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Footings (9)

Every deck and gazebo needs a good footing to ensure the structure remains level for years to come. In general, the size of the footing depends on the depth of the footing, the weight the footing must support, and the type of soil. If you don't size the footings for the conditions at hand, you run the risk of having a foundation that sinks over time and produces a sagging deck surface. Not good.

Just like sizing beams, the first task at hand is to determine the weight on the footings. This is a two step process. First, the area of floor the footing supports is figured out. And second, this area is converted into the weight that's on the footing. These steps aren't hard to do. You can read more about them by clicking on the beam in the 3-D diagram above.

After figuring how much weight the footings will need to support, it's important to determine the soil type. We frequently hear people comment that they've got really "good" soil because it's tough to dig. Unfortunately, this isn't a good test of bearing capacity. When soils heavy in clay are dry, they are tough to dig. However, these soils can't support as much weight as soils with more sand and gravel - they have poor bearing capacity.

The reason clay laden soils don't bear weight well is that clay is one of the smallest sized soil particles. When soil with a lot of clay-sized particles gets wet, it easily shifts. More specifically, heavy clay soils have a non-adjusted load capacity of 1000 pounds per square foot (psf). On the other hand, soils consisting of a lot of sand and gravel have an unadjusted load capacity of 2000psf. To help you determine if you've got a poor, medium, or good soil, look over the soil properties table.

Once the load on the footings and the soil are determined, it's a simple matter to look up the required footing size in BestDeckSite's footing tables. These tables factor in soil type, the load, and are also adjusted for the depth and size of the footing. Including all these factor is important in order to arrive at accurate bearing capacities.

The reason why it's important to factor in for the footing size and depth is that if you double the size of the footing, it more than doubles the load bearing capacity. This relates to the shape of the bubble of soil below that supports the footing. Likewise, deeper footings in the same soil can carry more weight. The deeper the footing, the more soil there is pressing down and around the soil directly under the footing.