Place a stepladder or extension ladder under the rafter end with the most damage. If you are using an extension ladder, ask your helper to hold the bottom of the ladder while you measure and repair the rafter tail.
Measure from the end of the rafter tail to the area of good wood with a tape measure. When measuring, extend the tape measure a few inches over the good wood. This helps ensure a solid fit for the replacement ends.
Cut 2-by-6-inch boards to the measurement with a circular saw. Count how many rafter ends require replacement, and cut the same amount of boards to the same length.
Mark off the first rafter end at the original measurement. Draw a line at the mark using a speed square to ensure a straight line. Cut the rafter end off at the line with a reincorporating saw, taking care not to cut through the roof sheathing.
Cut in between the top edge of the rafter end and the bottom of the roof sheathing with the reciprocating saw. This cuts through the nails securing the sheathing to the rafter end.
Pull the old rafter end away from the roof eave. Hold the replacement against the end of the remaining rafter. Make two lines on the side of the replacement rafter end and the end of the rafter 1 inch from the top and bottom edges. This is where your wooden dowels will insert into the wood.
Drill holes in the end of the rafter adjacent to the marks with a 1/2-inch drill bit. Drill holes on the matching end of the replacement rafter end in the same manner. Each hole should be at least 4 inches deep.
Cut the 1/2-inch diameter wood dowels to 6-inch pieces. Squeeze a few drops of wood glue into the holes on each piece of wood. Tap the wood dowels into the holes on the end of the rafter with a hammer.
Apply wood glue to the end of the rafter and on the top edge of the replacement end. Position the replacement end over the exposed dowels, and tap the end of the rafter end until the opposite end is flush against the rafter.
Repeat for the remaining rafter ends that require replacement.