Your Deck Needs a Solid Foundation Your Deck Needs a Solid Foundation

Even though you may not see them, the posts (or footings ) holding your deck form it's solid foundation and are vital for a strong, safe structure. A solid foundation means having the correct number of posts as well as ensuring they're properly located and solidly fixed in the ground and can't shift over the years. Here's some thoughts on creating a solid foundation for your deck.

How Many Posts do I need?

  • Unfortunately there isn't a simple answer to this seemingly simple question. In general, you need posts at the corners of your deck and about 8" apart under the structure.
  • However, different deck designs require different amounts of support. For example, a deck with a hot tub will need a lot more support than a deck built low to the ground, or an odd shaped deck with multiple angles will need more support than a square or rectangular layout.
  • The final decision of how many posts your deck needs is your local building department, who will ensure your deck has the proper number of posts when you get your building permit .


  • Once your design has been approved, you know how many posts and how far apart they must be. Now, using string and wooden stakes you can determine and mark their exact locations.
  • Starting at a corner close to your house drive a stake with a piece of string attached into the ground then stretch it the width of your deck, and drive a second stake in at the far side (keep the string parallel to your house). Mark the proper spacing of the posts onto the string then repeat, the process running a series of strings parallel to your home until you reach the far end of your deck.
  • Finish laying out our posts and create a matrix by running strings spaced properly apart but now running perpendicular to your home and over the first strings . Your deck posts should be located below where the strings cross each other.

Post Holes

  • Post holes need to be dug deep enough to ensure the bottom of the hole is below the frost line so as to prevent any movement. In cold climates the frost line is about 4' down but you can find out the frost line in your area by check with your local building department.
  • Ensure the diameter of your post holes is 4" wider than the width of the post. (i.e.a 4X4 post will require a 12" hole). Also, slope the sides of the holes so the bottom is wider than the top (to prevent the post from ever riding up).
  • Stand the post straight up in the hole, then pour in concrete while using a scrap piece of lumber to get rid of any air bubbles.
  • Use a level to ensure the post is straight (plumb) and adjust it if necessary, then allow the concrete to set up for a day or two.
  • Overfilling the hole with concrete and rounding off the top will drain rain water away from the post.
Murray Anderson is an experienced freelance writer whose work has appeared on numerous web sites, as well as in newspapers and books in both the US and Canada. He is often cited as an expert on home related topics.

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