Are downtown’s rooftops access free?

Been living in Houston for over 5 yrs, I'm from a country where we don't have skyscraper and I've always wanted to go into one of those building in downtown, go all the way up, enjoy the view and take pictures but those buildings look pretty classy and I've been afraid to ever try to go in. So I wanted to know is the access to the rooftops free? Do all of those buildings have rooftop access? Are there rooftop restaurant with decent pricing or my bill will always be over $1k?