Like in many other cities, it’s getting harder to find an apartment in Berlin. Rents have gone up almost 60% over the last seven years, and vacancies are so scarce that the city decided to ban Airbnb rentals of full apartments in May. But the city has around 55,000 unused roofs. So two Berlin-based architects are now experimenting with a new design for tiny houses that are designed to quickly install on rooftops and easily…