Deck Footing Size Chart: Sizes, Depths, Types & Safety
Everything you need to read a deck footing size chart correctly — definitions, footing types, depth-by-frost-line data, sizing formulas, safety rules, and pros and cons — in one detailed reference.
What Is a Deck Footing?
A deck footing is the buried concrete base that sits beneath a deck’s support posts or piers. Its job is to spread the deck’s weight, plus everything on it, across enough soil area that the ground can safely carry the load without settling, sinking, or shifting. Think of it as the foundation of a foundation: the post holds up the beam, the beam holds up the joists, and the footing holds up the post.
A typical footing has three parts:
Base / Pad
The wide bottom section that contacts undisturbed soil and spreads the load out, often set on a gravel bed for drainage.
Pier / Column
The vertical concrete shaft (often poured in a cardboard sonotube) that rises from the base up to grade level.
Post Base Hardware
The galvanized bracket set in or bolted to the top of the footing that connects the wood post to the concrete.
Why Deck Footing Size Matters
Footing size is not a cosmetic detail; it is a structural safety calculation. A footing that is too small for its load and soil conditions can settle unevenly, tilt, or crack, causing the whole deck to slope, rack, or separate from the house. Three forces drive the required size:
- Load — the combined dead load (materials) and live load (people, furniture, snow) that each post must carry.
- Soil bearing capacity — how much pressure, measured in pounds per square foot (psf), the native soil can support without compressing.
- Frost depth — how far the ground freezes each winter; footings must extend below this line so frost heave cannot lift them.
Types of Deck Footings
Before using any deck footing size chart, it helps to know which footing type applies to your project, since sizing tables assume a particular installation method.
Sonotube / Poured Concrete Pier
A round cardboard form is set in a dug hole and filled with concrete. The most common footing type for elevated, attached decks and the type most size charts are built around.
Poured Concrete Pad (Spread Footing)
A square or rectangular slab poured at the base of a pier, used when very wide load-spreading is needed on weaker soils.
Precast Concrete Deck Blocks
Factory-made pyramid or cube blocks set on grade. Fast to install but generally limited to low, freestanding, non-frost-critical decks where local code allows it.
Helical / Screw Piles
Steel shafts with helical plates twisted into the ground below the frost line. Popular for difficult soils, sloped lots, or minimal excavation projects.
Continuous / Strip Footing
A long poured footing running under a wall or a closely-spaced row of posts, common on porch-style or ledger-supported deck framing.
Engineered / Structural Footing
Custom-sized footing designed by a structural engineer for heavy loads, hot tubs, multi-story decks, or unusual soil conditions that fall outside prescriptive charts.
Deck Footing Size Chart (Diameter, Depth & Soil Bearing)
The table below is a general-reference deck footing size chart based on common prescriptive values (50 psf combined design load). Use it to understand how tributary area and soil bearing capacity drive footing diameter — then confirm exact numbers with your local building department.
Footing Diameter by Tributary Area & Soil Bearing Capacity
| Tributary Area (sq ft) | 1,500 psf soil | 2,000 psf soil | 2,500 psf soil | 3,000 psf soil | 4,000 psf soil |
|---|---|---|---|---|---|
| 20 | 16″ | 14″ | 13″ | 12″ | 10″ |
| 40 | 22″ | 20″ | 18″ | 17″ | 15″ |
| 60 | 28″ | 24″ | 22″ | 20″ | 18″ |
| 80 | 32″ | 28″ | 25″ | 23″ | 20″ |
| 100 | 36″ | 31″ | 28″ | 26″ | 22″ |
| 120 | 39″ | 34″ | 31″ | 28″ | 24″ |
Footing Depth by Frost Climate Zone
| Climate Zone | Typical Frost Depth | Minimum Footing Depth |
|---|---|---|
| Zone 1 — Mild (Gulf Coast, S. Florida) | 0″–6″ | 12″ (bearing minimum) |
| Zone 2 — Moderate (Mid-Atlantic, Pacific NW) | 12″–24″ | 24″–30″ |
| Zone 3 — Cold (Midwest, Northeast) | 24″–42″ | 36″–48″ |
| Zone 4 — Very Cold (N. Plains, N. New England) | 42″–60″+ | 48″–60″+ |
Quick Reference by Deck Size
| Deck Size | Typical Post Spacing | Typical Footing Diameter |
|---|---|---|
| Small (under 100 sq ft) | 6–8 ft | 12″–16″ |
| Medium (100–200 sq ft) | 8 ft | 16″–20″ |
| Large (200–400 sq ft) | 8–10 ft | 20″–24″ |
| Very large / multi-level (400+ sq ft) | Varies | 24″+ or engineered footing |
How to Determine the Right Footing Size (Step by Step)
- Find your tributary area. Multiply half the joist span by half the beam span supported by each post to get the load area in square feet.
- Check your soil bearing capacity. Look up presumptive values with your local building department, or use a geotechnical report if the site has poor or unusual soil.
- Find your local frost line depth. Your permit office or a regional frost-depth map will give the minimum footing depth for your area.
- Cross-reference the deck footing size chart. Match your tributary area and soil bearing capacity to read off the required footing diameter.
- Confirm with your local building department before excavating or pouring, since amendments to the base code are common.
Is It Safe? Deck Footing Safety Rules
Correctly sized footings are one of the most important safety factors in deck construction, since the footing is the only thing transferring the entire structure’s load into the ground. A properly designed footing, poured to the correct diameter and set below the frost line, is considered safe and code-compliant. Key safety rules include:
- Always pull a building permit for attached or elevated decks — inspectors check footing depth before it is covered.
- Never rest a post directly on soil, pavers, or an undersized block for anything beyond a very small, low, freestanding platform.
- Bear on undisturbed native soil, not loose fill, unless the fill has been engineered and compacted.
- Use a post base connector rated for uplift and lateral loads, not just gravity load.
- Have footings inspected before backfilling — this is the single easiest way to catch an undersized or too-shallow footing.
Skipping proper footings, or guessing at sizes instead of using a validated deck footing size chart or engineered calculation, is not considered safe practice for any structure that people will stand, sit, or walk on.
Advantages of Properly Sized Deck Footings
Advantages
- +Prevents settling, tilting, and uneven decking surfaces over time.
- +Resists frost heave when placed below the frost line.
- +Distributes structural load safely across native soil.
- +Extends the deck’s usable lifespan and reduces maintenance calls.
- +Passes building inspections and supports resale value and insurability.
Disadvantages & Common Mistakes
Disadvantages
- −Requires excavation, concrete, and curing time before framing can continue.
- −Oversizing wastes concrete and labor cost unnecessarily.
- −Undersizing risks cracking, shifting, or code violations.
- −Soil variability across a single lot can make a single chart value inaccurate without testing.
- −Skipping inspection hides problems until the deck is already built.
Uses & Applications of a Deck Footing Size Chart
A deck footing size chart is used by homeowners, contractors, and designers across several common scenarios:
- New deck construction — sizing footings before a permit application or excavation.
- Deck additions or expansions — matching new footings to an existing structure’s load path.
- Porch, pergola, and gazebo foundations — any post-supported outdoor structure follows similar logic.
- Hot tub or heavy-equipment decks — verifying whether prescriptive charts apply or an engineered footing is required.
- Permit and inspection prep — giving inspectors a clear basis for the footing sizes shown on submitted plans.
- DIY planning and budgeting — estimating concrete volume and excavation depth before ordering materials.
Frequently Asked Questions
A deck footing is the concrete base buried below a deck post that spreads the structure’s load over enough soil area to prevent sinking, shifting, or frost heave.
Soil expands as it freezes. A footing placed above the frost line can be pushed upward each winter, cracking the concrete and tilting the deck. Placing the base below the frost line keeps it on stable, unfrozen soil year-round.
There is no single standard size. Most residential footings range from 12 to 24 inches in diameter, depending on tributary load area and soil bearing capacity, as shown on a deck footing size chart.
Depth depends on the local frost line. Mild climates may only need 12 inches, while cold climates can require 42 to 60 inches or more.
Calculate the tributary area supported by each post, multiply by the design load (typically 50 psf), then divide by the soil’s bearing capacity — or read the value directly off a deck footing size chart.
Many codes set a practical minimum around 8 to 12 inches, but actual requirements are usually larger once tributary load and soil bearing capacity are considered.
Precast deck blocks are allowed in some areas for low, ground-level, non-frost-susceptible decks, but not where footings must extend below the frost line or support attached, elevated structures.
It may be enough for small, lightly loaded posts on strong soil, but larger decks, wider post spacing, or weaker soil will need a bigger diameter.
Presumptive values in prescriptive tables commonly range from 1,500 psf for soft clay to 4,000 psf for dense gravel, unless a geotechnical report says otherwise.
Many jurisdictions don’t require it in small residential footings, but rebar is commonly added for extra strength, larger footings, or frost-prone regions — and some codes do mandate it.
It depends on post spacing and beam spans; most decks use one footing per post, spaced roughly 6 to 10 feet apart along the beam line.
An undersized footing can settle unevenly, crack under load, or heave with frost — leading to a sloped deck, loose posts, or structural failure over time.
No. Skipping footings is not considered safe for any attached or elevated deck, since posts resting directly on soil or pavers will shift and sink over time.
Many homeowners pour their own footings for simple, low decks under permit, but complex soil, tall decks, or heavy loads are safer left to a structural engineer or licensed contractor.