Why do houses have shingles and slanted roofs, but most other buildings have flat tops?

Question

Why do houses have shingles and slanted roofs, but most other buildings have flat tops?

In: 8443

Answer 1

As many have pointed out flat roofs allow for mech/plant on the roof. But in many commercial or residential tall buildings, developers will build to the absolute max height allowed by the land zoning. So they could either have an extra floor with 10 more sellable apartments (or offices) or use that space for an elaborate roof.