Deck Footing Calculator
Use this calculator to estimate the minimum required area and dimensions for your deck footings based on your deck's size, expected loads, and the bearing capacity of your soil. Proper footing design is crucial for the stability and safety of your deck.
(Typical residential: 40 psf)
(Weight of deck materials: 10-15 psf)
(Typically 4-9 for a standard deck)
(Consult local codes or soil report. Typical: 1500-3000 psf)
Understanding Deck Footings
Deck footings are the foundational elements that transfer the weight of your deck, its occupants, and any snow load directly to the ground. They are critical for preventing settlement, frost heave, and ensuring the overall stability and safety of your deck structure.
Key Factors in Footing Design:
- Deck Dimensions: The length and width of your deck determine its total area, which directly impacts the total load.
- Live Load: This is the weight of people, furniture, and other movable objects on the deck. For residential decks, building codes typically specify a minimum live load, often 40 pounds per square foot (psf).
- Dead Load: This refers to the permanent weight of the deck structure itself, including framing, decking boards, railings, and any fixed features. A common estimate for dead load is 10-15 psf.
- Number of Footings: The more footings you have, the more the total load is distributed, potentially allowing for smaller individual footings. However, footing placement must also align with beam and post locations.
- Soil Bearing Capacity: This is the maximum pressure your soil can safely withstand without settling. It's measured in pounds per square foot (psf). This is perhaps the most critical factor. Different soil types have different capacities:
- Soft Clay/Silt: 500-1000 psf
- Sandy Clay/Loose Sand: 1000-2000 psf
- Medium Clay/Dense Sand: 2000-3000 psf
- Hard Clay/Gravel: 3000-4000+ psf
How the Calculator Works:
This calculator first determines the total weight your deck will impose on its foundation by summing the live and dead loads and multiplying by the deck's area. Then, it divides this total load by the number of footings to find the load each individual footing must support. Finally, it uses the soil bearing capacity to calculate the minimum required surface area for each footing to safely distribute that load to the ground. It then converts this area into equivalent minimum diameters for round footings and side lengths for square footings.
Important Considerations:
- Local Building Codes: Always check with your local building department. They will have specific requirements for footing depth (to get below the frost line), size, and reinforcement.
- Frost Line: Footings must extend below the local frost line to prevent frost heave, which can lift and damage your deck. This calculator does not determine footing depth.
- Structural Engineer: For complex deck designs, large decks, or challenging soil conditions, consult a licensed structural engineer.
- Tributary Area: This calculator provides a simplified approach by distributing the total deck load evenly. In reality, footings supporting beams will carry more load than those supporting joists directly. For precise design, a tributary area analysis is needed.
- Safety Factor: Building codes often include safety factors. The calculated minimums here are theoretical; actual required sizes might be larger.
Example Calculation:
Let's say you have a 12 ft x 10 ft deck (120 sq ft) with a standard live load of 40 psf and a dead load of 10 psf. You plan to use 6 footings, and your soil has a bearing capacity of 1500 psf.
- Total Deck Area: 12 ft * 10 ft = 120 sq ft
- Total Load per sq ft: 40 psf (live) + 10 psf (dead) = 50 psf
- Total Deck Load: 120 sq ft * 50 psf = 6000 lbs
- Load Per Footing: 6000 lbs / 6 footings = 1000 lbs per footing
- Required Footing Area Per Footing: 1000 lbs / 1500 psf = 0.667 sq ft
- Minimum Footing Diameter: Approximately 11.1 inches (for a round footing)
- Minimum Footing Side Length: Approximately 9.9 inches (for a square footing)
This means each of your 6 footings would need to be at least 11.1 inches in diameter or 9.9 inches square to safely support the deck on this soil type.