Question On Raising A Ceiling

I have an attic with rafters and plan on installing a hipped tray ceiling in the living room. I’m familiar with structural ridge beams for vaults but I’ve also read many places that you can simply install rafter ties along the lower third of the rafter to replace the ceiling joist. I was planning on hiring a structural engineer but wondered if he was going to simply say the same thing I’ve ready numerous places. In other words I wanted to find out if this was a uniform rule or if there was more to it. Thanks for any advice.