You will also need underlayment materials such as water-resistant felt, decking if part of the underlying roof is water-damaged, and new flashing. Or should we have a contractor cut into our soffit under the eave to install the vents there. Also, at least 2 contractors never said one word about our lack of ventilation suitable for a ridge vent. Factors that affect cost include the type and grade of material used, the complexity of the roof including the slope and number of valleys it has. In gen...