Replacing A Roofing Shingle
This technique will let you replace a single roofing shingle without damaging the one above it
When an asphalt roof shingle is badly ripped or damaged, don't try to patch it back together. Instead, remove the entire shingle and replace it with a new one.
Slip a flat bar under the damaged shingle and pry out the nails that hold it in place. Then, yank out the old shingle and slip in a new one. One tricky part of this repair is that in order to nail down the new shingle, you must lift the shingle that's directly above it. In many instances, the old shingle is too brittle to bend far enough out of the way so that you can swing the hammer.
Here's the solution: Place a wood block on the roof just below the lower edge of the new shingle. Slip the pry bar under the shingle and rest it on the wood block. Stand a roofing nail under the flat end of the bar, as shown. Then strike the bar with the hammer to drive in the nail. Once the nail is about halfway in, you usually can remove the pry bar and simply hammer it home.