When you need a longer load-bearing beam, you can splice a new section onto the end of the beam. You may have to extend a beam because the original end rotted, was infested by termites or because a remodeling removed the original beam end. Extend a beam only if replacement is impractical, because a spliced load-bearing beam will be weaker than an intact beam.
Measure the amount of beam extension you need. The beam extension must be the same height and breadth as the main beam. Square off the ends of the main beam and the beam extension that will join to it. Butt the extension against the main beam.
Cut two side plates to fasten to the beam on each side. Each side plate should be equal to the height of the beam and ½ the width of the beam. The side plates should extend at least four times the width of the beam on each side of the joint. For example, extending a 4-by-8 inch beam will require two wooden side plates 8 inches tall, 2 inches thick and at least 32 inches long. The side plates can be longer for added strength.
Drill four bolt holes 2 inches apart through the side plates and beam on either side of the joint. Stagger the holes. Spread exterior-grade wood glue over the inner surfaces of both side plates. Set one plate on each side of the joint. Place a washer on each bolt and pass the bolts through the holes. The bolts should be long enough to extend out on the other side of the beam. Place a washer on the other end and thread on a nut. Tighten the nuts with a wrench until you can't tighten them any more.