I am doing a little side work for a customer and I have a questions that hopefully someone can help me with. The house they live in used to have a concrete slab carport. The previous owner had the carport enclosed and made into a spare room. The contractor put the plate and the studs directly on the slab and also did not use sheathing on the outside. When it rains hard the water sits right there on that wall and the water seeps in under the sill plate and soaks the carpet, pad, insulation and drywall. I know I need to flash this to keep the water out but I am not exactly sure how to do this without tearing down the existing wall. Any help would be greatly appreciated. Thanks.