Insulation for old flat tile roof with cathedral ceiling – GreenBuildingAdvisor

We have a 1961 Midcentury Modern home with a combination flat clay tile gable and flat roof. We just replaced the old EPDM with PVC on the flat roof and discovered during that process that the tar paper and insulation under the tiles that overhang the flat roof is degraded. We’ve never had a leak in this part of the house (purchased in 2012). The tiled roof covers a living room with a cathedral ceiling and the space between the ceiling and roof in small, we think about 8” (based on the space between the ceiling and flat roof).

We’re wondering if this may be the case for the entire gable or if the condition of the underlayment may be restricted to the edges. What is the best way to find out what the condition of the underlayment is? Would it make sense to replace all the insulation even if it isn’t all degraded?

If so, we would appreciate any advice on the best underlayment and ice shield that will give us the most R value given that we’re unable to add any insulation above the ceiling. The tiled roof has a stone chimney at one end but no pipes or venting in a 6b planting zone (PA).

What we were thinking is to remove and clean the tiles, lay new insulation and ice and water shield, and replace the cleaned tiles. Does that cover what we should do in this situation? Should the ice shield cover the entire roof deck? Would this plan cover code requirements?

We’re also wondering if the copper flashing needs to be removed to add a drip edge. If so, can we reuse it or do we need to replace this too?

Thank you so much for your advice.