I am planning to cover a rather ugly concrete slab by our back door
with slate paving:

https://dl.dropbox.com/u/50920090/IMG_0578.JPG

If you look at the picture you can (hopefully) see that the front part
of the slab has slightly dropped but it hasn't moved in the six years
we have been here. The raised part of the slab at the back covers a
tunnel under house through which part of the mill stream flows.

As a result, I am unwilling to remake the slab as I don't want to
disturb what is underneath. Does the team think it will be possible to
put a bit more mortar underneath the slate at the front to compensate?

Are there risks to the tunnel from the increased weight of the slabs?
My thought is that it will be sufficiently spread so as not to be a
problem. The tunnel underneath has an arched brick roof and is about
four feet wide.

Presumably I shall have to kill any algae on the slab before I start.

Thanks for your views.

Jonathan