Reroofing over lightweight insulating concrete starts with evaluating the existing layer and identifying the assembly beneath it. Retaining it may be part of a proposed scope, but should be a documented decision tied to the selected roof system. The phrase “roof over existing” does not explain condition, compatibility or attachment.
Establish what is actually present
Ask for existing drawings and a roof investigation that identifies the insulating layer separately from the supporting deck. Note whether previous repairs created different conditions in different areas. A building with several additions may not have one uniform assembly, even when the finished roof surface looks continuous.
Johns Manville publishes separate roof-deck engineering guidance, while GAF illustrates hybrid assemblies using lightweight insulating concrete. These references show why the substrate must be considered within a complete system. Neither establishes that a particular existing roof is suitable without project-specific investigation and current manufacturer direction.
Separate retention from removal decisions
Request a roof plan identifying known unsuitable areas, areas proposed for retention and locations needing further evaluation. Ask what findings would change those boundaries. The contractor should explain who reviews the condition and how the owner will receive the evidence before additional work is authorized.
For example, a bid might retain most of the insulating material but include localized removal around documented problem areas. Compare that scope with a full-removal alternative using the same assumptions about drainage, flashings and access. Otherwise, a lower price may simply reflect a different demolition boundary rather than a more efficient version of the same job.
Document attachment and transitions
Have the proposal name the intended attachment arrangement and the documents supporting it. Ask how transitions between retained and rebuilt areas will be handled. The change in layers should appear on a detail, not remain an informal field instruction.
Agree on the process for concealed-condition photographs, quantity tracking and written approvals. At closeout, keep the final area map with the roof assembly description. That record is especially useful when the building later receives equipment work or another localized repair.
Perkins' repair-versus-replacement discussion helps owners frame the investment. Its commercial roofing information explains the starting point for a proposal. Bring records of past leaks and repairs so the investigation can focus on actual building conditions. The goal is a defensible retained-substrate scope, with clear alternatives if the investigation finds something different.
Manufacturer references: Johns Manville: Systems engineering roof decks; GAF: Hybrid roofing system solutions.