The CBS Early Show did a recent story on the subject called "Deck Dangers" that is a must see.
Many of the homes I inspect routinely have decks with serious defects. Sometimes these are the original decks installed at the time of construction by "professionals", and approved by the building department, while others were installed after initial construction by homeowners, or someone they hired.
It is estimated that approximately 50 percent of all decks in the United States have serious deficiencies which could lead to deck collapse. The critical connection point is a framing member called the "ledger board". The way this board is attached to the structure makes a huge difference in how safe the deck structure is.
In many cases the ledger boards are simply attached to the structure with nails. While nails will perform adequately holding the shear or vertical load, they do not provide adequate resistance against lateral movement along the horizontal plane of the deck system.
This is where lag bolts or lag screws come into the picture. Having adequately sized lag bolts/ screws, properly placed, in a sufficient number makes a positive connection to the structure of the home, reducing the possibility of a deck collapse.
A good source of information regarding deck construction has been produced by the American Forest and Paper Association in conjunction with the American Wood Council, and the International Code Council. The document addresses deck construction as based on the minimum requirements of the 2006 International Residential Code, as well as detailing the "best practices".
This document can be found at Prescriptive Residential Wood Deck Construction Guide;
At a recent home inspection I found a deck ledger board that not only had no lag bolts or lag screws, but had very few nails. The photo below shows the ledger board sagging at the center. This deck is in imminent danger of collapse.
Harold Miller, Licensed Home Inspector, Miller Home Inspection. Providing home inspections in the Everett, WA area. 425-501-2382