I am having a regency hearth heater installed and need to extend the existing fireplace hearth. It will sit (no legs) on the existing hearth with 2 inches to spare, therefore I need to extend it. The current brick hearth is 16"x5’ and 1/2” above the adjacent wood floor(floor is tapered up to the hearth in the front) so I want to pour concrete (I was told flexibond would be good to use and would not crumble) to make a 3x5’ hearth that is all level. How do I make a form for the concrete and what do I put in the existing fireplace opening to contain the concrete at the edge( I cant nail 2x4's there(brick) or can I?). I only expect minimal concrete there though because that is the higher elevation. Also, what do I put at floor level to keep concrete from seeping under the form? I was told the concrete will automatically settle to level(Is that true?). I intend to put 1 sheet durock board overtop (how should I attach?) and then tile. In the end the hearth will sit higher that the fireplace bottom(because of layer of durock and tile) but the hearth stove sits on the hearth it is not an insert so I don’t know if that is a problem. It will have a surround piece around it so you won’t be able to see in the fireplace. As far as r-value, I looked at my crawl space and it is a column of cement blocks under the existing fireplace and hearth to the ground so I think I will be ok. The extension is just the clearance area needed in front of the door and then some.