It depends on your locale - in southern cities this number is going to be quite a lot higher than 10-20% since a decent number of even detached houses have flat

It depends on your locale - in southern cities this number is going to be quite a lot higher than 10-20% since a decent number of even detached houses have flat roofs because snow isn't a thing - but heading into northern altitudes only apartment/condo towers will tend to have a flat roof (and some of them even have peeked roofs due to the snow).