How to join beams

Updated February 21, 2017

Joining wood beams together is done to expand the length of a beam for a specific distance. You can do this by creating a scarf joint. A scarf joint is a type of splice used to extend wood beams while keeping them flush and even. To do this you need to cut a half lap at the end of the beams.

Place the beams that you need to join on a pair of saw horses. Measure the width of one beam and then multiply the measurement by eight.

Measure the length of the first beam to the measurement in Step 1 and make a mark with the pencil. The measurement for a 4-inch-by-4-inch beam is 32 inches. Repeat this step on the second 4-inch-by-4-inch beam.

Measure down 2 inches from the 32-inch mark and make a second mark on each beam. Cut down to the 2-inch mark on each beam with the circular saw.

Place one of the beams on a table saw and cut it in half to the 32-inch mark. This type of cut is known as a half lap. Repeat this step on the second beam.

Place the two half laps together to create the scarf joint. Make a pencil mark every 4 inches down the length of the scarf joint. Drill a hole through the beam on each mark with the power drill.

Insert a 5-inch carriage bolt into each hole. Place a washer and nut on the end of each bolt. Tighten the nut with the socket wrench.


Stagger the position of the bolts in the beam to ensure strength. The lap joint can also be secured by attaching metal plates to each side of the beam.

Things You'll Need

  • 4-inch-by-4-inch beams
  • Saw horses
  • Tape measure
  • Carriage bolts
  • Washers
  • Nuts
  • Socket wrench
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About the Author

Cameron Easey has over 15 years customer service experience, with eight of those years in the insurance industry. He has earned various designations from organizations like the Insurance Institute of America and LOMA. Easey earned his Bachelor of Arts degree in political science and history from Western Michigan University.