How Deep Should Deck Footings Be?

Backyard decks require sturdy foundations or footings.

Deck footings are the concrete mini foundations that provide support to your backyard deck. Footings serve as the anchors on which the rest of your deck structure floats. Deck support posts are mounted within your deck footings and serve as the attachment points between the structural elements located above and below ground. When installing deck footings, depth is a key consideration. The soft soil, more prevalent near the surface, may be softer and more likely to shift or wash away. It is also important that your deck footings are located below the frost line to prevent freezing and cracking over time.

Things You'll Need

• Post hole digger
• Shovel
• Wooden post
• Tape measure
• Tubular concrete molds
• Work gloves

Instructions

1. Check with your local building authorities to find out the frost depth in your area. The frost depth differs from place to place based on seasonal temperature averages, the height of the water table and the soil makeup. The frost line in your area will determine how deep you should dig your deck footing post holes.

2. Check with local building ordinances to discover where your deck footings can be placed legally. If you place the footings in the wrong locations, you can end up being fined and having to move the deck to a new location. If water, sewer, gas or power lines run through your property, you will also have to consider their location before breaking ground on your concrete footings.

3. Use your post hole digger to make a hole into which your concrete footing will be poured. Dig the hole deep enough to allow all of the concrete to sit 6 inches below the frost line. Dig each hole so that the bottom fourth of the hole is wider than the top. The proper shape for a footing is an inverted cone. The extra width at the bottom, allows the concrete to disperse its load more efficiently and will create a more stable foundation.

4. Use your shovel or a piece of wooden post to flatten the bottom of the hole. Remove any rocks, loose soil or other debris from the bottom of the hole to ensure a firm foundation.

5. Dig each hole so that it has a surface opening between 12 and 24 inches. The larger the hole, the easier it will be to access and to pour concrete into.

Tips & Warnings

• Use tube-shape concrete molds to shape and pour your deck footings. The molds are left in the ground after the footings are poured and also help reduce the mess involved with the pouring process.

• Never handle wet concrete with your bare hands. Concrete contains lime, which is highly corrosive and can cause burns and other skin issues. Always wear work gloves.

• Placing your deck footings above the frost line can result in shifting and collapse as the temperature grows warm and the soil, which was once hardened by frost, softens. Post movement of up to 1/4 inch or so is expected and will not harm your deck.

References

• Family Handyman: How to Build a Solid, Frost-Proof Deck Footing
• Ask the Builder: Deck Piers and Footings
• Deck Magazine: Footings from New England to California
• University of Minnesota: Deck and Outdoor Structure Footings
• Decks: Deck Footing Frost Depth Map
• This Old House: How to Build a Simple Deck
• Danny Lipford: How to Build a Deck Foundation
• Google Books: Popular Mechanics: Post Hole Footings