Cricket flashing is installed where the roof intersects a chimney or a curbed roof penetration. The cricket diverts water around, while the backer flashing provides a weatherproofing transition material right where the backside of some type of penetration intersects the roof. A recommended best practice is to install a cricket when a chimneystack is more than 30 inches wide (the NRCA recommends 24 inches wide). Thank you InterNachi www.nachi.org. www.swordroof.com