Deck Railing Post-to-Deck Framing Connections
The North American Deck and Railing Association (NADRA) recently launched an effort to certify home inspectors in the inspection and evaluation of decks, and last month I became the third inspector in Washington State to achieve that certification. I applaud NADRA’s efforts as decks and railings are often homeowner-built, rarely built to contemporary safety standards, and can be downright dangerous.
The Consumer Product Safety Commission conducted a study from 2003-2007 and found that over 33,000 people were sufficiently injured to warrant an emergency room visit from the structural failure of decks, porches, railings and stairs… and the number from 2007 was up over 30% from 2003! It continues to rise, and for comparison, in 2014 wind and flood events sent 924 people to the emergency room while decks, porches, railings and stairs sent 8,560 people to the emergency room.
There are many areas of potential failure of deck components; in this blog I’ll take on the connection between railing posts and the deck framing. I’ll focus on other areas in blogs-to-come.
All decks 30” or more above the ground, measured as much as 36” away from the deck, require a railing. In residential construction, the railing should be a minimum of 36” tall. The IRC (International Residential Code) requires that that 36” tall railing be able to resist a load of 200 lbs. in any direction. What does that mean? It means that when Uncle Billy has too much to drink at the cookout and trips and falls against the railing, it should not fail and send him to ground!
The real challenge becomes one of physics. When Uncle Billy hits that top rail, he exerts a 200 lb. force (not counting momentum). However, at that moment the rail post begins to act as a lever, and the 200 lb. force applied at the top becomes a 1700 lb. force at the point at which it is connected to the deck framing.
Structure Magazine, for its July 2007 edition, ran a series of tests on this post-to-deck connection. They added a safety factor and tested for a 500 lb. force. Commonly seen ½” lag screws failed by withdrawing at 178 lbs. Also commonly seen ½” bolts with washers failed at 237 lbs., meeting the code but with no safety factor. The bolts typically failed by pealing away the band joist from the deck joists.
The only attachment that worked was one in which the load was transferred to bolts laterally… a shear force. Think of the force pulling sideways on a bolt rather than along its length. The correct installation has two bolts going through the post and band joist. The top bolt also goes into a metal bracket, which is then itself bolted to the adjacent joint, placing those bolts at a 90 degree angle to the force. It’s a bit hard to describe in words, but here is a link that will take you to a Simpson Strong-Tie document that should make it clearer. https://www.strongtie.com/deckties_decks/dtt_tie/p/dtt
If you are attaching a post to a deck to act as railing support, do you really want to save a few dollars on hardware and not use the only approach proven so far to keep Uncle Billy upright?
For general information about deck construction, I recommend you go to https://www.strongtie.com/products/deckcenter and download the Deck Connection and Fastening Guide.
To quote from the old television show Hill Street Blues… Let’s be careful out there!