The cost of re-shingling a roof merely because it is convenient to do so does NOT get factored into the Solar Credit.
The cost of structurally reinforcing a roof would be factored in, and the cost of specialized highly reflective shingles that are designed to enhance the solar panel output would be factored into the credit.
Kenley makes an interesting point. If it is IMPOSSIBLE to install the solar panels without replacing the wood shake roof, that could possibly make a good argument for including it in the credit.
