I know there are many theads on this, but there is one thing I don't understand. I have a typical cast iron insert with cast iron fireback (detachable) and a large builders opening. Question is when do I backfill with cement/vermiculite? After I have installed the insert and surround or before? If I try and do it afterwards I am working through a very awkward and restrictive opening which is large enough to shovel the stuff down, but will be difficult to create sloping top. Before, and I have the problem of shuttering the stuff and getting the shape correct. I'm sure I'm just being stupid, but anyone done this before?