Deck posts are the vertical supports that transfer the weight of the deck structure and its occupants safely to the ground. Establishing the correct distance between these posts is important for the safety and longevity of the structure. Posts bear the combined weight, known as the total load, and channel it into the foundation below. This load includes the dead load (the permanent weight of the deck materials) and the live load (people, furniture, and snow).
The Structural Role of Beams
The distance between deck posts is determined by the maximum allowable span of the horizontal beam (also known as a girder) that rests on top of them. This beam collects the load from the decking and supporting joists before distributing that weight evenly to the posts. Post spacing must be less than or equal to the beam’s maximum span capacity to prevent excessive deflection or structural failure.
The size of the beam directly corresponds to the distance it can safely cover between two posts. A larger beam, such as a built-up assembly of two or three dimensional lumber pieces, supports a heavier load over a greater distance than a smaller beam. The required beam size is calculated based on the total area of the deck the beam is responsible for supporting, known as the tributary area.
Determining Maximum Post Distance
Maximum post distance is governed by prescriptive span tables found in residential building codes, which are calculated based on standard engineering principles. These tables assume a live load of 40 pounds per square foot (psf) for residential decks, plus a 10 psf dead load for the materials. The calculations also account for the species and grade of lumber used, as different woods have varying strength properties.
A common structural guideline suggests that a built-up beam can safely span a distance in feet roughly equal to its nominal depth in inches, assuming it is made from common lumber like Southern Pine. Following this general rule, a double 2×8 beam would have a maximum post distance of approximately 8 feet. Similarly, a double 2×10 beam is often limited to a 10-foot span, while a double 2×12 beam can extend up to 12 feet between posts.
More precise calculations, which account for the exact length of the joists the beam is supporting, often result in shorter maximum spans. For example, a built-up 2×10 beam supporting joists that span 10 feet may have its maximum post spacing reduced to approximately 7 feet 9 inches. A built-up 2×12 beam under the same load conditions might achieve a span of about 9 feet 2 inches. Selecting a conservative spacing that is closer than the calculated maximum provides an extra margin of safety for the finished deck.
Supporting the Posts and Meeting Local Standards
The final post spacing must adhere to requirements for ground support and local regulations. Every post must rest on a solid footing or pier designed to prevent vertical movement. These footings must extend below the local frost line to prevent ground freezing and thawing from lifting or shifting the posts, a phenomenon known as frost heave.
The required diameter and depth of the concrete footing depend on the post size, the soil type, and the total load being transferred from the deck. Local building departments set the legal maximum post spacing and provide the prescriptive span tables that must be followed during construction. Obtaining a building permit and securing required inspections ensures the deck’s design meets all mandated safety and structural standards for the specific geographic location.