Pitchcards are transparent pieces of plastic, usually 4×8 inches which has some markings on it that will determine the pitch of a roof. Basically, you just stand at the sidewalk infront of the house and hold it up so it aligns with the roof and you can determine the roof's pitch.

For a simple illustration about this roofing tool, you can refer to this post –> http://www.roofing.com/about3088.html&highlight=