According to InterNACHI, over 2 million decks are built and/or replaced each year in the United States, 40% of which they estimate are actually safe.*
Decks are structures that need to be designed to resist stresses and they should not add weight load bearing to the house itself. Simply because they look easy to build, many folks construct them without a proper design or permits, which can cause big problems later on. Decks have to hold the weight of snow/ice, people, furniture, and can get considerably heavier during heavy rainy seasons, depending on the material used. They also need to be constructed in a way that prevents small animals (cats, dogs) and children from falling, this includes having proper infill spacing and graspable handrails.
The definition of a 'deck failure' is anytime the use of a deck leads to injury, including total deck collapse of course, but also includes the more common (but less dramatic) rail failure. Very few cities/townships perform investigations into the cause of the failure, and media outlets are generally more concerned with injuries rather than on the causes of collapses.
Here are a Few Interesting Facts About
Deck Failures courtesy of InterNACHI:
More decks collapse in the summer than during 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.