Like others have said, there is supposed to be a piece of card board or foam board up there and then the headliner material gets glued to that.

My truck did not come with a headliner, and when I tried to find one all of them were either warped or cracked. Talking to my friend that owns an upholstery shop, he suggested gluing it directly to the metal roof. He does it all the time.

I thought about it for a while and decided not to in case I wanted to change the color of my interior. But, I did like the idea if it were going to be semi-permanent. The metal roof has some nice contours to it IMO and gluing the liner material right to it would look pretty good IMO.

I finally found a good donor piece of foam board from a later model truck and he glued a nice piece of headliner material to it for me and it came out really nice. The only thing you need to be aware of is that 87+ sun visors have an extra clip out near the end to keep them from flopping around. 80-86 do not use these clips and you will have to fill the holes in the foam board it they will show through the headliner material.

Either that, or use the later model sun visors and the clips that go with them. I choose not to do this because they did not offer the color I needed (Fawn) in 87+ trucks and the saddle tan was too dark for the rest of my interior.

One more thing: Make sure that your cab has the necessary square holes in it for the clips on the trim pieces. It is not so easy to add a headliner to a truck that did not have one from the factory. The trim pieces and those metal clips are what holds the headliner in place.