Last week, we took a pretty close look at how deck footings should be set to a depth below the frost line so that they’re not destabilized by frost heave, a phenomenon where subsoils will lift upwards (basically expanding in all directions at the point of freezing) when freezing. Water, when it turns to ice, will expand in volumetric size with significant force and power, enough to lift up and destabilize not just decks and concrete footings but also buildings as well. For that reason, it’s a requirement of the building code to build both footings for decks and house footings or building footings to a depth below the frost line. In our area here in Washington DC, that depth is about 30 in below grade. We took a close look at that this past week and discussed a lot of the details around the frost line in our area. Today we’re going to talk about soil destabilization and the effect on footings.
In the picture below you can see a view of a multi-story deck that runs up the exterior facade of a multi-unit housing development. Multi-story decks are required to be built with a more robust strength in the load path to carry the weight of not just the one level, but also the additional levels above. Unlike many of the decks on the residential row homes in Washington DC, this multi-story deck does not have a roof above.
A secondary effect of not having a roof above is not just that the deck itself gets constant exposure to rain water, but the footings as well also have rain water exposure at all times of precipitation. This leads to concentrated areas of hydration at the subgrade footings.
Here, in this case specifically, the concentration of rainwater has led to wash out. If you look at the rear footing in the picture below, at the post against the exterior brick facade of the building, you can see that the soils have washed away at this location.
You can also see that an area under the edge of the footing is recessed below or inside of the footprint of the footing, which indicates that the footing is not set deep into the earth. It looks like it’s a very shallow footing. Shallow footings like this are a prime example of the wrong way to build a deck foundation and footing. The columns that support the load of the multi-story deck, in this example, need to be set on a deep subsurface, below the frost line. As waters continue to wash away the surrounding soils, and even the subsoils below this footing, it will lead to destabilization of the deck. The buildings and decks at this location are not new, but they’re not nearly as old as the majority of buildings in Washington DC. For that reason, it’s even more of a shame that it was built improperly because it will deteriorate and degrade on an accelerated schedule which will lead to the need for reconstruction, well below or within its intended lifespan.
In the next picture below you can see that there are two walking levels on this deck, meaning that it’s really built three levels high with a walking surface at the third level. Even though it doesn’t have an additional structure of a roof, it still has a significant overall load and is intended to not just support the dead load of the construction materials themselves, but also the live load of occupants or users of the deck and the building. The building code is ostensibly written and crafted to protect people from risks and danger. This is an example where as the deck degrades and deteriorates, it will be of risk to the occupants and users of the decks.
In the photo below, taken at a closer distance, you can see the area washed out below the widest part of the concrete footing. At the time of the original construction, a square frame was set around the top of the footing excavation. The soil now has washed out farther below that area.
A base plate separates the top of the concrete footing from the bottom of the wood post but also, and this detail, we see another defect because the base plate is undersized for the size of the post. That particular base plate is intended to be used for a 4×4 post, but in this case, the deck is supported by a 4×6 post.
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
