Extra Tall Deck Posts

Challenges and Structural Solutions for Extra Tall Deck Posts

Today, we’re looking at a relatively small footprint rear deck, but one of the interesting unique details of this deck is that it is built with extremely tall posts. The deck is approximately 2.5 stories above the adjacent grade.  In many cases, we see tall decks, similar to this, but in most of those circumstances, here in Washington, DC, it is a multi-story deck, so not just is it tall, but it’s built in two or three levels overall.  The difference here though is that the posts of the deck run all the way from the top floor of the building down to the ground.

Extra Tall Deck Posts
The structural bearing capacity and load path is altered when a deck is particularly tall.  In other words, if all other things were the same but this deck were lower or closer to the ground, you could size and support the deck post slightly differently.

The next picture below shows a different deck, built similarly in some ways, but particularly different in a very important way which we will explain. We’re just using this photo as a point of comparison, in this particular case, but we will look at this deck more closely, in another blog, here on our website.

If you look at the lower portion of the framing closest to the front, as shown in the picture below, there is a joint and a separate section of the deck built at the left hand side. The joint runs across the facing rim joist.  By comparison, that is a very weak connection and likely a structural defect in the construction of this deck. However, the main point here is that it would be very difficult to properly support a deck like the one in the picture below if it were done with a very tall pair of posts and nothing else.  

lower portion of the framing - the Extra Tall Deck

The tall deck though that we’re looking at is shown more closely in the picture below. You can see that there are only really two posts carrying the entire outer edge of the deck.  The ledger board joins the deck to the building itself, and at that point there is a significant amount of structural support. However, the ledger board alone is not enough to support the deck because the deck must also be supported at the edge farther away from the building.

tall deck

The deck posts are very thin, like a pair of spider legs, and since the deck is particularly tall, it has a greater need for sway bracing and wind resistance. But with the long and tall skinny 4×4 post, this part of the framing looks to be insufficient.  Also, there’s another concerning detail that when you look at the connection between the deck rim joist and the post, you can see that the rim joist has not been shouldered into the post.

Shouldering, is cutting a notch into the deck post so that the rim joist or beam can sit both directly on top of the post and nestled into or against a portion of the side of the post as well.  This is a more stable and secure connection, reducing the risk of the joist slipping, rolling, or becoming unstable over time.

This is particularly important in decks with tall, thin posts, as shouldering helps resist lateral forces, such as wind or sway, by creating a more rigid structure. Shouldering also helps resist lateral movement which can lead to structural failure under load.  

The picture below zooms in on the connection between the top of the post and the underside of the deck rim joist, at the yellow circled area. Here you can see, not just at the outer edge, but also at the side.

underside of the deck rim joist
The next picture below shows a different deck, but also, like the deck above, the deck below is built with a very tall posts system. Here though, it’s built a little bit differently. Instead of just being built with individual tall skinny post extending from the ground all the way up to the underside of the deck at the top level of the building, here, they also have installed a robust and strong system of support bracing.

This bracing is built both at an angle from the post to the underside of the deck above and in an x-form, as large cross braces that interconnect at two points on each of the two posts. This is particularly important because this type of bracing can prevent the deck from racking, swaying, and will maintain a degree of rigidity, even with all of the types of external forces of movement and sway.

external forces of movement and sway.

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.  We are based right here in Washington,  DC and 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 

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