Question: Can I install a mechanically attached TPO system over an existing ballasted system?
Short answer, yes, but you must remove the existing ballast first. However, there is a more important question that should be addressed in this situation: Why was the previous roofing system ballasted? Ballasted systems are generally installed for a reason, such as:
- The underside of the deck is visible from below and fasteners would disrupt the aesthetics
- The deck is post tensioned concrete and fastening into the deck could sever one of the tensioning cables
- The roof deck is concrete and mechanical attachment was too labor intensive
- The deck is pre-cast or pre-stressed concrete and mechanical attachment could cause spalling of the underside of the deck
- The building owner preferred the look of a ballasted versus mechanically attached roofing system when viewed from adjacent buildings
- The design professional/owner wanted to cut cost
Understanding the reasoning behind ballasting the previous system could prove very valuable by preventing a costly mistake or design flaw in the replacement system.
Whenever recovering an existing single-ply system, regardless of whether it is ballasted, fully adhered or mechanically attached, there are critical steps that must be taken:
- The existing single-ply membrane should be sliced into 10’ by 10’ squares before installing new insulation and/or a cover board.The existing membrane is cut into a grid pattern to prevent moisture from getting trapped between the two roofing membranes
- Removing and replacing all wet materials in the existing roofing system
- Installing a new layer of insulation or cover board
Have a roofing problem or question that you would like answered in the blog? Submit questions to MHBlog@mulehide.com.