Often in our field studies we encounter multiple layer roofs. The reasons for this vary. Multi-layered roofs have advantages and disadvantages:
- Cheaper than a tear off roof installation
- Not as messy as a tear off roof installation (less cleanup required).
- Faster installation than a tear off roof.
- Difficult to detect any leaks
- Adds weight
- Gets hotter"
- Affects inspection reports
*- Each layer of roofing has insulation properties which much be accounted for. The "R" value is a measurement of the effectiveness of thermal insulating materials.
R value calculator - https://www.designnavigator.solutions/CRC.php