Covering over an exposed basement

[ooberandy]

I have an outbuilding which i want to demolish, as its beyond repair. Underneath the outbuilding extends the basement from my house.

The outbuilding has a large 'hole' which exposes the basement. I was thinking of using a beam and block floor to cover this large hole and finish with paving slabs.

What would be the best way of waterproofing the beam and block area?

Many thanks
Andy