For the vast majority of decks, the post sizes are either 4”x4” or 6”x6”. In some cases for relatively small decks with limited spans, and adequate or limited space runs between post, 4×4 post might be appropriate, but in most cases, for the next we build, particularly where elevated to a upper level or a bit more than a few feet above grade, such as an entire story above grade, it’s in most cases better to use 6×6 posts.
Pressure-treated Southern yellow pine is the predominant material used for deck framing and posts in the Washington D.C. region. This wood species is common and often preferred because of its availability and its ability to readily accept and its compatibility with chemical preservative treatments. Southern yellow pine is also known for its relatively high strength-to-weight ratio, making it a common choice for structural applications like exterior deck construction.
By comparison, there are a variety of different exotic hardwoods which have significantly higher strength characteristics, but they are less available locally and have a significantly higher weight which changes their weight to strength ratio. Typical exterior framing lumber in our region’s dense grain structure contributes to its strength, and its ability to hold fasteners well adds to its suitability for deck construction. By comparison, strong or hardwoods, even locally available hardwood such as red oak which has a very dense grain structure, will often split when pierced with fasteners without pre-drilling pilot holes.
When pressure-treated, Southern yellow pine gains significant resistance to decay, fungal growth, and insect infestation, extending the lifespan of the deck structure. However, it’s important to note that the pressure treatment process can slightly alter the wood’s physical properties, potentially causing some initial warping or twisting as the wood dries and acclimates after installation. Almost all types of softwoods and hardwoods require an extensive, long duration milling, stacking, drying, storing, and shipping process. Then, once the material arrives to a local lumber distributor, these materials are often stored again for long periods of time. During that time, the pressure and moisture contents regulate or neutralize within the individual boards which can result in twisting and warping. For this reason, almost all of these types of materials require a degree of selecting and culling.
The strength characteristics of framing materials, including deck posts, in our area, are governed by the International Building Code (IBC) and the International Residential Code (IRC). These codes provide standardized guidelines for many types of construction practices, essentially laying a framework for enforcement of safety and structural integrity across various building disciplines. (But, enforcement of the building code is left to DCRA and related DC offices which are almost entirely unable to effectively manage their mandate. This is one of the many reasons it matters so much to use a good quality minded and focused contractor, like Dupont Decks and Patios.)
For wood framing members, the codes reference design value tables that specify the allowable stress for different wood species and grades. These tables take into account factors such as bending strength, compression strength parallel to grain, compressive strength perpendicular to grain, shear strength, and modulus of elasticity. For deck posts specifically, the primary considerations are usually the compression strength parallel to grain (as posts primarily bear vertical loads) and the bending strength (to account for any lateral forces).
When determining the appropriate size for deck posts, several factors come into play. The primary considerations are the height of the post, the load it needs to support, and the spacing between posts. The load a post must support is influenced by the size of the deck, the materials used for decking and railing, and any additional features like built-in seating or hot tubs. The height of the post is crucial because taller posts are more susceptible to buckling under load. As a general principle, the taller the post, the larger its cross-section should be to resist buckling. This is why 6×6 posts are often preferred for elevated decks or those more than a few feet above grade.
Here, in the picture below, you can see a type of warehouse of a modern distributor such as our industrial and commercial building materials suppliers. Often, when creating purchase orders and selecting proper construction materials, the process can seem a bit overwhelming until you’re very familiar with all of the options and varieties and details of the wide array of building materials.
To illustrate how these factors interact, let’s consider a specific example. Suppose we have a deck that’s 10 feet above grade, with posts spaced 8 feet apart, supporting a total load of 50 pounds per square foot (a typical design load for residential decks). Using the allowable stress design method and referencing the appropriate wood design values, we can calculate the required post size. For a 4×4 post (which has an actual size of 3.5″ x 3.5″), the calculation might show that it’s inadequate for this height and load, as it may exceed the allowable stress or have a slenderness ratio that’s too high, increasing the risk of buckling. On the other hand, a 6×6 post (actual size 5.5″ x 5.5″) would likely provide sufficient strength and stability for this application.
The calculation process involves determining the total load on each post, which is the deck area supported by the post multiplied by the design load (in this case, 8 feet x 8 feet x 50 psf = 3,200 lbs). This load is then compared to the allowable compressive stress of the wood, adjusted for the height of the post. The slenderness ratio, which is the ratio of the post’s unbraced length to its least cross-sectional dimension, is also calculated. If this ratio exceeds certain limits (typically around 50 for wood posts), the post’s capacity must be further reduced to account for the increased risk of buckling.
It’s worth noting that while these calculations provide a solid basis for post sizing, they represent a simplified version of the complex structural analysis that might be performed by the designer. In practice, many jurisdictions have prescriptive requirements for deck construction that specify minimum post sizes based on deck height and other factors. These prescriptive requirements are based on conservative interpretations of engineering principles and are designed to ensure safety across a wide range of common deck configurations. For instance, some local codes might mandate the use of 6×6 posts for any deck more than 6 feet above grade, regardless of the specific load calculations.
In the example in the picture below, although this deck has a relatively wide span from side to side, it’s only a short distance away from the exterior wall of the building where it is tied to the building with a ledger board. Also, the relatively low overall height of the deck, in this case, allows for a smaller post which saves a bit of cost.
The choice between 4×4 and 6×6 posts (or even larger sizes in some cases) depends on a complex set of factors including deck height, post spacing, supported load, and wood species characteristics. While 4×4 posts may be sufficient for smaller, lower decks, the increased stability and load-bearing capacity of 6×6 posts make them a more versatile and often preferred choice, especially for elevated decks or those with significant loads. As with all aspects of construction, it’s useful to work with a contractor like our company to think about the strength and longevity of the deck structure.
In this coming week, we will write and publish another blog, here on our site, that looks closer at the differences between the nominal versus real dimensions of framing lumber.
Our company understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . 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
