Locate the rotten wooden beam in the attic of your house.
Push the tip of a screwdriver into the rotted area to see how far it penetrates. Repeat the process at multiple locations to determine if the beam can be fixed. If the screwdriver goes in more than 1/4-inch, you must call a licensed contractor to replace the beam.
Wipe down the wooden beam and clean the rotted areas with a rag. Measure the length of the beam so a new beam can be cut to size. Mark the desired cut location on a new beam and line up the blade of a circular saw with the mark on the beam. Cut the new beam to length.
Spray wood epoxy over the damaged area of the old beam, starting on one end and working to the other.
Line up the new beam with the old one. Drill and screw the two beams together with a power drill and wood screws. Space the screws every 8-inches to be sure the new beam is securely attached.