On Sun, 27 Dec 2009 04:32:11 -0800 (PST), Paul
wrote:

Thanks both. Blackjack is a tar-like bitumen that I assume acts as a
waterproof barrier between the two layers of concrete.


The problem is, it doesn't connect with the breached membrane to
provide a replacement continuous damp proof membrane.


To be fair, the
two ends of the run are dry, and where it wasn't covered with carpet
it is now dry, so I assumed that his method worked OK, and it was just
concrete under the carpet that was not dry as it was covered.


They are dry because the rising moisture is ventilated, whereas under
the carpet ...


The original plans had drains going around the house, but this would
have meant the gradient was to shallow as the run was too long. In
order to do what we wanted, they had to go across the lounge (and this
was with the approval of the Building Inspector).


The approval of the Building Inspector would have depended on
effective measures being taken to provide effective damp proofing that
you don't appear to have. Did the Building Inspector personally
witness the installation of the drains and any attempt at providing
effective damp proofing? No, I thought not.

The lesson to be learned here is that any work that breaches a damp
proof membrane should include carefully designed and installed
measures to ensure the continuity of that damp proof membrane in the
finished job. The word "ensure" implies that the work should be
supervised by a construction professional, especially the critical
elements of the work that involve re-establishing the damp-proofing by
effectively connecting any new damp proof membrane to the existing.

No doubt several people on here (the usual suspects) will froth at the
mouth at the thought of spending money to employ a professional -
usually an architect or structural engineer - to supervise the work.
However, your experience is a prime example of why saving a relatively
small amount of money by placing your complete trust in a builder is
perhaps not the most sensible option.

It might be OK, if only builders could be more easily held to account,
and therefore tended to be competent. However, the fact is that there
are very, very few cases where builders have been successfully sued by
property owners for the cost of putting right their incompetent work.
So the incompetence continues. And ever more property owners are
faced with paying the cost of having the work done again to the
standard it should have been done to in the first place.