Today we are starting a two-part series on building proper deck footings. We will discuss the purpose of footings, guidelines on sizing, depth of burial, and heuristics related to capacities.
A deck can provide a great outdoor living space and add value to a home, but only if it has a solid foundation supporting it. Many deck failures and collapses are caused by inadequate footings that can’t properly support the weight of the deck. Skimming the codebook will show there is such a heavy focus on preventing detrimental collapse and failure. Without proper planning and appropriate configuration, mistakes and misunderstanding happen that lead to dangerous conditions. For this reason, constructing proper footings and foundations should be a priority from the very start of a deck build project.
In this article series, we’ll cover the basics of deck footings and foundations, including the importance of proper footings, footing requirements, materials, and how to calculate load and build secure footings that will reliably support a deck for many years.
The outline for this week’s and the following week follow below:
This Week:
- Why Proper Footings Are Critical
- Footing Size Requirements
Next Week, in Part Ii
- Footing Depth
- Calculating Deck Load
- Digging and Pouring Footings
Why Proper Footings Are Critical
Deck footings provide the base that transfers the deck load into the ground. They consist of pads or underground blocks or piers, usually made of concrete that spread the weight of the deck over a larger area to prevent settling. Without adequate footings, the posts and beams have nothing solid to rest on, leading to sagging, instability, and potentially a collapsed deck. For example, if you were just set wooden post directly on the dirt with the load of the deck on top of the post, over time as the ground changes back and forth between varying states of moisture content with cycles of participation, the posts would penetrate and eventually enter into the ground and subside allowing the deck to lean away from the building until the point that it collapsed or subsided lower on the edge away from the building. As well, posts without solid and significant footings can heave upwards with moisture changes and with frost heave when the land freezes.
Proper footings accomplish several things:
- Transfer the deck load into the deeper, greater compacted ground, rather than the soil directly underneath, which cannot support the weight on its own.
- Anchor the posts and prevent lateral movement and shifting.
- Keep the deck level by preventing differential settlement.
- Provide adequate support in multiple soil conditions and climates.
- Allow for frost movement in colder climates, like here in Washington DC without our bitter cold winters.
Deck footings must be sized and dug down to the proper depths to achieve these goals based on factors like soil type, deck size, and weight loads. Many building codes, like the adopted ICC code, here in Washington, DC, require inspection of footing excavations before further deck construction can begin.
Footing Materials: Concrete is the most common and recommended material for deck footings. Pre-mixed concrete is readily available and provides high strength and durability. In most cases, it is important to use concrete rated for at least 3,000 PSI compression strength.
For a variety of reasons, such as convenience, limiting waste, time savings, concrete tube forms are available that need to be placed in the footing holes before pouring concrete. Tube forms create round footings. For square footings, you can build wood framing at the area above grade. We recommend, when placing footings in an unpaved area, to set the footing tops slightly above the grade level to keep the post an extra small distance above the plane of the ground or grade.
Other footing and footing top options include:
- Poured concrete footings – Using forms allows full customization for unique footing shapes if needed.
- Concrete blocks – Stacked blocks can support lighter decks, but may not provide enough support for larger decks.
- Concrete piers – Pre-cast concrete piers can provide sturdy footings, especially for elevated decks.
- Screw piles – Steel screw anchors drilled into the ground provide anchoring and adjustability.
Footing Size Requirements
Building codes and local AHJ’s dictate minimum footing sizes based on the deck’s square footage. Typical requirements for wood-framed decks may be approximately as follows:
- Small decks under 200 sq ft – 12 inch diameter round footings or 16×16 inch square footings.
- Medium decks of 200-500 sq ft – 15-16 inch diameter or 20×20 inch square footings.
- Large decks over 500 sq ft – 18-24 inch diameter or 24×24 inch square footings.
These sizes are based on assumed deck load capacities and typical residential soil conditions, each deck must be specifically calculated as it applies to the configurations and spacing, and loads required. Larger footings may be required for heavier deck loads, weak soils, or cold climates. It’s necessary to carefully consider the building codes for each specific detail of footing size requirements.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. You can contact us today for more information.
