We removed the entire roof and roof deck. We replaced the roof deck with two layers of opposing plywood taping the seams We also built a level roof deck over the eastern 2/3 of the roof.  Where the deck meets the parapet walls we caulked and taped to insure air and water tightness.  We then sandwiched six (6) inches of Roxul insulation with one (1) inch of polyisocyanurate insulation.  We then added another layer of opposing plywood to which the roofers applied water and ice shieldand finally a modified bitumen roof.