Be careful. I’ve heard Propane tanks are not allowed within 10ft of any housing structure in DC, so on a roofdeck would be a bit too close. I only know this b/c a friend needed to get a natural gas line run so he could have a grill on his top floor deck. I could be wrong, but it may be something you want to check. I doubt anything would happen, but if anything ever does, insurance may not like it.
Also, be sure to store and use propane tanks with their valves up.