Concrete walls and pillars must rest atop suitable footings and foundations.
The strength and stability of a concrete footing is vital to the column or wall that rests atop it. The footing must be big enough to distribute the weight of its load across a sufficient patch of rock or soil. The larger the footing, the more concrete you'll need. Prepare a little extra concrete to ensure you have enough.
Soil
The key considerations for footing size are the load-bearing value of the soil and the weight of the house. Soil load-bearing strength is often measured in pounds per square foot, or psf. The bigger the house and the weaker the soil, the broader the footing must be. If the soil has a load-bearing capacity of 4,000 psf and the house has traditional wood-frame construction, for example, footings for walls must be at least 6 inches wide. The Concrete Network website lists the necessary dimensions for footings beneath various types of walls and on various kinds of soil.
Unexpected Events
No matter how careful you are in choosing the dimensions for a footing and planning ahead, the construction will seldom follow the drawings and plans precisely. Excavation may end up removing large rocks and forcing the contractor to dig a little deeper than he had anticipated, or rain may force him to go down farther to find firm soil. Consequently, you may need to recalculate the dimensions of your footing after excavation and take into account the new figures for the depth.
Calculations
Once you have definite figures for the width, length and depth of your footing, you can figure out how much concrete you need. Concrete is generally measured in cubic yards. To calculate how many cubic yards are required, make sure all your figures for the dimensions are expressed in feet, not inches, and multiply length by width by depth and divide by 27. The result will be the volume in cubic yards.
Excess
It's often helpful to have a little more concrete than you need. A little additional concrete doesn't cost much, but running short may mean paying your work crew extra money, incurring charges from your ready mix supplier or ending up with a weaker joint between the end of one pour in the footing and the beginning of another. If you calculate that you need between 1 and 5 cubic yards, add 0.5 to be sure you have enough. For 6 to 10 cubic yards, add 1 cubic yard to your order, and for 11 to 20 cubic yards add 1.5 cubic yards of concrete.
