Deck Footing Size Calculator

Deck Footing Size Calculator

Enter the square footage of deck surface that this single footing will support. (e.g., 5 ft x 5 ft = 25 sq ft)

Typical residential deck load is 50 psf (40 psf live load + 10 psf dead load).

Poor Soil (Soft Clay, Loose Sand) – 1000 psf Average Soil (Medium Clay, Sandy Loam) – 1500 psf Good Soil (Dense Sand, Stiff Clay) – 2000 psf Excellent Soil (Hardpan, Bedrock) – 3000 psf Consult local building codes or a geotechnical report for accurate soil capacity.

Calculation Results:

Total Load per Footing: — lbs

Required Footing Area: — sq ft

Minimum Square Footing Side Length: — inches

Recommended Footing Size: — inches x — inches

function calculateFootingSize() { var deckAreaSupported = parseFloat(document.getElementById("deckAreaSupported").value); var designLoad = parseFloat(document.getElementById("designLoad").value); var soilBearingCapacity = parseFloat(document.getElementById("soilBearingCapacity").value); if (isNaN(deckAreaSupported) || deckAreaSupported <= 0) { alert("Please enter a valid positive number for Deck Area Supported."); return; } if (isNaN(designLoad) || designLoad <= 0) { alert("Please enter a valid positive number for Deck Design Load."); return; } if (isNaN(soilBearingCapacity) || soilBearingCapacity <= 0) { alert("Please select a valid Soil Bearing Capacity."); return; } var totalLoadPerFooting = deckAreaSupported * designLoad; var requiredFootingArea = totalLoadPerFooting / soilBearingCapacity; var requiredSideLengthFeet = Math.sqrt(requiredFootingArea); var requiredSideLengthInches = requiredSideLengthFeet * 12; // Round up to the nearest practical inch for recommendation var recommendedSideLengthInches = Math.ceil(requiredSideLengthInches); // Ensure minimum practical size, e.g., 10 or 12 inches if (recommendedSideLengthInches < 10) { recommendedSideLengthInches = 10; } else if (recommendedSideLengthInches % 2 !== 0) { // Round up to nearest even inch if odd recommendedSideLengthInches++; } document.getElementById("totalLoadOutput").innerHTML = "Total Load per Footing: " + totalLoadPerFooting.toFixed(0) + " lbs"; document.getElementById("requiredAreaOutput").innerHTML = "Required Footing Area: " + requiredFootingArea.toFixed(2) + " sq ft"; document.getElementById("minSideLengthOutput").innerHTML = "Minimum Square Footing Side Length: " + requiredSideLengthInches.toFixed(1) + " inches"; document.getElementById("recommendedSizeOutput").innerHTML = "Recommended Footing Size: " + recommendedSideLengthInches + " inches x " + recommendedSideLengthInches + " inches"; } // Initial calculation on page load for default values document.addEventListener('DOMContentLoaded', calculateFootingSize);

Understanding Deck Footings and Their Importance

A deck footing is the foundation element that transfers the weight of your deck safely to the ground. It's a critical component for the stability, longevity, and safety of any deck structure. Without properly sized footings, a deck can settle unevenly, crack, or even collapse, especially under heavy loads or adverse weather conditions.

Why Footing Size Matters

The size of your deck footing directly impacts its ability to support the deck's weight without sinking into the soil. If a footing is too small for the load it carries or the soil it rests on, it will exert too much pressure on the ground, causing settlement. This can lead to structural issues, uneven decking, and potential safety hazards. Proper sizing ensures the load is distributed over a large enough area to prevent soil failure.

Key Factors Influencing Footing Size

1. Deck Area Supported per Footing: This is the portion of the deck's total surface area that a single footing is responsible for supporting. Larger supported areas mean more weight, requiring larger footings.
2. Deck Design Load (psf): This refers to the total weight per square foot that the deck is designed to carry. It typically includes a "dead load" (the weight of the deck materials themselves) and a "live load" (the weight of people, furniture, snow, etc.). For residential decks, a common design load is 50 pounds per square foot (psf), comprising 10 psf dead load and 40 psf live load.
3. Soil Bearing Capacity (psf): This is the maximum pressure the soil can withstand without yielding or settling excessively. Different soil types have vastly different bearing capacities. For example, solid bedrock can support much more weight than loose sand or soft clay. Local building codes often specify minimum soil bearing capacities for various soil types, or a geotechnical report may be required for precise values.
4. Frost Line Depth: While not directly calculated by this tool, it's crucial to note that footings must extend below the local frost line. This prevents "frost heave," where freezing and thawing of moisture in the soil can lift and shift footings, causing significant structural damage to the deck.

How to Use This Calculator

Our Deck Footing Size Calculator simplifies the process of determining the minimum required footing dimensions. Here's how to use it:

• Deck Area Supported per Footing: Estimate the square footage of the deck that each individual footing will bear. For example, if your deck joists span 10 feet and your footings are spaced 5 feet apart along a beam, each footing might support 50 sq ft (10 ft x 5 ft).
• Deck Design Load: Use the standard 50 psf for most residential decks, or adjust if your local codes or specific design requires a different value.
• Soil Bearing Capacity: Select the option that best describes your soil type. If you have access to a soil report or local building code information, use those precise values. When in doubt, err on the side of caution and choose a lower bearing capacity.

The calculator will then provide the total load on the footing, the minimum required footing area, and the minimum side length for a square footing. We also provide a recommended practical size, rounded up to a common dimension.

Important Considerations

This calculator provides a general guideline for footing size based on common engineering principles. However, it is not a substitute for professional advice or adherence to local building codes. Always:

• Consult Local Building Codes: Footing requirements vary significantly by jurisdiction. Always check with your local building department for specific regulations, minimum sizes, frost line depths, and inspection requirements.
• Consider Professional Engineering: For complex deck designs, challenging soil conditions, or if you are unsure, consult a licensed structural engineer or a qualified deck builder.
• Account for Other Loads: This calculator primarily focuses on vertical loads. High decks, decks in windy areas, or those supporting heavy features like hot tubs may require additional considerations for lateral loads and uplift.

By using this calculator as a starting point and combining it with local expertise, you can ensure your deck has a safe and stable foundation.