Excavating the Site

      

Although this article deals with "Concrete Slabs," we've included info. on concrete patio basics. This way you can gain the knowledge of both projects-slabs and patios-in the same dose. Both are really constructed in about the same way, using about the same technology.

Working with large amounts of fresh concrete is a hot and heavy job. It takes some skill; however, it takes more planning and organization than special talent.

AN OVERVIEW:
Before starting any concrete project, be sure you check local building code regulations so you can build the project to the correct specifications. Building codes are designed to ensure the longevity of the project. This is especially true in areas subject to ground heave caused by freezing temperatures. An improperly prepared site will result in a cracked concrete surface. Building codes respond to these conditions, so don't ignore them.

LOCATING UTILITIES:
Before any construction, determine the location of all underground utilities.

The major concern here is that deep footings may conflict with the utility service. Water, gas, sewage, and telephone lines may force you to incorporate special construction or to relocate the project. If your home is fairly new, your local building inspector probably has a copy of the utility hookup locations. If not, check with the customer-service representatives of the local utility companies. If there is a conflict, discuss your options with the service in question.

EXCAVATING THE SITE:
Lay out the dimensions of the slab and excavate as needed. The depth and width depends on the project and the specifications in local codes.

The subgrade needs to be uniform throughout to correspond with temperature and moisture changes. Level the earth surface, so there are no deep holes or protruding objects. Remove all large rocks, roots, and other debris. Then tamp the subgrade, either with a hand tamper or a mechanical tamper you can rent.

The pitch-
A concrete slab must be pitched, or set at a slight angle, to ensure necessary drainage. Pitch must angle away from a house or other structures, such as a garage. This is especially important in a patio or driveway project.

A proper pitch is usually 1/4-inch per foot, depending on the amount of rainfall in the area. Codes will tell you this. An example: if the slab extends out 15 feet from a house, the pitch would be 15 feet. X 3/4-inch = 3-3/4 inches. Therefore, the edge of the slab farthest from the house rests 3 3/4-inches below the edge nearest the house.

Creating the Pitch-
Once you have completed the excavation, you create the pitch. Example: drive Stake A at a corner nearest the house. On this stake, mark the proposed height of the slab. Most on-grade slabs are about 2 inches above the ground. Straight out from the house, pound in Stake B at the point corresponding to the end of the slab farthest from the house. Stretch a chalk line at the mark on Stake A to Stake B. Check the string with a line level. Now measure down from the string to a distance equal to the required pitch. Retie the chalkline at this point.

The pitch will affect the angle of the slab, but not the thickness. Therefore, you will have to excavate high spots at the high points and backfill this earth to the low spots so the height remains uniform throughout. The string across the stakes will serve as a guideline.

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