My parents just went through this in SF, I believe wood shake roofing is just that, wooden roofing shingles. They are a fire hazard, a perennial issue during home inspections, and an occasional issue for home insurance on resale. My parents came across this as they were researching getting solar. Oddly, solar became a particularly good deal for them, not just for the reduced energy costs, but because the solar company contracts with a roofer to replace roofs that need it as part of the panel installation. My parents had needed a new roof for a while, and thanks to the contract negotiation by the solar provider and the credit they provide, my parents are only paying $2200 out of pocket for a new roof. In San Francisco that is a good deal. That includes removal of all shakes and shingles, replacement of the sheathing below, and new asphalt shingles. Reroofs generally come in two styles, a new layer of shingles over the old (cheaper) and a replacement of the sheathing and then new shingles, better as the shingles are in better contact with the sheathing. Adding solar may be an extra benefit and you can kill two birds with one stone.