How to Build a Concrete Pad

Robert Korpella

Homeowners install concrete pads to create solid, level surfaces for purposes such as driveways, mechanical system mounts and detached garage foundations. A pad differs from a foundation meant to support larger structures, such as homes.

Finishing begins before concrete is completely dry.

Although sometimes referred to as a pad, foundations are partly underground and have deep footers, while a true pad is mainly built above ground. Installing a pad, especially a larger one, is tough work, but you stand to save 30 to 50 percent by doing the work yourself. Most of the work occurs before the concrete is ever poured. The proper pre-pour efforts ensure a reliable, long lasting pad.

Site Preparation

Check your local building codes for any nuances involving concrete pads, such as allowable locations on your property and required dimensions. Once you’ve located the site for your pad, dial 811 on a phone before you do any digging. Alerting the utility companies ahead of time saves damage to buried electrical and other lines. Clear the site of grass and other vegetation, and about 8 inches of topsoil if your site has loamy or clay soil. If you are building a large pad -- one intended to support walls and a roof -- dig a trench along the perimeter of the pad, which provides a foundation strong enough to hold up these structures.

Adding Forms

Forms hold wet concrete right where you want it during pouring. Build them along the outside edge of where the pad will be. Construct forms from two-by-twelve lumber if you are building something like a garage floor, and two-by-six lumber for smaller pads that do not have trenches built along the perimeter. Anchor the forms in place with two-by-four stakes driven straight into the ground along the outside of the form. Support those stakes with two-by-four kickers driven into the ground at an angle, and resting against each stake. The forms should be mostly level, save for a slight grade of about a quarter-inch per foot that allows water runoff.

A Strong Base

Achieve proper drainage and add a base to the concrete by adding granular fill, such as limestone gravel. Tamp the fill to compact it. Smaller pads may need just a few of inches of fill, but larger pads require up to 5 or 6 inches. Add a layer of 6-mil plastic sheeting on top of the fill for pads that will accept walls. The plastic provides a vapor barrier that prevents concrete from wicking moisture into the finished building. Add rebar and wire mesh, which give even more strength to the pad and help dissuade concrete from cracking over time.

The Concrete Order

Tell the concrete supplier what purpose the pad will serve so the supplier can provide the correct concrete mixture. The supplier sells concrete by the yard. Multiply the length, width and depth measurements of your pad, and divide that number by 27 to arrive at the yards of concrete needed for your pad. Add 5 to 10 percent for waste and shrinkage.

Pouring and Finishing

Since concrete is very heavy, you cannot shovel or rake it very far. Start pouring along the forms furthest away from the truck hauling the load, then move outward, spreading the material as evenly as possible. Use a screed -- a long two-by-four you can pull across the wet concrete -- as a means of flattening the material. A bull float worked over the surface removes screed marks, and an edger gives a slight rounding to the edges. Once the concrete has dried enough to support your weight, cut straight grooves about 10 feet apart on large pads. Cut these grooves to a depth of 25 percent of the thickness of the concrete slab. They relieve some of the stress in concrete pads, reducing cracks.