Have you been using a typical "A" frame roof until now and not joining perpendicular section together? If you have an L shape, the corner pieces in the build menu will help you fill in the odd shape, you will have to experiment. To avoid a super high roof, you can use the 26 degree shingles or build it up from all of your walls simultaneously.

It's a little hard for me to describe, but try building a 4x4 structure and have 2 regular shingles in the middle sections all around and use the corner pieces in the four corners. It takes practice, at least it did for me. I hope that helps.