Concrete Basics: Footers and Slab Preparation

A solid foundation is the basic underlying principal to which all concrete slabs achieve their strengths. Footers of thickened concrete filled with steel for strength create base for the slab to rest upon and are designed to take the weight of not only the slab, but the entire structure built on top of it.

In this article I'll explain the process of digging a proper footer and grading a slab once the forms have been set. If you haven't set the forms yet, I suggest reading the other articles in this series.

Once you have your form boards set, you can begin to shape the interior of your slab. A few questions should be answered before you get out the shovel. How much dirt do you need to remove or add to your slab to make sure only four inches of concrete is poured? The answer to that question can be solved by running a few string lines. Tack a nail into the back side of one corner of your form boards. Do the same on the other side of the slab but at a slight angle. Tie a string line tight between these two nails. Now measure down from the string line to the dirt in various spots moving the string line as you go along.

The measurements will give you a rough estimate of how much dirt needs to be removed or added. If it needs to be removed, find a good spot for it. If you have to add dirt, dig it out from the footers and throw it onto the slab.

Once you get a nice square pointed shovel, mark the end of the shovel down the length of the handle the same measurement as your footer is. Now you won't need to alternate between using a tape measure and a shovel.

Mark the width of the footer in the dirt with your finger. I like to cut an over sized piece of wood and drive a nail into it at the desired length. Then I run the board down the length of the footer, dragging the nail into the dirt. It helps create a good line over varying terrain.

Start digging. It doest get any less technical than that. Use the dirt you need or get rid of it. Keep using your shovel handle to check the depth. Use a pick ax to dig hard soil. Rocks can be removed or left in place if they are too large to remove. Exceptionally soft soil should be excavated no matter what the depth. If you can shove a piece of #5 rebar into the soil with your hand more than a foot, you need to keep digging. Use the board nail board guide to maintain the proper width of your footer as you continue your way around the slab.

Once you have the footers dug to the proper length, its time to correctly grade the slab. Stretch the string line across the forms once more and measure down to achieve a uniform depth of four inches. Using a metal rake helps to grade down loose soil over partially flattened terrain. Spread the dirt wherever it's needed as you work from one end of the slab to the other. Don't worry if dirt falls into the footers a little. You're going to need to clean them out again anyways.

Once the slab is graded, drop back down into the footer and put a 45 degree edge between the footers edge and the slabs surface. This will thicken the edge of the footer slightly to tie in the slab. This design is called monolithic.

Clean out any remaining dirt from the footers and you are now ready for the slabs surface penetrations to be put in. These include and underground electric or gas and any plumbing pipes that may go into the structure. While I won't go into that in this series, I would also recommend hiring a professional for those services.

Once all the slab preparations have been completed its time to call in the pest control. If you plan on building any structure or butt it up to any structure, then it needs to be poisoned for termites. Termites can easily find refuge under any slab that's not poisoned. From there they launch an attack on any wooden structure above it.

Once the soil has been sprayed, immediately cover it with construction mil plastic. This plastic is used for several reasons. The first and foremost is a vapor barrier. This prevents water from entering through the concrete into the structure above. If you don't have a structure, such as a driveway, then you don't need plastic or termite poisoning.

Seal all the plastic laps and penetrations with duct tape to prevent the termite poison from evaporating and keep the ground water in the ground. Once you have the plastic down its time to place the steel.