There are a multitude of different parts and components in deck construction, and every deck is a little bit different, there are multiple different types of configurations and layouts. However, through all the different varieties and variations, there are some pretty consistent structural elements that are required to support a deck and make it stand strong. Although these elements can very slightly from one deck to another, they are generally consistent in certain ways. Most decks are built with a joist system, with horizontal decking laid on top of that joist system. Most joist systems are bound on three sides by a system of rim joists. In our past blogs we talked about some of the steel structural connectors required to build a deck in modern times. Those connectors will tie many of these components together in a redundant but important way, for reinforcement.
The deck level itself is generally held up or supported by a girder / beam that runs perpendicular to the joist with post or vertical support columns which bear upon individual footers or footings.
Today we will look at some diagrams we created to explain these elements and provide for their description to really understand how these components work together in a structural context to support a deck.
(Although decks are not incredibly complex, a structural support system is required to support an elevated deck above grade. We will continue to examine this diagram more closely in upcoming articles.)
We’ll talk about the following items, in this 6-part series, and today we will focus on Footings:
- Footings
- Columns
- Girders
- Joists
- Ledgers
- Rim Joists
Footings
Footings form the foundation that supports the deck structure above. These, mostly, below-grade components transfer the cumulative weight of the deck, any live loads like people and furniture, and environmental forces down into stable, undisturbed soil. Properly sized and constructed footings are essential for preventing deck settlement or failure over time. They are needed both to support the mass of the deck from above so that it does not sink deeper into the ground over time and also needed to secure the deck to the ground and resist uplift type forces from winds and other factors.
The most common footing materials are poured concrete. There are some alternative options for precast concrete pier blocks/panels but they are generally too small for most DC type decks. Poured concrete footings are formed by digging an appropriate hole and filling it with a concrete, in many cases that concrete must be reinforced with (intentionally) deformed steel. Local codes specify minimum diameter and depth requirements based on anticipated loads and soil conditions.
(This diagram shows two different typical methods of building a post footing, the diagram to the left with the post set on top of the footing is increasingly more common.)
The most common methods of connection of the post-based to the concrete footing are through a mechanical plate connection or embedment. There are a variety of reasons why the method shown on the left with a post-based connector may be preferred. Some contractors, specifically historically, preferred the method to the right where the post is set into the concrete footing, but for the reasons we describe here it’s increasingly less common.
In cold winter climates, like ours here in Washington, DC, footings must extend below the frost line to prevent heaving, (generally about 30” in our local DC region, BUT each deck and each type of construction has specific AHJ requirements based on a multitude of factors so check carefully and thoroughly). Precast concrete footings avoid having to pour concrete on site but still provide the necessary bearing area, but they are rarely a sufficient option for elevated decks and significantly sized decks.
If you look closely at the diagram above you will notice that both of the footing options are configured so that the bottom of the respective footing is set below the level of the frost line. That frost line varies depending on the region. Here in Washington DC, building codes and AHJ requirements generally require a 30-in depth. People generally follow those rules without really questioning the climate science, but with extreme weather spikes, temperatures could possibly even go lower than normal ranges. This depth of 30 inches is intended to include all possibilities, based on recent decades and centuries of historic knowledge of temperature ranges in our area, but it’s possible that temperatures could go even lower and the impacts could be significant. Frost heave can be forceful enough to displace a significant building foundation!
For enhanced strength and longevity, there are several best practices for deck footing construction. Adding reinforcing rebar (in a grid pattern in some configurations) increases the footing’s tensile strength. Setting the anchor bolts or pre-formed boots for connection to wood posts or manufactured post bases during the pour can avoid the need for concrete drilling later, but requires a batter board or adjacent layout prior to footing construction. In some cases, it is helpful to use inset tube forms, depending on soil conditions.
Historically many builders set the post directly in the footing, but as time goes on this practice will be less common, for a variety of reasons (such as changing rules on wood preservatives and increasingly more advanced post base innovation, and respective building code acknowledgement).
The concrete pudding shown in the diagram below and the steel post-base anchor connection is one of several types available through Modern structural connectors. This particular type must be embedded in wet concrete and cannot be set by the typical drilling process after the concrete has cured. It is a strong connection from both the points of stability and uplift resistance. The base is manufactured with a one inch standoff which allows the wood to be separated from the concrete just enough to prevent the typical end grain wicking of moisture up from the concrete into the wood post which will lead to rot. By including this standoff in the base plate it significantly can help resist moisture wicking and preserve the post for a much longer lifespan.
(This diagram shows one of the several varieties of Steel post base anchor connection points.)
Oversizing the footings, slightly, compared to minimum code requirements adds extra load distribution into the soil – preventing settlement issues. Using a gravel or compacted base underneath further improves stability. In cases of poor soil quality, techniques like an elevated post base or speciality helical pile footings may be required.
Paying close attention to proper depth, material quality, and reinforcement when installing deck footings ensures the entire structure has a robust, durable foundation to rest upon for decades of safe use. Taking the time for quality footing work is a wise long-term investment.
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 call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us
