How to Build a Slab Foundation

A slab foundation functions as a base for new homes, garages, sheds, gazebos, and other construction projects. To build such a foundation, one needs to have at least a basic construction skill set. Building a well-made slab foundation will determine the longevity of your project and potentially the stability of your home or other structure. It requires a lot of patience, knowledge, and skills to make one. Here are some basic steps to follow.

Step 1 - Plan the Project

Determine how wide the slab foundation will be. Its width will depend on the ground area as well as on the design of the structure it will support. The slab foundation should be the same dimensions as that of the planned building. You will need to be precise in these measurements and change them if the home design evolves.

Step 2 - Prepare the Ground

Remove any impediments on the ground. They include sticks, rocks, and pieces of debris. Levy the ground using either a backhoe or a pick and a shovel. Backhoes make the work much easier to complete. You can complete this work yourself or higher out to the get the job done.

Mark the perimeter of the slab foundation. Drive stakes at every corner around the perimeter to suit this purpose. Measure the perimeter to check if it fits the dimensions that you pre-planned. Check and double check for accuracy. Tie a string around one stake and connect it to the other stakes to mark the perimeter in a straight line. Make sure the string is equal distance from the ground all the way around.

Dig footers around the perimeter afterward. Facilitate digging by using a backhoe, but you can also work with a pick and a shovel. The depth of the footer will depend on the estimated weight of the entire structure to be built. The footer will serve as a foundation for the slab foundation, providing support to the building as well.

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Foundation construction is an integral part of any building project. Although pouring a slab can be dirty back-breaking work, the process of preparing an area for a concrete slab is relatively easy. Here are a few of the things that will need to be done to prepare an area before any concrete is poured. It is important to note that the following article explains the things needed to prepare for an above ground house foundation slab. Below ground slabs and slabs needed for commercial buildings, may require different preparation techniques.

1 – Frame the Area for the Slab

Using 2-inch by 12-inch boards, make a frame around the area where the slab is going to be poured. This frame is referred to as the form. Premade forms can be used, but are often more expensive than using regular boards. Once the form is in place it must be reinforced with stakes and 2-inch and 4-inch boards so that when the concrete is poured its weight does not bow it.

2 – Level the Ground within the Form

Shovels and rakes must be used to completely level the ground within the form. Use a laser level to ensure the ground is totally flat.

3 – Dig Footings

All along the inside of the form, about 3-feet in from the perimeter, a trench that is 1-foot wide and around 7-feet deep must be dug. To ensure that the trench is dug in a straight line, it is common to run a string from one end of the form to the other. The trench is then dug directly below this string. It is important to dig the footing trench correctly because it is what keeps the slab from being able to tip back and forth as the soil gets wet, freezes, etc. Similar trenches must be dug in the areas that will sit beneath any load bearing walls of the house that is to be constructed on the slab.

4 – Tamp the Ground Down

To compact the soil as much as possible so that it is a stable base for the foundation, a tamper or jumping jack must be run across it. These tools slam a heavy weight against the soil over and over again forcing the particles in the dirt close together. On extraordinarily big jobs, a steam roller is sometimes used for this. Be sure to tamp the trenches dug for the footings, and remember: there is no such thing as too much tamping.

5 – Prepare Rebar

Push stakes down into the footing trenches so that their tops stick up to about the middle of the trench. Along these stakes rebar must be tied so that once the concrete is poured, the rebar will float in the middle of the trench and reinforce the footing. Once the rebar is placed correctly in the footing trenches, a rebar grid must be built above ground. Once again, use stakes to raise the rebar up off the ground. The rebar grid should cover the entire area within the form and each bar should be about 2-feet apart. This will create a series of 2-foot by 2-foot squares. Lash the rebar together with rebar ties and begin mixing the concrete that is to be poured.

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Step 3 - Pour Cement on the Footer

Mix mortar and sand to create the cement, sticking to the manufacturer’s instructions. Pour the cement into the footers to cover the entire perimeter of the foundation. Allow some time for the cement to dry.

Step 4 - Lay the Foundation

Surround the whole perimeter with 2x4-inch or 2x6-inch wooden boards to lay the foundation. Make sure the wooden boards form the entire lengths and widths. They will hold the concrete that you pour to create the foundation. Nail the wooden boards together at the corners and make sure they can support the concrete.

Before you start pouring the concrete, lay the rebar across the foundation to make the concrete structure a lot stronger. Afterward, fill the entire area with concrete mix. Typically this comes directly from the concrete truck and will be poured from deeper sections to more shallow sections. Using a trowel or a piece of wooden board, level the poured concrete. Allow the concrete to cure completely before construction begins. The curing time will depend on the type of mortar that you use and the weather at the time of the pour.

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A slab foundation is a simple concept. Basically, it's a concrete pad poured on the ground so in theory, it's well within the skill set of a competent DIY'er. However, because you don't get any 'do overs' when pouring a foundation most people wisely opt to contract the job out to a professional. However, to give you an understanding of what is involved in the process here's an overview of how to pour a slab foundation.

Pouring a slab foundation

• Start by laying out the outline of your foundation. Figure out where the corners are located and drive stakes into the ground at each corner, then run string between the stakes. These strings will be the outline of your foundation.
• Inside the strings dig a trench 18” wide and at least 2' deep (in colder climates you need to go down deeper to get below the frost line – usually 3 to 4 feet). This trench will form the footings for your foundation and provide the support for the walls of your building.
• Around the trench build wooden concrete forms using 2” x 10” boards. Brace the forms every 2” using wooden stakes behind them.
• Install rebar into the perimeter trench in a continuous line at least 2 inches from any outside surface. Overlap the rebar and join the pieces together using rebar wire to keep the rebar line continuous. Lift the rebar from bottom of the trench by setting it on 'rebar chairs' every 12 inches.
• Spread a layer of sand or gravel covering the inside of the slab form 4 to 6 inches deep. Level the gravel then cover the interior of the surface of the pad with 6 mil plastic to help control moisture infiltration after the concrete pad is poured.
• Install rebar in a grid pattern again being sure to keep it elevated using 'rebar chairs'.
• Conduit for electrical wiring, water and drain pipes is installed above the plastic sheet making sure they are up from the bottom of the slab before pouring the concrete slab is poured.
• Fill the footing forms with concrete and use a piece of 2” X 4” to level it. Install anchor bolts every 12”.
• Continue pouring concrete into the center of the form creating a concrete pad 4” to 6” deep and level with the top of the exterior footings. Use long handled trowels to smooth the interior pad.
• Allow the poured concrete to set up for at least 48 hours before removing the wooden forms.

Local building codes will actually dictate the specific details of how your slab foundation is poured in your area. Details such as the thickness of the slab, depth and width of the footing, size and location of the rebar as well as the type of sand or gravel are commonly specified in local codes.

Murray Anderson is a veteran freelancer whose work has been appeared in books, newspapers and newsletters as well as on numerous web sites in both the United States and Canada. He writes on a wide range of topics including home, consumer, and personal subjects as well as general business and Marketing specific topics.