Rooftop solar is one of the most promising tools for drawing down greenhouse gas (GHG) emissions and is cost-competitive with fossil fuels in many areas of the world today. One of the most important criteria for determining the suitability of a building for rooftop solar is the current age of its roof. The reason for this is simple -- because rooftop solar installations are long-lived, the roof needs to be new enough to last for the lifetime of the solar array or old enough to justify being replaced. In this presentation we will present a data-driven method for determining the age of a roof from historical satellite imagery, removing one of the last obstacles to a fully automated pipeline for rooftop solar site selection. We estimate that a solution to this problem will accelerate the deployment of rooftop solar, and lead to an additional 760 megatons of CO2 displaced by rooftop solar between 2020 and 2050.