Footings
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Deck Footing Size Chart

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 live for load (these are variable loads that are dynamic such as the weight of people and furniture) and 15 lbs per square foot for dead load (this is the weight of the materials used for the construction of the deck) for a total of 55 lbs per square foot total load. 

If you were building a 10x10 deck attached to a house with 2 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 1375 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

                                                      24           9400        6200       4700
 


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