Bookworth, I don't want to be pedantic but thousands of houses have floor joists below the DPC level, but different construction methods mean that they are safe from rising damp.
Obviously some will be damp, but this is usually because the joists have been built into the walls below the DPC (allowing damp to strike the timbers) whereas latterly the joists were placed about an inch from the walls and supported on "sleeper" walls which should have had some form of damp proofing between the brickwork and the bottom of the joists.