Review on Efficiently Replace or Install Roof Vents, Side Mount Vents, Plumbing Stacks & Refrigerator Vents with Camco Universal Vent Installation Kit (25003) by Joe Calabro

Using putty makes the installation clean as the screw holes may not line up, which is good. Once you've removed the roof debris from the previous fan, set the new fan in position and mark where it will be. Remove the fan and stick putty directly into the marks covering the old holes. Install the fan and secure with new screws. If you then apply Dicor self-leveling sealant to the new screws and edges of the front panel, you will have a clean install.