Deck Posts: Ensuring Stability with Off-Centered Intermediary Positions
If you look at the buildings in the picture below, you can see that every single one of these houses lack a rear backyard. Instead, they have a deck, built above grade, coinciding with the middle level rear kitchen, and each one of the rear properties have parking spaces at the ground instead of a yard or patio or garden.
They use the rear undeveloped part of the property as driveways to garages under the houses Whether or not you think it’s a good use of space, it does make sense to have a garage Here in a busy and crowded little city like Washington DC, space is at a premium and some neighborhoods are particularly difficult for parking. As nice as it might be to have a backyard to use as an outdoor gathering space or even just as a garden or personal little sanctuary, sometimes it’s worth more to have an extra parking area.
Street parking can be tough. Sometimes finding a parking spot at night and sometimes other drivers hit cars along the side of the road and or sometimes cars get broken into. Vehicles are much safer in garages.
These decks are built just high enough and with just enough space between the structurally supporting deck post to fit an average sized vehicle. However, if you look closely you may notice that the three posts supporting the girder below the deck are not equally spaced. If they were equally spaced, then the middle deck post would end up being installed directly where the vehicle needs space to drive past on its way to the garage. Also, just to note, these driveways can also be used as parking spaces themselves. But if a post were to be installed in the middle of the driveway then it would neither work as a driveway nor a parking spot.
Instead of leaving the deck post directly in the middle of the driveway, the builder has staggered the center post and moved it over closer to the post at the outer side edge. When an intended center or intermediary post is moved over off center, it decreases the support capacity. However, in the case of these decks and many others that will be looked at today in today’s blog article, the center post still has an important functional purpose and the off-centering does not significant degrade from the structural support.
In the last sentence in the paragraph above, the word *significant* is important. To move a post off center, it does diminish the structural impact of the post. In this case, we have three posts. The girder that supports the deck above, runs parallel to the ledger. The end posts bear the near entirety of that weight. They may be close to their current capacity for the weight and span of the girder that they can carry.
With a bigger girder, both probably thicker in width and taller in height, two posts can support a longer girder length without having to increase the size of the post or the size of the footings which support the posts. However, in this case, the inverse scenario of increasing the girder to be able to run the intended full length without intermediary support would require a much taller and thicker girder and it would be counterproductive to install a very tall girder because it would push the new girder size down below the minimum head height clearance requirement. Vehicles have to be able to enter the garage and therefore need to have clearance up to a particular height.
You can see in the picture below, that there is a significant amount of space available before the girder is so low that it comes lower than the garage door header. So in this example, we have sufficient space, but in other examples, that available clearance might be limited. By making a girder taller, it could restrict the opening size for a vehicle to pass through into the garage. At the center point, the post can have a significant role in decreasing the load on the end post on both sides.
At the closest set of post, on the left side of the picture below, where these posts run below the deck girder, you can see that knee bracing has been provided between the rear side of each post and the rear face of the girder.
This bracing prevents shifting between these specific posts and the girder, and also helps reinforce the unsupported joint between the post on the outside or other side of the girder and the girder itself.
In this coming week’s next blog, we will continue looking at a very similar aspect of the same topic related to the configuration and proper layout of a deck to accommodate a vehicle parking and or driveway underneath to access a garage.
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 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
