  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 paper we present a
data-driven method for determining the age of a roof from historical satellite
imagery, which removes one of the last obstacles to a fully automated pipeline
for rooftop solar site selection. We estimate that a full solution to this
problem would reduce customer acquisition costs for rooftop solar by
$\sim$20\%, leading to an additional $\sim$750 megatons of CO$_2$ displaced
between 2020 and 2050.
