July 15th, 2009
More than 2 million decks are built and replaced each year in North America. InterNACHI estimates that of the 45 million existing decks, only 40% are completely safe.
Because decks appear to be simple to build, many people do not realize that decks are, in fact, structures that need to be designed to adequately resist certain stresses. Like any other house or building, a deck must be designed to support the weight of people, snow loads, and objects. A deck must be able to resist lateral and uplift loads that can act on the deck as a result of wind or seismic activity. Deck stairs must be safe and handrails graspable. And, finally, deck rails should be safe for children by having proper infill spacing.
A deck failure is any failure of a deck that could lead to injury, including rail failure, or total deck collapse. There is no international system that tracks deck failures, and each is treated as an isolated event, rather than a systemic problem. Very few municipalities perform investigations into the cause of the failure, and the media are generally more concerned with injuries rather than on the causes of collapses. Rail failure occurs much more frequently than total deck collapses; however, because rail failures are less dramatic than total collapses and normally don’t result in death, injuries from rail failures are rarely reported. Here are some interesting facts about deck failure:
- More decks collapse in the summer than in the rest of the year combined.
- Almost every deck collapse occurred while the decks were occupied or under a heavy snow load.
- There is no correlation between deck failure and whether the deck was built with or without a building permit.
- There is no correlation between deck failure and whether the deck was built by a homeowner or a professional contractor.
- There is a slight correlation between deck failure and the age of the deck.
- About 90% of deck collapses occurred as a result of the separation of the house and the deck ledger board, allowing the deck to swing away from the house. It is very rare for deck floor joists to break mid-span.
- Many more injuries are the result of rail failure, rather than complete deck collapse.
- Deck stairs are notorious for lacking graspable handrails.
- Many do-it-yourself homeowners, and even contractors, don’t believe that rail infill spacing codes apply to decks.
A proper deck inspection relies heavily on the professional judgments of the inspector. This article will help improve the accuracy of those judgments.
Many thanks to InterNACHI staffer Lisa Vega for creating the awesome deck graphics.