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Responding to Deck Failures

Dec 2006 DeckWorld Magazine

BY JANET ARDEN

A deck failure can be as simple as a board that gives way or a sag that develops at one corner. It can also be as dramatic as the whole structure pulling off the second story of a house.

You've seen the headlines and heard the news reports:

A deck on a house in the 800 block of South Elm collapsed in the midst of a 30th anniversary party Sunday afternoon, sending five guests to the hospital with various injuries, none serious.

The good news, of course, is that no one was seriously hurt. Unfortunately, that's not always the case. Of the deck collapses that were reported in 2006, 94% resulted in personal injuries. The actual number of deck failures is very difficult to know. Many deck failures go unreported because emergency crews were not summoned or the deck was not occupied at the time it went down. Additionally, decks that were clearly unsafe and were repaired or replaced before they collapsed are not recorded either.

DeckWorld talked at length with Michael Morse, founder and president of DeckLok. Morse has been studying deck construction, building codes, and ways of developing a failure‑proof system for almost 25 years.

Morse points to studies that show over 90% of deck failures are linked to a failure in the connection between the house foundation and the deck. Metal fasteners (whether they are bolts or nails) embedded in wood depend on the resistance of the wood fibers to hold the screw or nail in place, attaching the deck ledger to the house. But this assumes that both the fasteners and the surface they are gripping have the appropriate construction integrity.

For starters, these same metal fasteners offer a pathway for water and then rot in the wood they are supposed to be holding. Seasons of freezing, thawing, and heat cause wood to dry, split, or even rot. Screws may loosen or pop out. Nails provide a much weaker connection to begin with and are more susceptible to popping. As Morse points out, it only takes a slight shift in the surface to begin a lateral pull on nails or screws. For example he points to how easy it is to remove a nail from a piece of wood, once it has started to withdraw. The lesson here, he says, is that once fasteners start to pull out, it does not take much for them to pop the rest of the way.

What about the building code?

According to the International Code Council (ICC), for the most part, "there are no prescriptives for decks in the International Residential Code. They would be treated as floor construction and would follow Chapter 5 of the IRC especially the floor joist spans table."

Specifically, the 2006 International Residential Code, R502.2.2 Decks, stipulates: "Where supported by attachment to an exterior wall, decks shall be positively anchored to the primary structure and de­signed for both vertical and lateral loads as applicable." It goes on to describe attachment methods and structural supports.

At its recent 2006 hearings in Orlando, the International Code Council Code Committee adopted a prescriptive method for attaching the deck ledger to the house, but it does not specify which part of the house this is. It even specifies the number and placement of bolts.

Morse points out, however, that most decks are inspected the day they are complete - when the materials are new and they have not been subject to weather or weight. The damage comes when this same structure has been left in the elements - wet, cold, snow, heat‑for a decade or two.

By Morse's definition, a successful deck is one that remains structurally sound throughout the service life of its wood. Engineering for decks, he says, needs to account for the fact that wood changes over time.

Morse believes there are three key facts to keep in mind in preventing deck collapse:

1. Follow the load path to the footings. According to the International Code Council's Residential Building Code, the "design must be capable of transfer­ring all loads from their point of origin to the foundation."
2. Do not assume you have created a structural link if you have not seen the structure. Often‑through no fault of the homeowner or builder‑the deck is not tied to a structural member of the house. The band joist must be specifi­cally attached to resist being pulled out by the deck. The deck ledger may be fastened with the correct number of ap­propriately sized lag bolts, but they may only be holding on to siding or an un­supported band joist. There must be a structural link to the house foundation.
3. New construction methods that use materials such as manufactured "I" joists do not necessarily tie structurally into the band joist. In this case, a deck can collapse and take the band joist with it.

What about vinyl and composite materials? Vinyl and composite decking materials attach to the same deck underpinnings as wood decking does, but with some differences. The hidden fastener systems these materials use hold decking in one direction, making the stability of the substructure that much more important. Morse says any structure needs to accommodate the characteristics of the new product.

There are no bad guys here. Morse is quick to praise the craftsmanship of deck builders. In fact, he says, most of them do a beautiful job - for 95% of the deck. He believes the last 5% - fastening the deck to the house - is where there's room for improvement.

© DeckWorld Magazine

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