Anatomy of an Inherently Dangerous Deck, Part I & II

Anatomy of an Inherently Dangerous Deck, Part II

By: Brian Foley

Whenever I go out of town, I notice the differences in deck construction, particularly in other states that don’t have a robust or uniform building code like Virginia.  One of the most common mistakes I can easily see from my informal windshield survey (or handlebar survey when I’m riding my bike) are notched guard posts.  Statistics from deck collapses nationwide verify that one of the weakest points on a deck is the connection of the guard post to the deck framing (the other is the ledger board connection; see the July 5, 2017 post below).

Per the building code, in a worst-case condition, a guard post must resist a horizontal outward force of 200 pounds at its top.  For a 36-inch tall guard, that translates to a rotational torque (moment) at the connection to the deck of 600 foot-pounds.  The most important part of a post at that connection is its thickness – the exact location of the notch!

Most guard posts are comprised of wood 4x4s which have actual dimensions of 3½ inches by 3½ inches.  The average notch I’ve seen is roughly half of the guard thickness which equates to a 1¾ inches.  However, I’ve seen some truly butchered posts that were notched in both directions into a 1-inch by 1-inch section.  Considering a post with half its thickness notched, its ability to resist the rotation is not reduced proportionally.  Due to the way a section is analyzed, the actual resistance is reduced by 75 percent.  That reduction increases exponentially for every additional loss of section.

If you’re building a deck, do NOT notch the guard post, and follow all the connection requirements from the Fairfax County Typical Deck Details to ensure your guard system is constructed safely and can resist the required loads.

Learn more about deck safety in Fairfax County

 

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Anatomy of an Inherently Dangerous Deck, Part I

By: Frank Woeste

Deck collapses caused by failure of a nailed deck-ledger-connection to the structure or house have been followed since 2002, when researchers at Virginia Tech began to investigate and test critical structural elements of a residential/multi-family deck. After several years of tracking deck collapses, it became apparent that the primary cause of deck collapses was the connection of the deck ledger to the house made with nails only. In fact, I believe 90% of all deck collapses result from the failure of the deck ledger-to house connection, and of the 90% most are caused by nailed-only ledgers.

The focus of this article is existing decks with a “nailed-only” ledger connection. As will be demonstrated, a homeowner, home inspector, property manager, or other professional can quickly determine if the deck is dangerous—nails only in a deck ledger constitutes a “dangerous deck.”

Background For decades, the residential codes have specified an occupant deck load of 40 pounds-per-square foot (psf), which translates into about one average-size person locating a space of 2-ft. by 2-ft. In addition, every edition of the International Residential Code (IRC) since 2000 has required that decks be positively anchored to the structure to resist both vertical and lateral loads. Assuming a deck that is 14-ft by 28-ft, the code design load would anticipate the deck could be safely occupied by 98 people. However, until 2007, the IRC did not prescriptively specify how contractors should fasten or connect the deck ledger to the house.

 

 

 

 

 

 

 

 

Frank Woeste, P.E., is Professor
Emeritus, Virginia Tech University
and a wood construction consultant.
He can be contacted by e-mail: fwoeste@vt.edu

1 thought on “Anatomy of an Inherently Dangerous Deck, Part I & II”

  1. I greatly appreciate NADRA sharing this article with Members and the Public. The photo of a “nailed-only” ledger seemed to have helped laymen grasp what is meant by a deck ledger being connected to a house or apartment building with only nails. The complete article can be accessed at this site: https://goo.gl/r3sqQi

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