Re-decking is required when you have a cedar roof installed over slats and the roof you are going to install requires a solid deck. All shingles require a solid deck with the exception of #1 cedar shingles. If your roof has two layers of singles with the bottom layer being #1 cedar over spaced decking and the top is either shake or comp, and you remove both layers, then you must install a solid deck to accommodate the new roof...if it is going to be anything other than #1 cedar shingles. You may lay 7/16" wafer board or 1/2" CDX plywood directly over the lathe (spaced) deck.