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Unstable Deck

Unstable Deck Causes Discussed:


Anonymous said… Hi, I found your notes through, and I thought you might help me with a problem. We’ve built a 12 ft. by 14 ft. deck to hold a 500 lb. playhouse. We sunk posts at each corner and in the center, but it still wobbles. We basically have an unstable deck. Can we still add extra posts without sinking them into the ground?  Thanks! “Confused”


Dear confused, Thank you for the comment and question. Several issues may be occurring to create an unstable deck:

  1. Inadequate sized or supported floor joists
  2. Inadequately sized or braced support post or spaced too far apart
  3. Inadequate footing
  4. Deck is over 2′ above the ground Without knowing the answers to these questions it’s difficult to make diagnosis.
  5. Read my article Deck Safety – Repair vs Replacement

Here’s a few questions for you:

  1. How high off the ground is the deck?
  2. How thick are the posts
  3. Are the sunk posts sitting on concrete footings? If yes, how big and deep are the footings? [note: 2006 IRC, R403.1.4.1 exception 3 states, ‘Decks not supported by a dwelling need not be provided with footings that extend below the frost line.’]
  4. What size floor joists did you use?
  5. How far is the distance between posts? you mentioned that you installed them in the center – did you mean centered between corner posts or centered under the deck?
  6. Is there any diagonal bracing attached to the posts / beam? Assuming that you have built the deck properly and have it supported on proper footings I would then look at diagonal bracing before adding extra posts.

Note – Freestanding decks greater than 2 feet above grade must be able to resist lateral and horizontal movement by providing diagonal bracing. For a free standing deck, that means you need to install the bracing on all sides. Try installing 2 x 6 pressure treated, diagonal cross braces and secure them with 1/2′ x 4-1/2″ galvanized lag screws. See photo below.


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