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Installing a mechanically attached TPO system over an existing ballasted system

Posted: 1/18/2019

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


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