Since our first simple shelters, roofs were made of wood and straw, and had a slope to keep out the rain. If you needed to cover a building wider than the length of tree, you had to join timbers together to make a "structural beam", and the strongest shape was triangular.
The Industrial Revolution changed everything. New materials such as iron and steel could span farther than timber, and asphalt and plastic kept flat roofs dry. Cities ballooned in size, creating the need for bigger schools and office buildings. Travel brought us fashions in architecture from hotter cities with flat roofs.
Now we can build virtually anything, anywhere, but still many people expect houses to have traditional pitched roofs. In a nutshell, the answer combines the practical, technical, historical, cultural, material, sociological, ideological and environmental – that's architecture for you.
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