Why people shape roofs the same way

The roofs of houses look similar in profile, especially in villages and old settlements. Why? The question is about the common angle of inclination. The answer comes from heat transfer by natural convection. Inspired by the evolution of the sapiens toward more power from the adoption of artifacts, we show that the existence of common roof shapes can be reasoned from the needs of those who live under the roof. Is there a shape that, while reducing the heat loss to the ambient, reduces the human effort (gathering firewood, etc.)? We consider two classes of roof shapes, Λ and cone, and two roof sizes, small and large (respectively, two flow regimes, laminar and turbulent). In laminar natural convection, the common profile of the Λ and the cone looks shallow, with height/base ratios comparable with 1/4. In turbulent flow, the Λ and the cone look like an equilateral triangle. These findings reinforce the evolutionary record of human civilization toward economy of effort and longer life. They also present an opportunity for the future ‘energy design’ of buildings: the external shape has a significant effect on the heat loss from the building.