This is what I would do: I would remove few cedar shingles from each course along the outside-side of the cut and place some ice and water barrier in the corner. Maybe 12in along each side. That's the same stuff used for roofing and windows/doors, etc. Then put your corner board in place (square cross section of size slightly larger than depth of your shingles) and butt them up to it. Make sure when you replace the shingles you don't have any joints lined up. Any water that infiltrates the corner will find its way down via the water and ice barrier.Originally Posted by seaweed1
Make sure whatever you do at the bottom near the foundation doesn't allow water to get in around the sill.
When you replace your shingles, make sure you match the spacing of the old. As they get older and dry out/shrink they leave a little gap. If you butt the new (or reused old) ones up tightly it'll be obvious that its a repair even after they weather.