Synthetic Underlayment is typically used in mostly shingle roofing as a release between the two materials, wood and asphalt. Also, think of it as a lightweight protection against any damage on a stormy day. Let’s say a shingle or a tile should break in a storm. If the roof has a layer of synthetic underlayment over it, it has a secondary layer of protection that will resist the driving rain. It is also much stronger, and lasts longer than traditional felt paper, which makes for less possibility of damage during installation and throughout its life on your roof.