Existing hearth safety

[Mahoney86]

Pulled my ZC out today and now working on hearth. I am going to extend the hearth all the way back to the wall but I am concerned with the safety of the existing half. It is 2" blue stone slab sitting on top of 1/2" plywood and ontop of 2x4 framing. The inside is a hollow box. About 1/2 of my jotul f400 will be sitting on this area. I feel I should be pulling off the slab and putting a layer of DuRock where the plywood is but that 2" stone has decent r value

 

[begreen]

If your F400 has the bottom heat shield it only requires ember protection and the bluestone is ok. That said, if you want to have a bit more peace of mind there's no harm and little expense to adding a sheet of 1/2" Durock under the blue stone for additional protection. It would have the added benefit of seam protection at the joint of the old and new stone. If it doesn't have the bottom heat shield then the hearth needs to have an R=2.0 insulation value. An option then would be to put 5 sheets of 1/2" Durock NexGen under the stone.