The trim available for fiberglass edge trimming comes in different channel sizes for different thicknesses of material. If it isn't fairly easy to knock on, it may be the wrong size for your roof. I use a leather faced mallet for easy & damage free installation.
I buy mine at J&J Auto Fabrics in Rialto, CA, they have a wide range of sizes. Here is their website <<<jjautofabrics.com>>>