How to Size Concrete Deck Footings
Constructing a deck is not just an addition to your home; it's a gateway to outdoor living. However, the strength and longevity of any deck lie in its foundation—specifically, the concrete footings that anchor it to the ground. "Mastering Deck Construction: How to Size Concrete Deck Footings" serves as your essential guide to understanding the critical role of footings in deck construction and ensuring that your deck stands firm against the test of time and the elements.
A typical 24” concrete deck footing. This footing was sized at 24” instead of the typical 12” due to poor load-bearing characteristics of the soil in this yard.
The importance of accurately sizing concrete footings cannot be overstated. These foundational elements bear the load of the entire deck structure, distribute weight evenly, and prevent settling and damage that can occur due to frost heave and other soil movements. This guide will walk you through the key considerations, including soil type, load-bearing capacity, frost line depths, local average snow load, tributary areas, and local building codes, that influence the sizing of your deck's footings. Whether you're a seasoned contractor or a DIY enthusiast embarking on your first deck project, the insights provided here will equip you with the knowledge needed to make informed decisions, ensuring a sturdy and safe deck for years to come.
Embark on this journey with us as we delve into the intricacies of deck construction, starting with the foundation that keeps it all standing: the concrete footings. By mastering the art and science of sizing these crucial components, you'll be setting the stage for a successful deck project that not only enhances the aesthetic appeal of your home but also increases its value and functionality.
What is a Deck Footing?
A deck footing is a foundational component that supports a deck by spreading its load over a larger area. This is essential for maintaining the stability and safety of the deck structure. Deck footings are typically made of concrete and are installed below the frost line in the ground to prevent shifting caused by freeze-thaw cycles. The size and type of footing required can vary based on local building codes, the size of the deck, soil conditions, and the deck's height and weight load. Common types of deck footings include concrete piers, which are poured into cylindrical forms; precast concrete footings; and helical piers, which are screwed into the ground. Properly installed deck footings help ensure the deck is stable and durable over time.
This article focuses on the most common type of deck footings, which are concrete piers.
Footing vs. Pier
When we refer to a deck’s footing and its size, we are actually referring to the bottom portion of a concrete pier that makes contact with the soil.
The cylindrical portion of concrete in a deck footing is the pier, and its only purpose is to transfer the load down to the footing.
So when we say “size of footing”, we are really referring to the surface area of the bottom of the concrete pier. The size of the rest of the pier doesn’t have any impact on the footings load bearing capacity.
How to Size Deck Footings
Selecting the correct size of footing is imperative to avoid settling or failure. To properly size concrete footings for a deck, we need to know the following:
Total Structural Load Per Isolated Post
Total structural load per isolated post refers to the total weight each of the deck’s posts will support. Because each post is supported by a footing, this is also referred to as the total load per footing.
The Load Bearing Capacity of the Soil
The amount of weight or load the soil can support.
Step 1: How to Determine the Total Deck Load
To accurately size footings for a deck, we first need to find the total load of the deck. The total load of the deck needs to factor in the weight of the building materials themselves, the weight of deck furniture, accessories and occupants, and any potential snow load. Throughout this article, loads will be measured in psf (pounds per square foot).
For determining the total load of a deck, the below framework is common practice in Ontario, Canada.
10 psf dead load (weight of building materials)
+
40 psf live load (weight of furniture and occupants)
=
50 psf total load (50 pounds per square foot of deck)
The live load assumption in this example already accounts for some snow load, but we can check the average snow load for our specific building area. Standard practice is to replace the live load, with your region’s snow load data, if it exceeds 40 psf.
For example. If you live in Uxbridge or Port Perry, Ontario, Canada, the average snow load is 53 psf.
Therefore, your load calculation would look something like this.
10 psf dead load
+
53 psf snow load
=
63 psf total load (63 pounds per square foot of decking)
Let’s use an imaginary deck as an example. The deck will be located in Port Perry, Ontario, which has an average snow load of 53 psf. The deck will be 12’ x 20’ (240 square feet). To calculate the total load, we simply multiply 240 square feet with 63 psf giving us a total load of 15120 lbs.
This means that all of our footings and the soil beneath them need to support 15120 lbs safely. But we can’t just divide this number by the number of footings (it would be great if it were so simple).
To properly determine the load exerted on each isolated footing, we need to understand and be able to calculate tributary areas for a given design.
Tributary Areas
The portion of deck surface area carried by a single post and footing is called the “tributary area”. Because posts can be placed under almost any area of the deck, and additional posts can be added, the tributary area should be calculated for each post. Consequently, the appropriate size of footings can vary within the same deck.
The load on a single footing is determined by multiplying the surface area of its tributary area with the total load of the deck.
A standard 12’ x 20’ deck with ledger board divided up into tributary areas.
Step 2: Calculating Tributary Areas for a Deck
The first step to calculate the tributary areas for a particular deck is to examine the deck’s layout. You can draw the layout on paper, or use CAD software such as Sketchup.
Divide the surface area of the deck up into sections, with one per footing. Each section should terminate at the mid-point between foundations. These sections represent the tributary areas. It’s important to determine your mid-points based on the locations of your footings, and not the deck footprint itself. For decks with a ledger board, such as in the example above, we treat it as one foundation.
In the example above, the deck extends 12’ from the house, but the beam and row of footings are 10’ from the house. Therefore the mid-point for determining our tributary areas is 5’ from the house, and not 6’ as one might assume.
The tributary area also needs to include any associated cantilever, as seen in the example above.
Step 3: Calculate the Isolated Load on a Deck Post or Footing
Now that we know the total load of the deck, and have determined the tributary areas, we can calculate the isolated load on each post and footing. Let’s continue with the Port Perry example we have been using.
Area 1 is carried by a ledger board in the above example, and there are no footings, so there’s nothing more we need to calculate there.
Areas 2 & 6 share the same tributary area, so the footing requirements will be identical.
The same goes for areas 3-5. Because the tributary areas are all the same, we only need to calculate the footing requirements for one.
To calculate the isolated load for a tributary area, we simply multiply the area by the total load per square foot. Let’s work through it below.
Isolated Load on footings 2 & 6:
24.6 sq. ft. (tributary area) x 63 psf (total load per square foot) = 1549.8 lbs.
This means the isolated load carried by footings 2 & 6 is 1550 lbs. (rounded up to the nearest pound).
Isolated Load on footings 3-5:
28.5 sq. ft. (tributary area) x 63 psf (total load per square foot) = 1795.5 lbs.
This means the isolated load carried by footings 3-4 is 1796 lbs. (rounded up to the nearest pound).
Now that we have determined the isolated load carried by each post, we need to understand the soil we’re working with and its characteristics. After all, it’s going to be supporting our footings, and the entire deck.
Understanding Soil Types and Load Bearing Capacity
When it comes to sizing deck footings, the type of ground you’re working with plays a pivotal role. Soil types range from dense clay to loose sand (and everything in between), each with distinct characteristics that affect how weight is distributed and supported. The most important soil characteristic we need to consider when sizing footings is load-bearing capacity.
Load-bearing capacity refers to the ability of soil to support the weight of structures without undergoing settlement or failure, varying significantly across different soil types. Understanding these soil types, and more importantly, their bearing capacities is crucial in determining the appropriate size for your deck footings. To put it in simple terms, we need to ensure that the soil can hold up the weight of the loaded footing, without settling or collapsing.
Load-bearing capacity is often measured in pounds per square foot (psf). This is the unit of measure we will use for demonstration purposes in this article. A load-bearing capacity of 1000 psf means: that over a surface area of one square foot, a given soil type will support 1000 lbs before failure.
Common Soil Types
Clay Soil: Known for its high density and low drainage capabilities, clay soil expands when wet and contracts as it dries. This characteristic can lead to significant movement, which poses challenges for deck stability.
Sandy Soil: Sandy or gravelly soils offer better drainage than clay but have a lower load-bearing capacity. This means that while footings may not need to be as deep, they may require a broader base to distribute the deck's weight evenly and prevent sinking.
Loamy Soil: Loam, a mixture of sand, silt, and clay, combines the best aspects of each soil type, offering both stability and drainage. Footings in loamy soil can be of standard size, as this soil type provides a good balance of support and water management.
Rocky Soil: Building on rock provides an excellent foundation, as it's incredibly stable and capable of supporting significant weight. However, the challenge lies in excavation. Footings may not need to be as deep, but creating them can be more labour-intensive.
Types of Load-Bearing Capacity
While there are many more than two types of load-bearing capacity, for the purposes of this article and deck construction, we will separate the term into two different types: ultimate bearing capacity, and allowable bearing capacity.
Ultimate bearing capacity refers to the absolute maximum amount of pressure/load that the soil can support before failure occurs. This means that soil with an ultimate bearing capacity of 1000 PSF, will fail at a load of 1001 psf.
Allowable bearing capacity refers to the values we use in determining footing size and placement to support a specified load. For the design of deck footings, this typically means the ultimate load, divided by a factor of safety (typically 2 or 3). For example, soil with an ultimate bearing capacity of 3000 psf would have an allowable bearing capacity of 1000 PSF.
Step 4: How to Determine Soil Bearing Capacity for Deck Footings
There are various ways to determine the load-bearing capacity of soil, each with varying levels of accuracy, complexity, and cost. In very few instances are actual soil tests performed for sizing deck footings. It can be a costly and time-consuming process that most often doesn’t fit into the scope of an average residential deck project.
By far the most common practice for determining the bearing capacity of soil for deck construction is through visual assessment and educated assumptions. Your local building department or inspector can be a valuable resource for this, as they often have an intimate understanding of the soil characteristics in your area. When securing building permits, the local building department will often provide a load-bearing capacity value to assume for your calculations.
The chart below is an excellent resource for assuming the bearing capacity of soils.
Soil Types with Ultimate and Allowable Bearing Capacity. Source: DICA USA
If you aren’t comfortable assessing the soil characteristics on your own, various companies accept samples for analysis.
For the example we have been working through, we’ll assume the soil has an allowable bearing capacity of 2500psf. This is a common bearing capacity we encounter in Southern Ontario, Canada.
Step 5: Determine Deck Footing Size
Now that we have calculated the load on each post, and have determined the bearing capacity of the soil, we can calculate the size each footing needs to be to support the required load.
Let’s calculate the footing size needed for tributary areas 2 & 6, which share identical loads of 1550 lbs. each. To calculate the footing size required, we can use the following formula:
A = B/C
A = surface area of footing in sq. ft.
B = load on the footing in lbs.
C = load-bearing capacity of the soil in psf.
Let’s plug in our numbers for areas 2 & 6.
A = 1550 lbs. / 2500 psf.
A = 0.6 sq. ft.
This means our footings for tributary areas 2 & 6 need a surface area of 0.6 sq. ft. each. Now, let’s determine the diameter of our footing so it’s easy to work with.
Because our deck footings are cylindrical, we know the footing surface is a circle. Therefore, we can use the following formula to determine the diameter.
ø = 2 * √(A / π)
ø = Circle diameter
A = Circle area
π = Pi = 3.14159…
Let’s input our surface area of 0.6 sq. ft.
ø = 2 * √(0.6 / π)
ø = 0.87 ft
Great! We now know that footings 2 & 6 need a diameter of 0.87 feet, or 10.5” each.
Wrap Up
We’ve covered a lot in this article, and it’s okay to feel overwhelmed. If you ever feel stuck when calculating footing sizes, this article is an excellent guide to keep open and reference. The process may seem complex at first glance, but it can be broken down into a few easy steps.
Step 1: Draw your deck’s framing layout
Step 2: Determine the total load
Step 3: Find tributary areas
Step 4: Determine the isolated load for each tributary area
Step 5: Determine soil bearing capacity
Step 6: Calculate the footing size
Whether you're a seasoned builder or a determined DIY deck enthusiast, mastering the art of deck construction begins from the ground up—with footings that are sized to perfection. Failing to properly size deck footings can lead to costly repairs or even catastrophe.
Looking for a professional opinion? I’ve been building decks and deck footings in Ontario for almost 20 years! Whether you’re looking for a quote for your deck project, or just to chat, fill out the contact form below to get in touch. I look forward to hearing from you!