On cars without a sunroof, you have the sheet metal roof, a small air gap, and then the headliner - somewhere around 1/2 to 3/4" of an inch. On cars with a til

On cars without a sunroof, you have the sheet metal roof, a small air gap, and then the headliner - somewhere around 1/2 to 3/4" of an inch. On cars with a tilt/slide sunroof (most of them) the mechanism takes up somewhere around 1.5 to 2 inches.