At EZ-CRETE, we love this question because it means you are building your deck with care and attention, just the way we build all our products. And, just like using our products, it’s pretty straightforward to determine how many EZ-TUBE precast footings you’ll need once you determine the live and dead loads of your deck (or any structure for that matter) – and the tributary loads, or the portion of the ‘live’ and ‘dead’ load carried by the footings only, not the ledger board.
To determine the live load (that dynamic load that changes when people, snow, furniture etc. move on and off the deck) multiply the square footage of the deck by 40 pounds per square foot. Your live load may change depending on where you are located. Check with your local code official to make sure your live load falls within this range.
For your dead load, figure 15 pounds per square foot, for a total load of 55 psf (depending on your location).
Once you have determined the load, it’s time to figure out how much weight is carried by the footings only. Providing you are attaching one or more sides of your deck to the house, this is your tributary load.
For ease, let’s assume that you have a 10-foot square deck with one side attached to the house. Your deck is 100 square feet or 10′ x 10′ = 100 square feet. Because your house supports 50% of the load and the footings support the other 50%, divide your square footage by 2, 100 square feet/2= 50 square feet. If your structure is a stand-alone design simply don’t divide this number.
This is your tributary square footage. To figure out the weight supported by the footings multiply the tributary square footage by the total live and dead loads, in this case, 55 psf.
50 square feet x 55 psf = 2750# carried by the footings.
Now you can use the chart below to figure out how many footings you will need. It’s important to note that for stability at least two footings must be used. Space footings equally where possible.
Choose your soil type, then find the typical load by the footing type (with or without an EZ-Base). Note that generally code requires a base section. Check with your local code official.
Once you know how much load a single footing will carry in your soil type simply divide your tributary load by the weight typical for one footing to determine the number of footings you need.
For this exercise let’s assume we have sandy soil and code requires us to use a base:
2750# tributary load/ 5200# load per footing = We can get by with the minimum of two footings.
Now that you know how many EZ-TUBE footings you need, you can forget about mixing concrete and keep building, fast!
***Remember, this article is meant for informational purposes only. Different jurisdictions may require more or fewer footings. Always be sure to check with your local code official before building.***