Structural Load & Soil Capacity
Calculating Dead, Live & Snow Load
When determining footing requirements, it is important to determine the structural load that will be supported by each isolated post, as well as the load-bearing capacity of the soil.
When calculating the structural weight each post will bear, the dead weight of the structure itself must be combined with the anticipated live load, as well as the potential snow load.
The total structural load (including dead, live, and snow weight) must be spread across a wide enough soil surface—via a footing—so that the pounds per square foot at the base of the footing do not exceed the soil capacity. If overloaded, soil will give way and the footing will sink. This is true for footings made of any material. Because the soil is the weak link, footings made of engineered fiber-reinforced composites such as FootingPad equal the performance of concrete footings of the same diameter.
Estimating Structural Load
For structural load, these rules of thumb are commonly used:
Decks: 10 lbs. dead load plus 40 lbs. live load for a total of 50 lbs per square foot of deck surface. Replace the live load with snow load if it’s greater than 40 lbs. (Snow load can range from zero to 75 psf in the northern U.S.)
The deck surface area carried by a single post / footing is called the “tributary area”. Because posts can be placed under almost any area of the deck, and additional posts can be added, the tributary area should be calculated for each post. Consequently, the appropriate size of footings can vary within the same deck. See more here about tributary area determination: https://footingpad.com/decks-footing-size-calculation-guide/
Post frame buildings: 5 lbs. per square foot of roof surface area, plus snow load.
Estimating Soil Capacity
Soil capacity is measured in pounds per square foot (psf). The load-bearing capacity of soils range from 4,000 psf for sedimentary rock to as little as 1,500 psf for clay and silt. Soils that are a predominantly sand or gravel typically fall in the middle, from 2,000-3,000 psf. In very few instances are actual soil tests done. Most inspectors will know the approximate soil-carrying capacity in the areas they cover.
Deck and Post-Frame Load Calculation Guides
Use these guides if you’d like to manually calculate your footing size by post. These guides also offer information on formulas associated with deck and post-frame building construction, including snow load weight.