Last week we began looking at a six-part series on the topic of the structural elements and the load path of a typical deck and deck system. Today we will continue on the second part of this series looking at columns or structural posts.
The outline of the series follows:
1. Footings
2. Columns
a. Deck Post Materials
b. Deck Post Sizing
c. Deck Post Spacing
d. Connecting a Post to the Horizontal Girder Above
3. Girders
4. Joists
5. Ledgers
6. Rim Joists
The concept of columns as needed to support an elevated deck system, is relatively simple but there are some facets and details that are important to understand about columns. Today we’re going to talk about a few different areas of this topic: including materials, sizing, spacing, and the upper connection to a girder.
The picture above shows the combined elements of the load path but be aware that the footings shown are generally not in the proper context configuration or sizing and the girder mounting should be shouldered on top of the post.
Deck Post Materials
there are several popular material options to choose from, each with their own advantages and considerations. One of the most common choices is pressure-treated wood posts, typically pine or fir. Pressure-treating forcespreservative compounds deep into the wood fibers, greatly increasing rot and insect resistance. These posts are relatively inexpensive and can be painted or stained.
For improved durability and lower maintenance, some decks use naturally rot-resistant wood species like cedar or redwood for their posts. While more expensive initially, these woods have natural defenses that can enable decades of service life when properly maintained.
Increasingly, manufactured posts made from vinyl, aluminum, or fiber-reinforced composites are being used on decks. Vinyl posts are easy care and resist moisture, insects, and rot. Aluminum provides solid structural performance with clean lines. Composite posts blend wood fibers with polymer resins for good looks and weather-resistance.
Steel posts, either galvanized or coated, offer superior strength compared to wood. However, they ate more expensive and require footings with anchors specifically set for their concentrated loads. One of the biggest advantages of using a steel post system is that the post can easily be connected to a steel girder which can have a lower profile in some cases this is needed at the area under the deck to allow cars to pass under the deck and into a garage built into the rear of the building. Decorative hollow aluminum, wrought iron, or stainless steel posts provide support while complementing railings.
(The picture above shows the configuration and context for installing a built-up beam on top of a steel post with integral or connected structural plates.)
For almost all contemporary decks, not just very tall or structurally demanding scenarios, it is generally advisable to use heavy-duty post anchors or manufactured post bases attached to buried concrete pier footings or foundations. These rigid connections prevent racking and uplift forces.
No matter which primary post material is selected, using proper structural hardware like post anchors, bases, and caps is important. These connectors ensure the posts safely transfer loads into the footings below and beams/girders above according to code requirements.
The post material decision factors in budget, aesthetics, maintenance preferences, structural needs, and local environment. Selecting materials suited for the application results in safe, long-lasting deck posts.
Deck Post Sizing
Deck post sizing is determined based on factors like deck size, open span requirements, and load distribution:
Proper sizing of deck posts is critical to ensuring the structural integrity and safety of the deck above. Building codes provide guidance on minimum post dimensions based on the loads they must support and their height/spacing. However, many decks require larger posts than the bare minimum to handle real-world environmental conditions.
For relatively small, single-level decks, building codes often allow 4×4 inch treated wood posts at the outer corners where loads are lower and smaller. These smaller posts provide enough capacity for some decks under 100 sq ft or so, in some cases. However, as deck size increases, additional and larger posts become necessary to distribute the weight over more support points. A third line of posts running through the center is common on larger decks.
In cases where the deck design calls for an open span underneath, such as to allow parking access or a walkway, the post sizing must increase substantially. Spanning distances of 10-14 feet usually mandates using 6×6 inch posts as the primary supports to prevent excess sagging or deflection. These beefy posts have the strength to handle longer stretches unsupported by footings and posts below.
Post material also plays a role in sizing, it is one of the several linked variables in the sizing calculation. While 4×4 may suffice for wooden posts on a small deck, steel or aluminum deck posts often need a larger 6×6 dimension at minimum for comparable capacity due to their hollow centers. Manufacturers provide span rating tables to determine the required sizing.
For very large decks over 500 sq ft or multi-level/multi-span designs, the design loads must be thoroughly vetted. The design can analyze based on the dimensions and run calculations to specify optimal post sizes, types, and spacing to ensure long-term safety and performance based on the expected load scenarios.
Properly sizing deck posts from the start prevents issues like bounce, swaying, sagging, or even catastrophic failure down the road. Oversizing where feasible adds an extra margin of safety and longevity.
Deck Post Spacing
The spacing of posts supporting a deck’s outer rim joist is based primarily on two factors – the size/type of posts being used and the overall load they need to carry. Building codes provide maximum spacing tables or span ratings that specify how far apart posts can be placed based on their dimensions and expected weight load.
For example, with typical 4×4 wooden posts, some code tables may allow a maximum spacing of 6-8 feet between posts for light-duty decks under 40 psf live load, in certain cases. However, if the deck will have areas of higher concentrated load like a hot tub, spacing may need to be reduced to every 4-5 feet, in certain cases, to increase support.
With larger 6×6 posts, the allowable spacing can increase up to 8-10 feet in some cases for similiar load conditions. Manufactured posts like steel, aluminum or composites have their own spacing charts based on testing of their specific dimensions and load capacities.
In addition to meeting the posted span maximums, deck designers also need to consider concentrating posts near any heavy load points. For example, stair landings, locations for hot tubs/heavy planting containers, or areas designated for crowds may require closer post spacing locally to prevent overstressing.
Another spacing factor is whether the deck is cut into separate sections separated by points of lateral resistance (i.e. house walls). If so, each section can potentially be evaluated individually for post spacing rather than considering the entire length of rim joist as a continuous span.
(The picture above shows that a multitude of different factors have implications on sizing, including spacing, overhang, material height, and wood species.)
Furthermore, longer uninterrupted rim joist spans may require posts placed along the interior of the joist span rather than just the ends to prevent excessive deflection or sagging between the end posts.
Overall, deck post spacing aims to position supports closely enough to handle the cumulative deck loads without violating allowable dimensions in the code tables for the specific post type being used. Following these span maximums ensures adequate or sufficient strength and stiffness.
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
