Window to door, how to do the floor?

[Rick888]

We had a window changed to a bifold door and unsure how to fill the gap in the what was cavity , under the rubble is sand and it seems to stop about 12 inches down, do I need to put a plastic
membrane down too?

 

[Notch7]

View attachment 323623
We had a window changed to a bifold door and unsure how to fill the gap in the what was cavity , under the rubble is sand and it seems to stop about 12 inches down, do I need to put a plastic
membrane down too?

Form a U shaped trough with damp proof membrane and fill with concrete.

bear in mind that creates a cold bridge, but then if your house floor isn’t insulated, it makes no difference

 

[Rick888]

Thanks ok, no floor isn't heated it's concrete floor.