How to Attach 4x4 Posts to a Deck

Updated February 21, 2017

When building a deck, security and safety are key for any installer or homeowner. Therefore, many installers, builders or buyers choose to add a handrail to the deck to prevent falls or other safety issues. In many cases, these railings are supported by 4-by-4 posts, holding the handrail firmly in place and securing it to the deck to support weight. Install the posts into the deck joists before completing the main floor of the deck so that they attach securely to the overall support of the deck system.

Line up your first 4-by-4 post with one inside corner of the deck. Align the bottom of the post with the bottom of the deck joist so that they are even. Clamp the post in place with wood clamps, securing it to both sides of the corner joist.

Use a straightedge or level to check that the post is completely plumb against the joist and straight all the way up and down. If you find an area that is not plumb, slide thin plastic shims between the post and the joist on either side to create a plumb surface.

Drill 5/16 inch by 4 inch lag screws through the joist and into the post. Install two screws through each side of the corner, resulting in four screws holding the corner piece in place. Remove the clamps and check to make sure the post is secure.

Attach the remaining corner posts to the deck using the same method. From there, measure the distance between corner posts to determine how many other posts you need to install. Remember that the posts must be joined to the edge of the deck at joists; you may need to adjust distance between posts to make sure you can attach them to joists.

Attach the interspaced posts according to your measurements. For the remaining posts, you should only need two 4-inch screws, drilled through the joist or frame of the deck. Check that all posts are secure and do not shift when you apply weight before continuing with deck installation.


If possible, use treated wood for the posts. Treated wood will withstand weathering longer. You can also cover the posts with sleeves to change the style and protect the wood. Depending on the joist and frame system of your deck, you may need to trim or notch some of your posts. Use a jigsaw or circular saw for this work. Remember to always wear safety goggles when cutting wood.

Things You'll Need

  • Posts
  • Clamps
  • Level
  • Plastic shims
  • Drill
  • 5/16 inch by 4 inch lag screws
  • Measuring tape
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

Samantha Volz has been involved in journalistic and informative writing for over eight years. She holds a bachelor's degree in English literature from Lycoming College, Williamsport, Pennsylvania, with a minor in European history. In college she was editor-in-chief of the student newspaper and completed a professional internship with the "Williamsport Sun-Gazette," serving as a full-time reporter. She resides in Horsham, Pennsylvania.