I’m new to this forum, but have been an avid reader of Fine Woodworking and Fine Homebuilding for many years. I’m an electrical/computer engineer, but have always done all of my own work on the house and for others. Even helped build a couple of houses a number of years back.
Anyway, about 4 years ago, I replaced a failing deck on the back of the house, with a slightly larger one, used PT structural elements and Trex for the decking material. I did my own calculations on load and ended up with 7 supports (4×4) for the 56′ long deck. Each post is about 8′ long and is terminated with a adjustable post base (large threaded rod at the base of the footing). These are set into holes drilled into the top of 3′ deep 18″ sonotubes filled with concrete. The location of the tubes was a function of design restrictions (beam span), positioning around fir tree root systems, and a septic tank.
When I drilled the holes (using a bobcat with an auger) I wasn’t as precise as I needed to be. Two of the holes were about 3″ out of line with the load from the deck. I drilled the anchor holes in the proper line, being out of the center line of the footing.
Fast forward 4 years. Looking at the deck posts for another reason, I realized that the base of two of the posts had shifted by about 2-3″. So that the posts are no longer plumb. It appears that the footings are tilting away from the house, taking the base of the post with it. These two posts are the ones without an on center post base. Either this is an amazing coincidence, or the offset load has caused them to twist.
What I can’t figure out is:
- Why did they move? I know that the posts are off center, but I drilled a tight hole (had to drive the sonotubes into the hole). The frost line in my area is 18″, but ground hasn’t frozen since the deck was built. The whole area is sloped (in the direction of the tilt), and drains well. The base of footings were on clean, undisturbed soil. The drain lines for the septic system, are about 6′ away from the nearest footing.
- What can I do about this? The deck has a plate glass deck rail and will most likely not be fond of having the deck surface drop in some areas.
Any thoughts? Other than I’m glad that is not my problem!
Thanks in advance.