I really wonder how is that so. Even in my home country which is easily in the bottom 10% of Europe's poorest I lived in a building built in 1977 (!) with flat roof. It never had a leak until this year, and on the first sign of it tenants found some company to install some membrane and gave them 10 year no-matter-what warranty on it. And it was super cheap when divided on 30 owners. And we're talking about a country with average monthly salary of $400 or so.

I've heard this stated a few times - that flat roofs are meaningfully cheaper - but really? How? The tiny bit of extra material to add a bit of height to some ridge seems trivial. Where's the cost?