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How to Join Wooden Beams

A scarf joint can join wooden beams.
beam image by jbattx from Fotolia.com

Scarfing is a method of joining wooden beams where opposing ends are overlapped and fastened together. There are many methods of scarfing wooden beams. Which style you choose depends on the amount and direction of the load. The half-lap joint will be explained here and is useful for general purposes with fairly heavy timbers, such as four-by-fours. In vertical posts it resists compression stress.

Lay beams out on saw horses to facilitate accurate measuring and cutting.

Measure and draw the scarf position on the end of each beam. The scarf length is eight times wider than the beam width. In the case of four-by-fours, that is 32 inches in length.

Use a square to draw a line that is 2 inches deep and 32 inches long on the ends of the beam. This indicates the depth cut which will form the two overlapping sections of the scarf joint. Plan accurately to create a perfect bond between the two beams.

Saw down 2 inches on each beam at the 32-inch mark.

Cut each beam in half with a power saw. The cut will be 2 inches deep and 32 inches long. This is known as a half lap.

Lay the two beam cuts on top of each other to create the scarf joint.

Secure the joint with eight 5-inch bolts placed at 4-inch intervals. Stagger the bolt positions to ensure the strength of the four-by-fours.

Things You'll Need:

  • Saw horses
  • Measuring tape
  • Pencil
  • Square
  • Saw
  • Power saw
  • Drill
  • 5-inch bolts


A fish plate may be added before the nuts and bolts to add extra strength to the joint.


  • Consult an engineer to find accurate calculations of axial force computations for any load-bearing wooden beam project.
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