How to join beams

Written by cameron easey
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How to join beams
Wood beams can be connected with a scarf joint. (beam image by jbattx from

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.

Skill level:

Things you need

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

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  1. 1

    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.

  2. 2

    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.

  3. 3

    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.

  4. 4

    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.

  5. 5

    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.

  6. 6

    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.

Tips and warnings

  • 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.

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