What is a backer rod and why is it used?

A backer rod is a foam cylinder inserted into a joint before applying sealant. It serves two purposes: it controls the depth of the sealant (preventing waste) and creates the correct width-to-depth ratio for the sealant to function. Crucially, it prevents 'three-sided adhesion' by ensuring the sealant only bonds to the two sides of the joint, not the bottom, allowing it to stretch freely without tearing.