrete along with the sand.
Your structural engineer can help you determine the exact width and depth of the footer, but any load-bearing concrete structures have to have a footer under the wall. The footer extends out on either side of the wall below the soil and helps spread the weight of the wall. Without a footer, concrete walls and foundations will cut into the soil like a knife, causing rapid sinking of the home.
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