How Big Should the Footings Be for My Deck?

Footing Basics

• Deck footings support the weight of the structure, keep it from settling and shifting, and elevate the deck off the ground to prevent moisture damage and wood rot. The soil condition determines type and size of the deck footing. For instance, the depth where the ground freezes in winter, known as the frost line, affects how deep the footing must be. Deck footing must extend at least 6 inches below the frost line. Cold climates have a lower frost line in the soil, sometimes 4 feet or more in depth, so that the deck footing can be as deep as 6 feet. Solid soil content without much sand or rocks generally requires deck footing to be 6 to 8 inches thick and 12 to 16 inches wide.

Types of Deck Footing

• Deck footing is generally in the form of poured concrete or in precast form. Poured concrete footings in areas with a shallow frost line require a hole that is 12 inches across and 8 inches deep with a gravel base. If you live in a colder climate with a deeper frost line, you might be better off use premade tube forms for the footing, which are up to 12 feet in depth and 8 inches wide. Lighter decks with less vertical load and stable soil content only require the use of precast deck piers, which are generally tapered from a 6-inch base to a notched top of about 4 inches that holds the deck post.

Pier and Footing Types

• Larger decks, or those that sit on less stable soil types, can be supported with a combination of a pier and a footing. One type consists of a round footing attached to a cylindrical pier with rebar. Based on the minimum dimensions for deck footing, the footing will be 12 to 16 inches in diameter and 6 to 8 inches thick. This type of deck footing provides a very solid support for deck posts and will resist settling and shifting.

Local Codes

• You will have to get a building permit regardless of the type of deck you decide on. This would be a good time to talk to your local code enforcement agency to find out what type of deck footing you can use. They will also let you know how big your deck footing needs to be to satisfy the building codes.