Originally Posted By: TooManyWheels
You can not just glue it back up. The reason it is sagging is because the foam backing middle layer has deteriorated and no longer exists. If you try to glue the remaining material, the glue will come through to the surface. You can get new material and use the 3m adhesive, however.
Yep, correct. 01 Civic that has been rarely garaged started sagging and coming away at edge. Used a white craft cloth glue (ease of application) to delay the inevitable for awhile by gluing the edges back down, but it was only a stop gap. Once the sagging starts, it only gets worse. Had the headliner professionally redone for $100 last year.
3M adhesive spray is specifically made for the headliner, and is readily available. That said, the preparation to use it correctly (w/new hl material) is the most time consuming part. The actual application of the 3M spray is only a minor part.