As mentioned earlier, you can use deck blocks to build a small deck. Large decks or structures on unstable soil will require poured footings or perhaps even a continuous foundation of poured concrete around under the entire perimeter. If you have any doubt about soil conditions, ask someone at the local building-permits department. Whichever type of deck you want, the first step is always to draw and layout your design before calculating the material costs.
Make a simple drawing on grid paper. You can also figure out the exact placement of each deck footing. The drawing should include any nearby structures, trees, and utility lines.
The shape of your planned deck is another important consideration. Important — After marking the corners of your layout, you should always check for squareness by triangulation. Triangulation means measuring across the diagonals between each opposing corner. These diagonal measurements should be the same.
For decks with lots of angles, that means at least one footing for every angle or projection. The diameter of the footings commonly range between 8 and 24 inches. Most builders use inch diameter footings. Additionally, determine approximately how much total weight the deck will hold. Soils are classified as mostly gravel, sand or clay.
Gravel-type soils can support the most weight, typically up to 3, pounds per square foot. The weaker the soil the larger you should make the bearing surface of the deck footers. More beam means less footings. Whenever given a choice, I would rather increase the beam size and reduce the number of footings. Saving both time and money in building the deck.
A concrete footing will cost that and more. There are some rare situations where it is more advantageous to increase the number of footings not the depth of the beam. It may be better to dig an extra footing. Before pouring the additional concrete, is it possible to add an additional ply to the beam? Allowing you to keep the same beam depth but increase the strength, increasing the span. Something to consider. I look further into ways to lower deck beams close to the ground.
If you would like to read some other possibility, click here. The footings can support more, and the deck will look better with symmetrical spacing of the post and footings. The IRC allows deck beams to overhang the post, one-quarter of the backspan. Minimizing the required footings.
A word of caution, with beam overhang. It may be called the International Residential Code, but local code will overrule this at times. In Edmonton, Alberta , a deck beam can only overhang one and a half times the nominal depth of the beam. The size of deck footings is determined by soil bearing capacity.
Firmer soil requiring both smaller and fewer footings. 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.
Now you can use the chart below to figure out how many footings you will need. 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. Different jurisdictions may require more or fewer footings.
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