I have a new masonry fireplace under construction right now. They just built the firebox, set the damper, smoke chamber, and first flue tile. We are waiting the next county inspection. Where I would expect to see a flat or slightly concave smoke shelf is just an empty cavern. There is just an airspace between the back of the firebrick and the back wall of the fireplace. I asked the folks building the fireplace, and they said this was normal. Every drawing I have seen of a fireplace shows a flat or shallow smoke shelf. Mine is just a deep "V" that goes down about a foot. Is this OK? (It is a traditional fireplace, not a Rumford.) p.s. My other two masonry fireplaces have a flat smoke shelf. Thanks.