If I remember correctly it should be rubber roof membrane on top. To make sure the manual slide it adjusted correctly, check the gap between the inside wall and the lip on the slip out. It should be even, top and bottom. Also, with this type of slide it matters that you have the supports in the right place. One support should have a big "R" stamped on it. I think the stamp is on the bracket on the horizontal beam where it connects to the trailer. That one goes on the right side as you are looking at the slide when standing behind the trailer. The reason that it is important which side the supports go on is because there was a lot of variation with the trailers were built. It isn't unusual to see the brackets on the trailer at different heights on both sides.