I carried a spare on the roof of my former RV (sold in 2006). It was securely tied into a corner of the roof railing, with the tie-down ropes replaced annually. Got it up there by hoisting with a rope or pulling the RV next to something tall that I could climb on and then pushing tire onto RV.
Was this a good idea? Probably not. At the time, I thought it was better than no spare, which was the other option. I never had to use this spare in 15 years of backroad driving, maybe 80K miles.
If you took physics and remember about vectors, you know that putting something heavy up high is not a good idea.