By Paul Bianchina
Second-floor decks can be a great enhancement to any two-story home. A well-designed upper floor deck becomes an architectural focal point that offers increased outdoor living space, enhanced views and an opportunity to either tie upstairs and downstairs together or to create a private outdoor area accessible only to upstairs rooms.
Upper-floor decks also offer some engineering and construction challenges that differ somewhat from low, first-floor structures. While many of the construction techniques are similar, there are definitely some things to keep in mind with a second-floor deck.
Bracing
There are three primary forces at work on a deck - download, upload, and lateral load. Download, which is weight that is pushing straight down on the deck's supports, is pretty much the same for a second floor deck as it is for one on the first floor. Download weight consists of dead load - the weight of the deck structure itself, plus any permanent loads, such as a spa or a built-in barbecue - and live load - the weight of the people on the deck and any movable furnishings, plus any anticipated snow buildup during the winter.
In the design of a deck on any level, the decking, joists, girders and other supports need to be sized for the spans they'll be covering and the loads they'll be supporting. While download is typically very similar between a one-story and a two-story deck, the higher deck may require a larger foundation, additional bracing or other reinforcement.
Uploading, also called wind load, is the force placed on a structure by the action of the wind. With a large, flat structure such as a deck, wind can move below the deck and push upward with tremendous force. The higher the deck is, the more the wind can build up under it and push against it, so concern over the action of upload forces definitely increases as you place the deck further above the ground.
Uploading is more of a factor in some areas than it is in others, depending on local climatic conditions. In some regions, particularly along parts of the coast, uploading from wind forces can be a major consideration, and the deck needs to be designed with proper anchors and connecting hardware to hold everything together and in the right place.
The third force to consider is lateral load, which is movement from side to side caused by wind, seismic activity, or just the movement of the people using the structure. It's pretty easy to visualize that the taller something is, the more readily you can rock it back and forth. A deck sitting 12 inches off the ground will barely move from side to side, while one that's 12 feet above ground can sway pretty easily if not properly braced.
Adequately bracing an upper-floor deck is a major consideration, and the exact methods and materials depend on a number of factors. For example, a deck that is attached to a house on two sides is more stable than one that's only attached on one side, and a large deck with lots of occupants will want to move more than a small deck with less live load on it.
Another example is the weight of a spa. A completely filled and occupied spa can easily weigh in the thousands of pounds, and if forces can get all that weight moving from side to side, the effect on the structure can be devastating.
Railings
Another consideration with second-floor decks is railings. In most jurisdictions, any deck that is less than 30 inches off the ground does not require a railing. With a second-floor deck, railings are not only required, they need to be carefully designed for adequate support and stability.
Again, if you have a spa on that upper deck, the railings have to be high enough to protect the spa's occupants as well. This could mean railings that are four-feet-high or even higher, which presents an interesting challenge both structurally and aesthetically.
With any deck structure - and especially with one on an upper floor - proper design is absolutely essential. For starters, check with your local building department for specific load information in your area; then talk with your lumber supplier for more details on design. In many instances, it may even be necessary to obtain the services of an architect or a structural engineer to design a deck that safely meets the requirements of your particular application.
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