Why don’t they build decks/patios on roofs most of the time? It seems like prime space with the best views and fresh air? You could have a garden with great sunlight, chaise lounges for tanning, or if you’re a business you could have a rooftop restaurant/bar or an “open air” market. It just seems like a lot of prime square footage that goes wasted.

I’m guessing there are very good reasons why this isn’t done (weight of snow perhaps) or this would be a lot more common.