My husband added on a garage to the existing house/carport. He has now closed in the carport to try to create another room. The slab is not level and slopes. He is wanting to place a plywood sub floor over this but then it will cause the existing backdoor to be approx 6 inches lower then the floor. To open the door he will have to have a sweep section of the floor to continue to be the original concrete and then it will go into the sub floor. This appears to me to be causing a place that will be easy to trip, hard to clean and an unhandy design. Do you have another suggestion.