The Case of the Sinking Wood Deck

Posted by in Foundations, Wood | March 14, 2016

It was cold and rainy afternoon.  And, while gazing at the rain sheeting down the window, I was ruminating on my plans for the coming weekend.  My quite ruminations were interrupted by the loud, sudden ringing of my desk phone.  It was a call from a long time client.

We exchanged the usual pleasantries and caught up on each others lives quickly.  But, I could tell that something was bothering him.  He asked if I could check into something that didn’t seem quite right, a sinking wood deck.  He said it was reported that the deck had sunk under weight of snow and ice accumulations and snow and ice that had fallen onto the deck from a nearby roof. He wasn’t convinced of that report.  So, he asked me to look into it for him.

I was intrigued by the prospect of performing engineering calculations for the weight of the snow and ice accumulations and compare them with the capacity of the wood deck.  So, I accepted the assignment and made an appointment to examine the subject deck.

When I arrived I interviewed the eyewitnesses about the history of the deck, when the sinking was first discovered, and observations about the snow and ice accumulations.  Then it was time to get dirty.

First, I examined the top side of the deck and measured the amount of the sinking : five inches (Figure 1).  “That’s a lot of movement,” I thought, as I looked for new separations evidenced by relatively clean surfaces of wood that had been recently exposed by the sinking.

Figure 1  With a string line, I measured a five inch depression in the floor of the wood deck.

Figure 1 With a string line, I measured a five inch depression in the floor of the wood deck.

Then, I crawled under the deck to see what was going on with the framing.  I could see that one of the posts supporting the deck and the concrete cap at the base of that post appeared to have dropped relative to the other posts (Figure 2).  I noted that the bases of the other posts appeared to be relatively uniform in elevation (Figure 2).  I documented my observations with written notes and photographs.

Post in center of photo (directly under the depression) has dropped

Figure 2 Post in center of photo (directly under the depression) has dropped relative to the other posts.

I later returned to my office and reviewed weather records.  I used the weather information in combination with the reported depths of snow and ice to estimate the weight of the snow and ice accumulations on the deck.  I also considered the effects of drifting and snow and ice falling from the nearby roof.

My conclusions were that the weight of the snow and ice accumulation was much less than the weight which the wood deck was required to safely support according to the local building code.  Thus, the partial sinking of the deck was not due to an overload, but was due to another cause.

Considering that the post and its concrete cap had sunk into the soil relative to the other posts, my focus turned toward investigating the foundation-soil interaction.  The worn edges and discolored fracture surfaces of the cracking in the concrete cap indicated that the cracking had occurred years ago.  Since the cracking was due to the sinking, the sinking had also initiated years ago.

I had come to an inescapable conclusion – the sinking of the deck had initiated years ago and was due to progressive earth movements, such as soil consolidation, settlement, etc.

The case was concluded and I returned to gazing out my window – but this time, to sunny day.