I have a home being rehabbed. After peeling back 3 layers of asphalt composition shingles, the original roof is cedar shake. Rafters are 24" on center (real 2x6 rough cut - old 1920's home). There are firring strips that were installed onto the cedar shake to provide a level surface to install the comp shingles.

I am looking at two scenarios: (a) leave the cedar shake installed with firring strips and install 5/8" OSB on top of this, then install new 30-year architectural shingle roof or (b) complete tear-off down to the rafters and install 3/4" Plywood for support and install new roof.

The only concern I have with option "a" is using long enough nails to properly secure OSB sheathing to rafters and dealing with valleys (uneven surface after sheathing put on top of shake. I am thinking rolled paper thickness can compensate for this.

All thoughts appreciated.

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