BOB HEIDENREICH: Deck Math III
Part 1: How to calculate footing loads and the weight of a deck.
BY: BOB HEIDENREICH
If you’re a deck builder, you know that numbers are a common occurrence throughout your day-today schedule. Because of this, you also know that there are fast formulas for estimating lengths and loads—and that’s why knowing “deck math” can make or break a sale with a customer. Knowing your numbers means you know the art behind building a deck. A few months back, I discussed the benefits of learning quick math when it comes to estimating the lengths of a project. For this issue, I would like to discuss how to calculate a deck’s weight and footing load.
Weight is extremely important when it comes to building a deck. If you take a pen and push it into the ground, you wouldn’t have to exert too much effort to push it through the soil. On the other hand, if you take a pop can and try to push it into the ground, you’re not going to get the same results. That’s because the diameter of the pop can is wider than the pen. This analogy can be used when we talk about deck footings.
Different types of soils have certain amounts of resistance. For example, clay soil has about 13.88-pounds per square inch of resistance. In Minnesota, the ground is usually composed of sandy-gravel soils and clay soils, and we usually can use 2,000 pounds per square foot because of the soil compression strength. By taking that 2,000 pounds and dividing it by 144 to convert it into inches, we know that the soil can hold about 13.88-pounds per square inch.
Once you know how much the soil beneath a deck can hold, next you can figure out the weight of the deck. To calculate the weight of the deck, I’ll use a 10×10 deck for this example. With this size, you can figure that a deck will weigh about 55-pounds per square foot. Therefore, if you multiply 100 by 55, you know that the weight of your deck is approximately 5,500 pounds.
Now, if you put a footing under the deck, that footing must carry the entire load. Generally, a deck will have multiple footings. If you put a footing in each corner of the deck, half the load transfers to the house, while the other half transfers to the two footings. Because of this, each footing holds an area of 25-square feet (half the length of the deck multiplied by half the width of the deck).
If you would like to know how much weight each footing holds, you can multiply the 25-square feet by 55-pounds per square feet, giving you 1,375 pounds. However, if the ground is only capable of holding 13.88-pounds per square inch, you’ll next need to calculate how big of a footing you need. You can do this by dividing 1,375 by 13.88 (soil compression), and you’ll get 99.06 inches.
Because footings are usually circular, you’ll need to find the correct diameter that can hold the right amount of weight, and using a particular formula can do this. If you were to take a 12- inch circular diameter, you would calculate the amount of load it can hold by the formula π r². First take the radius (half of the 12-inch diameter—which is 6), and you square that number, resulting in 36. Next, you multiply this number by Pi (3.14), and you’ll end up with 113.4 inches. When that number is multiplied by 13.88, you’ll know that a 12-inch circular footing will hold 1,568 pounds.
As you can see, mathematic shortcuts can save you time and money on projects, as well as build your trust and reputation with the client.
For next issue, I’ll discuss how to calculate joists loads and how to work with different types of decks for those measurements. If you have any questions, feel free to drop me a note at email@example.com.