At EZ-CRETE, we love this question because it means you’re 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.
How to Determine The Live Load
To determine the live load (that dynamic load that changes when people, snow, furniture, etc. move on and off your 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.
How to Determine The Dead Load
For your dead load, figure 15 pounds per square foot, for a total of 55 psf (depending on your location).
Determine How Much Weight is Carried By The Footings
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 the house. Your deck is 100 square feet (10’x10’=100). 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 = 2,750# carried by the footings.
How Many Footings Do You Need
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.
|
Soil Type
|
Gravel
|
Sand
|
Clay
|
|---|---|---|---|
|
Allowable pressure on soil (psf)
|
3,000 lbs.
|
2,000 lbs.
|
1,500 lbs.
|
|
EZ-TUBE without Base Section
|
2,100 lbs.
|
1,300 lbs.
|
900 lbs.
|
|
EZ-TUBE with Base Section
|
7,900 lbs.
|
5,200 lbs.
|
3,900 lbs.
|
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:
2,700# tributary load / 5,200# load per footing = We can get by with a minimum of two footings.
Now that you know how many EZ-TUBE footings you need, you can forget about mixing concrete and keep buiding, fast!
***Remember: This article is meant for informational purposes only. Different jurisdictions may require more of fewer footings. Always be sure to check with your local code official before building.***
