How to build trailer side rails

Updated February 21, 2017

If you want your utility trailer to carry more than the existing sides on the trailer allow, you can build a set of side rails to increase the trailer's hauling capacity. Before starting the project, remember that trailers are designed with specific weight limits. Be certain to consider this when designing your side rails. If you choose lumber that is straight to prevent difficulties during the installation process, you can confidently complete this project with just a few electric tools.

Cut the rails and posts. Measure the distance from the front of the trailer to the back. Cut six pieces to this length from the 2-inch-by-6-inch lumber with an electric mitre saw. Cut 48-inch posts from the 2-inch-by-6-inch lumber as well. Cut one post for the front of the trailer and one back of the trailer, and at least two vertical pieces between the front and back.

Mark pilot holes in the rails. Lay the rails flat on a worktable. Measure from one end of the rails and make marks for each of the posts based on the measurements of the trailer. Put the framing square on the marks and draw lines across the rails. Make marks on the lines that are 1 1/2-inches from both of the long sides of the rails. Drill 3/8-inch pilot holes on the point where the marks and the lines cross.

Measure and mark the posts for the bottom rail. Measure from one end of the posts and make a mark equal to the distance from the trailer floor to the top of the existing sides. Make another mark that is 5 1/2-inches from that mark.

Secure the bottom rail to the posts. Separate the posts according to the lines on the rails and set the rails on the posts. Be certain the lines on the rails are in the centre of the width of the posts. Set the first rail on the lines drawn on the posts and secure the posts to it with the lag screws.

Affix the remaining rails to the posts. Set another rail on the posts so one of the long sides is even with the top of the posts. Secure it in the same manner as the bottom rail. Secure the third rail to the post midway between the top and bottom rails.

Drill the holes for the square U-bolts. Measure from the bottom of the posts and make one mark at 2 inches and another mark 3 inches below the bottom rail. Put the framing square on the marks and draw lines across the posts. Make marks on the lines that are 1 1/4 and 4 1/4-inches from one long side. Drill 3/8-inch holes on the points where the marks and lines cross.

Install the side rails. Get someone to help you hold the side rails on the trailer. Insert the square U-bolts around the posts on the trailer and through the holes drilled in the posts. Slip the washer over the threads followed by the nuts and tighten them down with a wrench.


Use neoprene nuts on the square U-bolts for permanent installation. Seal the side rails with water sealer to protect it from the elements.


Do not leave power tools within the reach of small children. Do not spray water sealer on the rails on a windy day.

Things You'll Need

  • 2-inch-by-6-inch lumber
  • Electric mitre saw
  • Framing square
  • Variable speed drill
  • 3/8-inch drill bit
  • 3/8-inch-by-2 1/2-inch lag screws
  • 9/16-inch socket
  • Ratchet
  • 5-inch square U-bolts with washers and nuts
  • Wrench
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About the Author

Michael Straessle has written professionally about the construction industry since 1988. He authored “What a Strange Little Man,” among other books, and his work has appeared in various online publications. Straessle earned a Bachelor of Arts from the University of Arkansas at Little Rock in professional/technical writing.