Water Absorption, Lateral Loads and Deck Collapse Risk

When deck collapse tragedies happen there is not always a clear-cut reason. It is mostly speculation of what caused the collapse, but here we look at two significant contributors and how you can prevent the chances of it happening to your deck.

Earlier this June there was a rather dramatic deck collapse in Pawleys Island, SC when a group of 25 – 30 people simultaneously gathered on a deck to take a photo in front of a rather impressive double rainbow. Although a rainbow is an innocent and inspiring motivation for a group selfie, this rainbow lured the unsuspecting group to a tragedy on an otherwise enjoyable day. The deck suddenly buckled like a trap door dropping over a dozen people 12 – 15 feet and sending them to hospital with a variety of cuts, contusions, concussions and broken bones after a massive rescue and recovery involving 10 ambulances, 6 fire trucks and many police officers.

The deck was at the ocean side Seaview Inn and had reportedly shown no signs of neglect or disrepair. A statement issued by the Inn on their Facebook page as reported by WMBF News states: “The best indication that we have is that there were over 30 people on the deck for this photo. This deck was not in disrepair or in a compromised state. It was waterlogged from the recent deluge of rain and simply could not hold the weight.”

The waterproofing professionals of Duradek know there are many reasons to keep a deck waterproofed, and this is an extreme example of the dangers that an unprotected deck is vulnerable to. While the wood on this deck may have appeared to have been stable, if it had been properly waterproofed it would not have absorbed the water and thus the extra weight to support.

Additionally, once water has invaded wood it is much more vulnerable to rot and can serve as a perfect breeding ground for molds and fungi witch will speed up the degradation of the wood exponentially. This results in a weaker deck and since this happens gradually, it can often go unnoticed unless you are maintaining or inspecting your deck regularly.

So IF weight was the real issue on this deck collapse, surely a little extra water in the wood would not be enough to take it down as suddenly and dramatically as it did. The size of the deck would indicate that it should be able to support the weight of 30 people, but a study conducted at Washington State University (WSU) may indicate that lateral load factors could have contributed to this tragedy as well.

Not a lot is known about the effects of lateral loads on decks, so WSU explored the topic and took a closer look at lateral loads imposed by three different sources on a 12 foot by 12 foot deck:

– by wind,

– by seismic forces,

– by occupancy.

As reported by Andrew Wormer at Profession Deck Builder magazine in the article Measuring Lateral Loads on Decks, the results of their experiment showed that neither wind nor seismic loads pose much of a design challenge except in hurricane and special wind regions. However, a lateral load due to occupant movement is another matter.

Building codes don’t address this except with grandstands, bleacher and stadium seating so the WSU team tested two types of occupant loads in their experiment on full sized decks:

– Cyclic side sway

– Impulse (run and jump stop)

Their findings showed that lateral loading form occupants can exceed worse-case design loads from either wind or seismic forces. This study also revealed the importance of screwed vs. nailed joist-hanger connections.

A key point from this experiment is that extreme occupant loading can occur anywhere on any deck; however we tend to see stories of deck collapse tragedies occurring most frequently in coastal areas where they are exposed to greater amounts of moisture so this would tend to indicate that inadequate moisture protection plays a significant role as well.

While traditional decks with slatted boards require regular maintenance to stay protected from moisture intrusion, decks protected with Duradek vinyl membrane are completely waterproofed and require no maintenance other than occasional cleaning. Duradek vinyl membranes are an approved pedestrian coating AND roofing membrane so you can be confident your deck is protected to the highest standards. After 40 years specializing in deck waterproofing, Duradek is the most reliable, low maintenance method of protecting your home and your family.

To find a Duradek installer near you, check out our Duradek Dealer locator page and request a free estimate for your deck renovation.