Headliner fabric has two layers: a foam layer and the fabric itself. The headliner fabric (fabric and foam backing) is glued to the headliner with a special upholstery glue. Over time with regular heat exposure, the foam deteriorates and the bond between the headliner fabric and the foam of the headliner fabric breaks down. As a result, the fabric of the headliner sags and hangs off the roof of your car.