While the elevation of a deck is an important design consideration, the space for footings depends more on the structure's overall design, local building codes, and the building site's conditions. However, deck height does impose limits on the types of footings that you can use for your project, because some footings inherently protrude above grade. An understanding of general deck construction and deck footings helps you determine the type of footing that suits for your low-level deck.
Deck Height Requirements
In many areas you can build a deck without any footings; this is called an "on-grade" deck, which means that the deck's frame sits on the ground's surface. If an on-grade deck is suitable for your area, you should construct the deck frame with preservative-treated lumber. If you must build your deck above grade, you should consult with your local building authority to determine footing requirements. Some municipalities don't require permits for low-level decks. Even if your project doesn't require a permit, you should follow suggested footing requirements and construction methods.
Formed and poured footings are the most common types of low-level footings. Conventional poured footings consist of custom-built framework, called concrete forms. Alternatively, you can purchase a prefabricated concrete tube form, which is a long, cardboard cylinder. Whether you use wooden framework or a tubular form, the footing begins with a trench. With tubular forms, you put the form in the trench and pour concrete into the form. With conventional forms, you frame a wooden border around the top of the trench and pour concrete into the hole. Conventional forms are adjustable to any height; you can even omit the framework and simply trowel the poured concrete at ground level. Likewise tubular forms are suitable for making footings as low as ground level, because you can cut them to any height.
Footing Diameter and Depth Requirements
Standard footing diameter is 12 inches. Since tube forms are uniform in size, they're the surest way to create an accurately sized footing. When you're building a conventional footing, it's difficult to dig a hole that's precisely 12 inches in diameter. This means that conventional footings usually require a little more concrete than tubular forms, because the conventional footing's hole is irregularly sized and the tubular form's is precisely 12 inches. Footing depth depends on local frost conditions. Freezing causes the ground to lift and settle, which pushes footings out of position if they don't extend below the frost line. In areas without frost, footings are often 6 to 8 inches deep. In areas with heavy frost, building codes might call for footings as deep as 5 feet.
Precast Footing Dimensions
Precast footings, also called precast piers, are solid concrete blocks with straps that hold deck posts. These blocks sit on-grade, either directly on the soil or on top of a ground-level poured footing. Precast footings are usually at least 10 inches tall and 10 inches wide. Therefore, by design, these types of footings extend at least 10 inches above grade. While 10 inches is relatively low, you must add the height of posts, girders and joists to your overall deck-height calculation. In other words, piers unavoidably add about a foot to your deck's height.
Spacing Between Footings
The space between footings depends on the overall weight or loads that your deck will support. Deck footings support two types of loads: dead loads, which consist of the deck's framing and construction materials, and live loads, which consist of the weight of people, furniture, hot tubs and other items. These loads determine the allowable span of the deck's beams or girders, which are the framing members that sit on top of the posts anchored to your footings. In general, larger beams support more weight and have longer spans. For example, 4-by-6 beams usually have a maximum span of 6 feet between posts and 4-by-10 beams can span up to 12 feet between posts. If you need help calculating loads, ask your local building department for a beam-span chart or consult a contractor, architect or engineer for design assistance. A 4-by-6 beam with posts and footings spaced 6 feet apart or less is common for general-purpose, low-level decks.