When building a porch foundation, the most important considerations are the weight the foundation must support, the soil conditions and the slope where you are building. First, estimate the weight the foundation must support, including the weight of the foundation; the weight of the porch; the weight of the roof, if there is one; the weight of the furniture; the weight of the people using the porch; and the weight of snow in the winter, if you live where it snows. Unstable soil conditions and steeper slopes require deeper footings of reinforced concrete. Footings also must go beneath the frost line to prevent frost heaves from pushing the footings up.
Estimate the heaviest weight that must be supported, then design the footings to support considerably more. For example, if you estimate the heaviest weight to be 2,500 pounds, build the footings to support 5,000 or 6,000 pounds. The added cost will be worth the added safety and life of the porch.
Dig the footing holes to a depth below the frost line. In some areas, this depth may be 4 feet. Frost heaves have tremendous force and can push the footings upward, causing damage to the porch.
Place a foundation tube in the hole to reach about 3 feet down. The concrete will extend through the tube to the bottom of the hole.
Mix the concrete to be thick. For example, use three parts cement to one part sand, then add just enough water to make a thick mixture.
Shovel the concrete into the foundation tube, filling it to the top. Once the concrete reaches the tube from the bottom of the hole, add two pieces of rebar to the concrete about every 6 inches until the tube is full.
Anchor the posts to the footings. Use either post anchors, which are placed directly into the cement, or a galvanized steel post base, into which you place an L-shaped anchor bolt in the middle of the footing. The shape of the bolt will hold it securely in the concrete. Push the post base slightly into the concrete. After the concrete is dry, put the washer and nut on the anchor bolt to hold the base securely in place.
Install 4 x 4 or 6 x 6 posts--depending on the weight you are building to support--to the anchors attached to the top of the footings. The porch framing will be attached to these posts.