Damaged constructional hearth
 

Hi, I am doing most of the legwork for fitting an inset gas fire for
my parents. They live in a terraced house built in 1910.
On removing the existing fire, surround and superimposed hearth I
found a badly crumbling constructional hearth underneath.
There is a 3' drop to the clay foundations and the constructional
hearth is in no state to bear any weight.
The house is built on reclaimed land and the foundations are below the
water table, hence water can sometimes be found under the floorboards.
Also, there appears to be nothing supporting the centre of the back
hearth.
The floorboards and joists are all dry and in excellent condition.
The new granite superimposed hearth is currently resting on the
floorboards, and covering the hole where the constructional hearth has
sagged.
Reading various threads it seems some people have simply removed the
constructional hearth (or at least a few inches of it) and fitted
floorbaords over the hole, then placed the superimposed hearth on top
of the floorboards. Other people have rebuilt the whole of the
constructional hearth.
What would you recommend?
Regards, Steve.