In general, it is not recommended to put a second layer of rolled roofing on top of an existing one. While it may be technically possible, there are several important reasons why it is not advisable:
Weight and Structural Load: Adding another layer of rolled roofing will increase the weight on your roof. Most residential roofs are designed to support a specific load, and adding an extra layer can exceed the original design capacity, potentially leading to structural issues or even roof collapse.
Compatibility: Different types of roofing materials may not be compatible with each other. Putting a new layer of rolled roofing over an existing one could lead to issues with adhesion and bonding, reducing the overall effectiveness and longevity of the new layer.
Warranty Voiding: Many roofing manufacturers void their warranties if the roofing material is installed over an existing layer. This means that you won't have the protection and support of the manufacturer if any issues arise.
Uneven Surface: If the existing rolled roofing has any imperfections, wrinkles, or uneven areas, these issues can transfer through the new layer, affecting its appearance and performance.
Potential Trapping of Moisture: Moisture can get trapped between the layers of roofing, leading to accelerated deterioration and the potential for mold and mildew growth.
Instead of adding a second layer of rolled roofing, it's generally best to remove the existing layer and then install a new one if needed. By starting with a clean and properly prepared surface, you can ensure better adhesion, prevent potential problems, and extend the life of your roofing system.
If you're considering a new roofing project or have concerns about your existing roof, it's advisable to consult with a professional roofing contractor. They can assess your specific situation and recommend the best course of action based on the condition of your roof and your budget.