Quick Answer: While your roof is open during roof replacement, the four upgrades worth considering are roof deck (sheathing) repair where rot is found, full synthetic underlayment plus ice and water shield at eaves and valleys, balanced attic ventilation (matched intake and exhaust), and new flashing at every penetration. These add 5% to 15% to your project cost, but determine whether the new roof reaches its full 25-to-30-year lifespan or fails early at the underlayer. Repla