For the last seven years, professor Frank Woeste has been studying deck failures at Virginia Tech. He and his colleagues, Joe Loferski and Cheryl Anderson, point to two critical points of failure for decks and balconies — ledger board attachments and railing systems. Both these connections are rarely detailed correctly, the researchers claim, and their failure can cause a sudden total collapse of the deck. Their research, summarized in Manual for the Inspection of Residential Wood Decks and Balconies (available from the Forest Products Society, www.forestprod.org/shop) indicates that many cases of deck collapse occur at loads much smaller than what the design should have supported.
The solution boils down to two rules of thumb:
Never use nails as the sole attachment to the house. Instead, use lags or though-bolt connections for all structural connections joining deck to house, girder to column, post to deck, or rail to post.
Never notch railing posts. The National Design Specifications, on which the model building codes are based, prohibits notching in the middle third of a post and limits notch depth to 1/6 the depth of the member in the outer thirds and ¼ the depth at the very ends, explains Steve Bean of the Southern Pine Council (SPC). This amounts to a notch of just ½ inch in a 4x4 and 7/8 inch in a 6x6 — hardly worth the effort. And even notching within these prescribed limits weakens the post by as much as 40%.
Avoiding Rot Notching posts without applying a topical treatment is also a formula for decay, Bean cautions. Water will penetrate between the joist and the post and will just sit there. If posts are notched, the cut should be coated with a preservative, such as copper naphthenate, which is yet another reason for avoiding notches altogether. The SPC's notchless railing details are noted in the illustrations on this page.
Rot is particularly problematic at the ledger board. Even with the right number and size of lag bolts at the ledger, this connection is prone to premature failure if the house wall deteriorates. Although the ledger itself is usually treated lumber, Bean says, most house sheathings are untreated. So the recommended detail is to leave a ¼-inch gap between the ledger board and the wall sheathing. “There just is no satisfying way to detail this to make a drainable connection,” Bean says. “The best detail is to use longer lags and a stack of washers, but this detail requires a larger, longer lag. In truth, it's not often done.”
For ground-level decks, Bean recommends using a second support beam and posts instead of a ledger board. “In essence,” he says, “you are building a free-standing deck close to the house rather than hanging it from the house.”