Carbon impact estimation software programs have simplified the processes for evaluating the carbon contribution of proposed buildings and can be relatively accurate down to building assembly. However, the simplifying assumption that a building’s embodied carbon is entirely a function of the production and installation, while a building’s carbon-in-use is the province of a building’s operational life can lead to misleading results, and hence, faulty decisions, when the lifespans of a building’s individual materials differ greatly from the building’s lifespan. The primary study became a way to point out those disparities between material life expectancy and carbon impact by studying the impact of three alternative roof assemblies.