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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Comments, Questions and Reviews

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Hi, I''m building a deck 35''x16'' that designed by Homedepot Do I need to have this deck attached to the house? I am planning to use 12" holes concrete filled footing. There are 29 footing posts for this deck Thanks. Will

posted by Will_V at 7/22/2014 11:49:23 AM


It really depends on the plan. If the deck is designed to be free standing (not attached to the house) it should have a beam and a row of footings about 2'' out from the house wall.

posted by mike at 7/22/2014 2:58:27 PM


Hi, could you clarify in your table if the footing diameter is a diameter or the sides of a square? Thanks.

posted by Timothy Larson at 7/12/2014 11:50:50 AM


It is showing the diameter of a circular footing.

posted by mike at 7/14/2014 11:01:48 AM


I''m building an attached deck. The ledger side is 16.5''. The joists will be roughly 10'' with a 1'' cantilever. I was planning on using three posts (one on each end and the middle one). Considering this should redistribute the weight more than only two posts, is it still necessary to use 12" diameter cement footings and 6"x6" posts? Or can I use 8" footings with 4"x4" posts? Any help is greatly appreciated...thanks!

posted by Ed at 7/1/2014 8:00:16 PM


I would recommend using 6x6 support posts and 12" footings. 8" footings don''t have enough surface area to carry these loads for most soil types.

posted by mike at 7/2/2014 9:04:20 AM


how do the load factors change if my "deck" is a roofed deck = porch?

posted by johno at 6/17/2014 10:14:12 AM


You will usually calculate 80 lbs per square foot for decks with porch roofs. Ask your local building department to verify. Some areas have additional requirements for heavy snow loads.

posted by mike at 6/17/2014 11:51:35 AM


My deck is 13''-6" wide x 28 feet long. Total design load is 18,900 The 28 foot dimension is along the house. Given the size & load, can I still assume that half that load (9,450#) can be carried by the ledger board to the house?

posted by mrz200 at 5/3/2014 11:49:15 AM


It really depends on the house wall construction. If you can flash and bolt your deck to a solid rim joist of the house with (2) 1/2" bolts every 16" on center you should be ok. Always check with your local building inspector.

posted by mike at 5/5/2014 9:00:53 AM


I am building a freestanding deck next to my pool that will be 16.5 x7. since it is rectangular I plan to put 6 footings (4 corners and 2 in the middle). is the load calculation the same for the outside footings as the middle footings?

posted by Tgib123 at 4/23/2014 10:58:49 PM


The calculation is the same for the corner footings. You still need to determine the tributary load which is usually considerably less than the center footings.

posted by mike at 4/24/2014 9:04:37 AM