A screened residential deck at a house on Pawley’s Island, S.C., collapsed during Memorial Day weekend, causing wedding rehearsal dinner guests to plummet 15 feet and injuring as many as 30 partygoers. According to local newspaper reports, two people with serious injuries were airlifted to hospitals in Charleston and Columbia.
Unfortunately, collapses like this have become a hallmark of the summer season for both older and newer homes, when fine weather beckons and large numbers of people gather on the decks. Homeowners and beach-home renters understandably treat decks as just another (outdoor) room of the house. And, by code, decks do have the same capacity requirement as indoor room.
According to experts, though, deck failures are the result of under-strength deck construction details, a problem compounded by seasonal deterioration caused by weathering and water intrusion.
Despite code requirements, typical support details for outdoor decks are far weaker in practice than the structure holding up the floor in the living or dining room. Instead of positive bearing of floor joists on a wall plate, most deck floor systems rely on a ledger board attachment to a house wall—which asks fasteners loaded in shear to carry gravity loads far beyond any load they are likely to carry in any other part of the home. The situation only gets worse as untrained and unsupervised carpenters often install the ledger boards with no idea of the limited capacity of the nails or screws that they are using.
In addition, ledger board connections may not be effectively flashed to prevent rainwater from entering the assembly and creating conditions that encourage rotting of wood and rusting of fasteners. So the deck-to-house connections weaken gradually over time, meaning that a deck that has withstood large parties in years past may suddenly fail under a smaller load this year.
While older decks are more vulnerable, new construction is by no means immune to the problem. Each year, more decks are installed with ledger-board attachment details that are inadequate to support the weight of the numbers of people that could be reasonably anticipated to gather on the deck.
Until recently, building codes have made little mention of residential deck structural attachment. Even the more recent International Codes only call for a “positive attachment” of the deck to the building, but do not provide specific prescriptive details.
Still, there are some useful deck construction resources available for builders and contractors.
The American Wood Council has posted a comprehensive construction manual for outdoor decks (PDF), which meets or exceeds the latest International Residential Code. The most critical details, researchers say, are the closely-spaced lag screws or lag bolts with nuts or washers between the deck ledger board and the house frame’s band joist, and the associated flashing details that protect this connection from water.
For a closer look at deck construction details, read this article by Virginia Tech engineering professor Frank Woeste at Hanley-Wood’s Coastal Contractor magazine.
If you’re concerned about the condition of an existing deck, look for Virginia Tech’s “Manual for the Inspection of Residential Wood Decks and Balconies,” produced in cooperation with the International Code Council.
Ted Cushman is a contributing editor to BUILDER magazine.
Learn more about markets featured in this article: Anderson, SC.