How to Size Concrete Footings for a Covered Deck

The very bottom of any structure is a concrete block called a footing. Footings are buried in the ground. Deck footings support concrete columns called piers that extend above the surface, and the deck is built on top of them. The size of the footings for your deck should already be specified by your local building codes. In general, footings must be below the lowest recorded frost for your location, on solid ground or rock, and they must be large enough and numerous enough to allow the earth or rock under them to bear the load of footing, the pier, the deck, the roof, the weight of any snow on the roof and the people and furnishings on the deck.

Calculator
Blueprints or plans

Instructions

- 1
  Calculate and total the weight of every element of your completed deck using your blueprints or plans. Add the weight of the maximum number of people who may stand on your deck and the weight of 2 feet of snow on your deck roof.
- 2
  Assume that each square foot of your roof will have to support 50 pounds of snow. Assume that your footings will be at least 8 inches thick. On average, concrete weighs 145 pounds per cubic foot, so a footing that is 2 feet square and 8 inches thick will weigh 387 pounds. Use the same average of 145 pounds per cubic foot to estimate the weight of your concrete piers.
- 3
  Estimate the load-bearing capacity of your soil. Rock can support 6,000 pounds per square foot. Silt and clay can support only about 2,000 pounds per square foot. When in doubt, assume your poor soil can only support about 1,500 pounds per square foot.
- 4
  Divide the total maximum weight of all elements of your deck by 66 percent of the load-bearing capacity of 1 square foot of your soil. For example, a 10,000-pound deck built on poor soil would need at least 10 square feet of footings.
- 5
  Divide the area of the footings you need for your soil by the number of footings in your building plan to determine the approximate size of each footing.

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