Deck and Outdoor Structure Footings
Outdoor structures such as gazebos, decks, porches, arbors, and pergolas require strong footings as a base to build on. If the structure does not have a strong and stable foundation, the posts and beams attached to it may fail resulting in possible injury to the occupants and loss of property.
Outdoor structures are supported by a basic foundation called footings. Footings eliminate settling and slippage, and help protect posts and beams from direct contact with the earth. Posts and beams that come in direct contact with the earth will rot more quickly and ultimately weaken the structure.
Footings also prevent frost heave. If moist soil freezes, it will heave upwards causing an upthrust of the soil. The movement of the soil can push up on the posts and the structure attached to it. Small movements of 1/4 inch or less are acceptable. When movement of more than 1 inch occurs, there is often structural damage to the gazebo, deck, porch, arbor, or pergola. If the structure is attached to a home, even more damage may occur. In order to combat this problem, footings are placed 6 inches below the frost line. The frost line is the maximum depth where the ground will freeze in the winter. In Minneapolis/St. Paul, Minnesota, footings are required by code to be between 54 and 60 inches deep.
Fig. 1. Adapted from Beneke
Many municipalities have code regulations on footing depth and width, type, and fill material. When building an outdoor structure, it is best to check with a contractor or the building inspector in your municipality regarding building codes. This can often be done while obtaining a building permit for your outdoor structure.
When building an outdoor structure, one must be patient digging postholes for the footings. If the soil is compacted, hard, or full of rocks, you may want to use a power auger to dig the holes. Don't be surprised if the project takes time. You never know what you will encounter digging in a residential or commercial site. In Minnesota before digging call Gopher One to mark all underground wires at 651-454-0002.
Footing Type Considerations:
In choosing a footing type, consider the following:
- Footing should be large and stable enough to provide a solid resting place for posts or beams.
- Footing should be placed below the frost line, usually 6 inches, to prevent frost heaving. A floating deck or stand-alone deck, one that is not connected to the house, will eliminate the danger of weather-induced damage to the house from frost heaving.
- Footing should rise at least 3 inches above ground level to keep the post dry.
- Reinforcing bar (re-bar) should be placed in the concrete to add strength to the footing (if necessary), especially if required by local code.
- Gravel or crushed rock should be compacted around the post to allow water to drain away from the footing, especially if drainage is poor.
- Digging the holes for footings: If digging in rocky or heavy soil, or if digging deep holes (below frost line), you may choose to use a power auger. If digging in sand, or if the structure requires just a few postholes, you may choose to use a manual posthole digger.
In areas that have no frost, dig a hole that will act as a concrete form. In areas with some frost, it may be necessary to dig a hole 12 inches in diameter and 8 inches deep. Fill the hole with concrete (make sure the concrete is at least 3 inches above ground level). If necessary, attach an anchor bolt that protrudes into a hole drilled in the center of the post when the concrete is partially set (Fig. 2). A galvanized metal post support may also be used to attach the post to the footing (Fig.3).
Fig. 2. Adapted from Beneke
Fig. 3. Adapted from Beneke
Using a tube form is generally the fastest and easiest way of creating footings. By pouring concrete into a tube form one can extend the tube above ground level to the desired height. Some municipal codes require tube forms for footings. Many building inspectors prefer this foundation form because the exact dimensions of the footing are easily determined. The forms come in many sizes. The most common size is 12 feet long by 8 inches wide. Most codes call for 6 inches of the form to be above ground level and 6 inches from the bottom of the hole. This allows posts to be above ground, and an anchor is created in the base of the hole, which helps prevent frost heaving. Tube form footings use less concrete than other methods because you need only fill the footing and tube area and not the entire hole. An anchor bolt, or a galvanized metal post support may be used to attach the post to the footing (see Fig. 2 and Fig. 3).
Fig. 4. Adapted from Beneke
Pre-cast footings are often used in areas that have little or no frost, or they can be used on top of a direct-pour footing. A pre-cast footing, or pier, is a tapered concrete block with a square, 3-inch depression in the topside. The depression holds the 4 x 4 post in place. Check with your municipality for codes regarding this type of footing. Prefabricated footings can be purchased at home improvement centers.
Fig. 5. Adapted from Dek-Brands
Below the Frost-line:
These footings arise from digging a cylindrical posthole that extends several inches below the frost line. For people in the Minneapolis and St. Paul, Minnesota area, the depth of the posthole is 54 inches to 60 inches.
The bottoms of these holes should be flared out to help prevent frost heaving. Flaring can be accomplished by chiseling out the sides with a large, long pry bar. A pry bar is also helpful for removing large rocks and small boulders from the hole. A power auger is advisable for this type of footing especially if there are many holes to dig. This type of footing is often used in the landscape industry for structures that need multiple posts with bracing between posts to hold the structure square.
Fig. 6. Adapted from Beneke
Wooden forms are typically used in larger holes where there is more room for the form to rest. Check with your municipality to see if wooden forms can be used and what spacing is needed above ground and below the end of the form. Building codes are generally similar to those for tube form footings.
Outdoor structure posts can be placed directly into holes dug in undisturbed soil, becoming part of the actual footing. Footings of this type should use water permeable compacted gravel in the holes around the posts to keep water away from the post. Crushed rock or gravel fill along the post will allow water to move through the subsurface and away from the post, thus keeping the post relatively dry, stable, and rot free. Tamping the fill next to and along the posts will help hold the post in place so that a frame may be attached to it. Using concrete around the post will hold any water that collects between the post and the concrete. For this reason concrete is not recommended over rock or gravel.
This type of footing provides structural support as well as lateral support. When using earth or gravel to fill the hole around the post, tamp the soil or gravel every 2 to 4 inches to stabilize the post. At the same time make sure the post is plum and as centered as possible. It is recommended that gravel be laid at the bottom of the hole to keep moisture away from the bottom of the inserted post. A concrete pad can also be used. Many city building codes call for this type of footing where the deck will be located high off the ground. Several nails can be driven part way into the bottom of the post before placing the post into the hole. This will hold the post in place after the concrete has hardened.
Fig. 7. Adapted from Beneke
Reinforced with Rebar:
All footings that are poured with concrete can be reinforced with rebar. Adding # 4 rebar (steel rod) to a concrete footing will increase the strength of the footing, and the footing will be less likely to crack or splinter. When reinforcement is necessary or desired, cut 2 #4 rerods that are 8 inches longer than the depth of the footing hole. Insert the cut rerods vertically into the posthole (the rerods should be a few inches apart from each other). Push the rerods into the hole until the end of the rerod is below the top of the footing.
It is also advisable to place 2-3 inches of gravel into the bottom of the hole to keep moisture from forming at the base of the footing. Most footings that are deep into the ground will call for rebar for added stability and strength. Depending on your area, climate, and local codes, you may be required to use rebar in your footings.
Fig. 8. Adapted from Beneke
Posts on a Concrete Slab/ Slab Footings:
There are situations where a concrete slab is itself already present. Posts are placed on top of the pre-constructed s lab. The cost of breaking up the slab and removing the debris can be as expensive as the structure itself. This type of footing can work well in frost-free climates, and when structures are low to the ground. In pre-constructed slabs, a galvanized metal post anchor can be affixed to the slab using Tapcon cement screws and then attached to the post. This will prevent the post from moving horizontally. It is advisable to talk with your local inspector about codes in your municipality before building.
Fig. 9. Adapted from Beneke
A solid foundation is needed for any type of deck or outdoor structure. Selecting the proper type of footing is important for structural stability. It is up to you, the builder, to decide what type of footing is most suitable for your project. In most situations, concrete footings that are below the frost line are recommended and will be required by code. Check your local codes, and talk to the local building inspector to find out what the footing requirements are in your area.
Remember that any type of post that is imbedded in soil and concrete will be prone to rot and decay over time due to moisture retention. In order to increase the life expectancy of the post, it is recommended to use post anchors that are above ground, such as a galvanized metal post support. Placing a roofing shingle between the post and the post anchor will protect it from moisture and decay and will extend the life of the post.
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Carter, Joe. 1996. Better Homes And Gardens Books: Decks: Your Guide to Designing and Building. Meredith Publishing Group, Des Moines.
Cory, Steve. 1997. Creative Homeowners Press: Decks, Plan, Design, Build. Creative Homeowner Press, Upper Saddle River.
Mills, Heather. 1999. Sunset Books: Deck Book - Everything You Need to Plan, Build, or Buy the Perfect Deck. 5th Ed. Sunset Books, Menlo Park.
Sauter, David. 2000. Landscape Construction. Delmar Publishing, Albany, New York. pp. 247-250.
Hometime. "Building a Deck: Laying Out Footings." http://www.hometime.com/projects/howto/deck/pc2dk05.htm
Hometime. "Three-Season Porch: Footings." http://www.hometime.com/projects/howto/porch/pc2por02.htm
RemodelingGuy. "Three-Season Porch: Footings." http://www.remodelingguy.com/footings.htm
This implementation report was developed by Jeff Fahrenholz, student, University of Minnesota Department of Horticultural Science.