I am helping a friend build a shed/fort for their 4 boys and need some advice on rafter framing. It is 12'X12' and he wants to put a second story for them to sleep in. He was thinking 10' walls with the second story floor at 8'. Would the floor of the second story act as ceiling joists so the rafters would not need ties? Sort of a cathedral? He wants the boys to be able to stand in the second story (at the peak). He was thinking of framing the 10' walls, then nailing a 2x4 on the inside of the walls all around and setting the joists on it. Like a balloon frame. Will all this be strong enough?