How many footings do I need for a 12x12 deck?

For an attached 12 x 12' deck, you'll need at least 3 footings, plus at least 2 more if you're planning on building stairs with it.

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Building a deck requires plenty of planning as well as the right materials and methods, but it’s worth the effort. Once you’ve completed the new deck, you’ll enjoy it for many years. Before starting to build, the first question is: How many footings do I need for a deck?

For an attached 12 x 12’ deck, you’ll need at least 3 footings, plus at least 2 more if you’re planning on building stairs with it. If your deck will be a different size, it’s easy to figure out how many you’ll need. Just decide its size and shape, then calculate and measure each element of the design as detailed step-by-step below. In this article, we’ll look at the types of deck footings and how to use them. We’ll also calculate the size and placement of footings for a 12 x 12′ deck.

Deck Types

Floating Deck

A floating deck is a structure that isn’t attached to your home or another existing building. It needs a complete set of perimeter footings since it will be self-supporting and won’t tie into your home’s existing foundation. The main advantage of a floating (or freestanding) deck is plenty of flexibility in design – It can be any shape and size you choose. The disadvantage is that you can’t save money by tying into an existing foundation.

Attached Deck

As the name implies, an attached deck is connected to some existing structure, such as a house, garage, or other building. The advantage is that you may be able to use fewer footings by tying into an adjacent foundation. If so, you could avoid some of the digging work and material costs for deck footings. The disadvantages are that you may need to work around exterior or underground plumbing and electrical lines. You might also need to build the deck in an irregular shape to fit existing property contours. Concrete deck footings are very popular. You can easily make them yourself using sonotube forms, concrete, and “reinforcing bars” (rebar). Building codes require the use of rebar to help support the weight of all attached decks.

Deck Blocks

You can achieve similar results by using precast concrete deck blocks or piers instead of pouring concrete yourself. Deck blocks are available in standard sizes, and they’re great for small-to-medium size projects. They usually have a concrete lug on top, so that you can attach wooden deck beams later. In a cold climate, you must check local regulations to ensure that your deck footings are placed below the frost line. In the coldest northern states, you may need to dig and place your deck footings as deep as 48” below grade. It’s the only way to avoid frost heaving.

What Factors Affect Selection of Foundation for Buildings?

Size

The size of your deck is the most important consideration. The bigger your deck, the more footings you’ll need. 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. The drawing lets you measure each dimension and precisely calculate the lengths of lumber cuts you’ll need. You can also figure out the exact placement of each deck footing. The drawing should include any nearby structures, trees, and utility lines. A drawing will also help when you check in with your local government’s building department. They’ll know that you’re a careful builder, and you’ll avoid potential building-code issues.

Shape

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The shape of your planned deck is another important consideration. If it’s square, you can easily lay-out everything using a cloth tape measure along with simple stakes and twine. 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. If not, your layout isn’t square and must be corrected. Otherwise, you’ll have problems when cutting and fitting lumber for deck beams, joists, and flooring.

Decks with Lots of Angles

If the deck has many angles or projecting areas, you’ll need more footings. 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 12-inch diameter footings. The most-common square footing sizes are 16 x 16” as well as 18 x 18” and 20 x 20”. Before calculating the size and number of deck footings, you’ll need to know the type of soil you’re building on. 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 3,000 pounds per square foot. Sand can support up to 2,000 pounds, and clay is the weakest at about 1,500 pounds per square foot because it doesn’t allow water to drain. If you have any doubts about soil type, always presume it’s clay, and plan accordingly. The weaker the soil the larger you should make the bearing surface of the deck footers.

Size and Number of Beams

The most commonly used sizes of beams are 2 x 6”, 2 x 8”, 2 x 10” and 2 x 12”. Many builders double these thicknesses by fastening together two pieces of 2” lumber using bolts to create a single beam. The larger the beams, the fewer the footings you’ll need to use. Still, even when using two pieces of 2 x 12” lumber, you should never span more than eight feet without support underneath. When you’re building an attached deck for your house, fasten a 2 x 12” securely at the correct level. It will act as a ledger along the entire side to help connect the wooden deck beams and joists to the existing structure. If you live in a damp climate, you should also attach metal flashing along the side of the existing structure to drain any water away from the wood-to-wood surfaces between the ledger and the deck beams and joists.

Size of Joists

Unless your deck is 4 x 4′ or less, you should place floor joists on top of the beams to support your flooring to avoid sagging later. When laying out the joists, you may plan for placing them on 12” or 16” centers. Although 16” centers are common in residential construction, it’s usually best to place joists on 12” centers. The flooring above will support more weight and be less likely to sag over time. To figure out how many joists you’ll need, divide your deck dimensions in inches by 12”, round up and add 1.

Stairs

Stairs and traffic pathways receive extra weight and vibration and should be supported by footers and beams. Place footings at each corner of the stairs and at two-foot intervals if stairs are longer than a few feet.

Check with Your City for Building Code Compliance

When building a deck or any other construction project bigger than a few square feet, you should always check with your city or village to make sure it complies with local rules. Take your drawing to the engineering department of whichever government agency issues building permits. Usually they’ll approve it immediately. That way, you’ll save plenty of time and money by avoiding potential legal headaches.

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Even if your deck is simple, footings should always be placed at a distance of not more than 8′ center to center apart. Hot tubs and other heavy objects will require additional footings and larger deck beams. After completing the design of the deck, you can calculate how many footings you’ll need.

Step 1: Calculate the Load

The load can be calculated based on the size of your deck, the soil properties, and what the deck will carry. For furniture and people, use 40 psf (pounds per square foot), and structural materials or dead load, add 15 psf. The total is 55 psf, multiplied by the square footage of the 12×12’ deck results in a total loaded weight of 7920 pounds. Remember, the ledger carries half the weight and transfers it to the attached building’s footings. The remaining weight is split between the supports planned. Therefore, take the total weight, divide it by 2 (=3960 lbs), and then divide it by the number of posts – usually 2 (=1980 lbs) or 3 (=1320 lbs). A 12-inch diameter footing would suffice for footings in gravelly soil, and the depth would depend upon frost levels. Footings can also be sized according to beam and joist sizes instead of being calculated based on deck loads.

Step 2: Calculate the Size and Number of Beams

The next step is to look at a handy deck-building table to decide on the size of your beams based on joist span. In this example, I’ll plan for 3 supports, and the beams will be made of two pieces of 2 x 8” lumber fastened together, and the span is 6’. Note: If your local building material supplier doesn’t have certain dimensions of lumber, you can always double the pieces to obtain the necessary total thickness. In each case, the bearing surfaces of the footings must be at least the size of your beam’s width. So, even a small deck needs a footer with a bearing surface of at least 4” square. Of course, it’s always best to use footings which are slightly larger than your beams. Keep in mind that you should never span more than 8’ without a footing and support underneath, even if your deck is small and light (i.e.without a swimming pool) and the beams are large – Eventually the deck will sag. Next, calculate the size of the footings. Some builders use their own rules-of-thumb to figure out the size of footings. The best option is to use a footing-size table based on the weight it must support, plus the lengths of your beam and joist spans. For our 12 x 12’ deck, we’ll use 12” round footings.

Step 3: Calculate the Number of Footings

Based on the dimensions in our example, the 12 x 12’ attached deck will need three 12” diameter footings, plus at least two more if you’re planning on building stairs.

Start Building Your Deck Today

Now is the best time to start building your deck. Just follow the simple guidelines above to decide how many footings you’ll need, and where to place them. Soon you’ll have a sturdy outdoor addition to your home.