In order to determine the proper size for your footings, you will need to establish how much total weight they are going to have to support and what kind of soil they are covering. To calculate the load, you should use 40lbs per square foot for live loads (these are variable loads that are dynamic such as the weight of people and furniture) and 15 lbs per square foot for dead loads (this is the weight of the materials used for the construction of the deck) for a total load weight of 55 lbs per square foot.

For example, if you are building a 10x10 deck attached to a house with two footings on the corners, you could calculate the loads for the footings in the following way. First, draw a line dividing the deck into two halves between the house and the footings. The load for the section nearest the house will be transferred back to the ledger board and carried down to the house foundation. The remaining half of the deck will again be split into two parts to be supported by the two corner footings. This is called the tributary load. If you multiply the area of this section 5’ x 5’, you will get 25 square feet. You can multiply this area by 55 lbs per square foot loading to come up with 1,375 lbs total load. Once you know the total load, you can use the chart below to determine the footing size for your soil conditions. Always be sure to check your calculations with your local building inspections department before digging.

__Circular Footings__

Maximum Allowable Load Per Footing, in Lbs

Soil Type __Gravel Sand Clay__

Allowable Pressure (Lbs /Sq Ft) __3000 2000 1500__

Footing Size (inches) 12 2300 1500 1100

13 2700 1800 1300

14 3200 2100 1600

15 3600 2400 1800

16 4100 2700 2000

17 4700 3100 2300

18 5300 3500 2600

19 5900 3900 2900

20 6500 4300 3200

21 7200 4800 3600

22 7900 5200 3900

23 8600 5700 4300