Understanding Concrete Footings
Footing Types Explained
Individual (Pad) Footings: These are isolated square or rectangular pads that support a single column or post. They are the most common footing type for decks, porches, and pole barns. A typical residential pad footing is 24 inches square and 12 inches deep, though sizes vary based on the load and soil conditions.
Strip (Continuous) Footings: Strip footings run in a continuous line beneath load-bearing walls. They distribute wall loads over a larger area of soil. For residential construction, strip footings are typically 16 to 24 inches wide and 8 to 12 inches thick. They form the base of most home foundations.
Combined Footings: When two columns are close together, a combined footing supports both on a single rectangular pad. This prevents the footings from overlapping and distributes eccentric loads more evenly. They are common when building near property lines where columns cannot be centered on individual footings.
Mat (Raft) Foundations: A mat foundation is essentially one large footing that covers the entire footprint of the building. These are used when soil bearing capacity is low or when individual footings would cover more than half the building area. Mat foundations spread the load across the maximum possible area.
How Soil Type Affects Footing Size
The bearing capacity of your soil determines how large your footings need to be. Weaker soils require wider footings to spread the load. Here are the presumptive bearing capacities from the International Residential Code (IRC Table R401.4.1):
- Clay: 1,500 pounds per square foot (psf) - the weakest common soil type, requiring the widest footings
- Sand: 2,000 psf - moderate bearing capacity, suitable for most residential footings
- Gravel: 3,000 psf - excellent bearing capacity, allows narrower footings
- Bedrock: 12,000+ psf - the strongest foundation material, footings can be minimal
For example, a column carrying 8,000 pounds on clay soil (1,500 psf) needs a footing area of at least 5.3 square feet, roughly a 28-inch square pad. On gravel (3,000 psf), the same column only needs 2.7 square feet, or about a 20-inch square pad. When in doubt about your soil type, a geotechnical report can provide exact bearing values for your site.
IRC Code Requirements for Footing Depth
The IRC requires all footings to be placed below the frost line to prevent heaving. Frost heave occurs when moisture in the soil freezes and expands, pushing footings upward and cracking the structure above. Frost line depths vary dramatically across the United States: 12 inches in the southern states, 36 inches in the Midwest, and 48 inches or deeper in northern climates like Minnesota and Maine.
Beyond frost depth, the IRC also requires a minimum footing thickness of 6 inches and a minimum width of 12 inches for one-story buildings. Two-story buildings require at least 15-inch-wide footings, and three-story buildings need 18 inches or more. Your local building department may have stricter requirements, so always check before digging.
Example Calculation: Perimeter Strip Footing
Suppose you are pouring a strip footing for a 25-foot by 25-foot addition, giving you a 100-foot perimeter. The footing will be 24 inches (2 feet) wide and 12 inches (1 foot) deep:
Volume = Length x Width x Depth
Volume = 100 ft x 2 ft x 1 ft = 200 cubic feet
Convert to cubic yards: 200 / 27 = 7.41 cubic yards
Add 10% waste: 7.41 x 1.10 = 8.15 cubic yards
You would order 8.25 cubic yards from your ready-mix supplier (they typically deliver in quarter-yard increments). At roughly $150 per cubic yard delivered, budget approximately $1,238 for the concrete alone.
Reinforcement Requirements for Footings
Most building codes require steel reinforcement in footings. For continuous strip footings, the standard is two horizontal #4 rebar running the full length, placed 3 inches above the bottom of the footing. Vertical dowels (#4 or #5 at 48-inch spacing) tie the footing to the foundation wall above.
For individual pad footings, a grid of #4 rebar spaced at 12 inches on center in both directions is typical. The rebar should have at least 3 inches of concrete cover on all sides to prevent corrosion. Use rebar chairs or brick fragments to hold the steel at the correct height during the pour.
Common Footing Mistakes to Avoid
- Not going below the frost line: This is the number one cause of foundation failure in cold climates. Footings that sit above the frost line will heave, crack, and shift, causing structural damage to everything above.
- Undersizing the footing: A footing that is too small for the load will settle unevenly, causing cracks in walls, sticking doors, and sloping floors. Always calculate the required bearing area based on your soil type and structural loads.
- Pouring on disturbed soil: Footings must rest on undisturbed (virgin) soil or properly compacted fill. Loose backfill will compress under load and cause settlement.
- Skipping the rebar: Unreinforced footings crack easily under tension. Rebar is inexpensive insurance against cracking from soil movement, frost, and uneven loading.
- Forgetting to inspect: Most jurisdictions require a footing inspection before the pour. The inspector checks dimensions, rebar placement, and soil conditions. Pouring without inspection means you may have to tear it out and start over.
Footing Calculator FAQ
How deep should concrete footings be?
Footings must extend below the frost line in your area. In the southern U.S., this can be as shallow as 12 inches, while northern states may require 48 inches or more. The IRC mandates that all footings rest on undisturbed soil below the frost depth. Check with your local building department for the exact requirement in your zip code.
What size footing do I need for a deck?
Most residential decks use 16- to 24-inch square pad footings that are 8 to 12 inches thick. The exact size depends on your deck's tributary area, live and dead loads, and soil bearing capacity. A standard 12x12 deck with 4 footings puts roughly 4,000 pounds on each footing. On typical soil, a 20-inch square pad is usually sufficient.
Do concrete footings need rebar?
Yes. Most building codes require at least two continuous #4 rebar in strip footings and a rebar grid in pad footings. Rebar prevents cracking from soil movement, frost heave, and uneven loading. Place rebar 3 inches from the bottom of the footing using chairs or supports, and maintain 3 inches of cover on all sides.
How much concrete do I need for footings?
Multiply length by width by depth (all in feet) to get cubic feet, then divide by 27 for cubic yards. For multiple identical footings, multiply by the count. Always add 10% for waste and spillage. Our calculator above handles all of this automatically and gives you material breakdowns for cement, sand, and gravel.
What is the minimum footing thickness?
The IRC specifies a minimum thickness of 6 inches, but the footing must also be at least as thick as it projects beyond the foundation wall. For most residential foundations, 8 to 12 inches is standard. Heavier structures or poor soil conditions may require thicker footings as determined by a structural engineer.