Flat Roof insulation advice

We have a flat-roof extension over 2 storeys which is a concrete slab covered with traditional felt and measures approx 2.5 x 5.0m. There are two issues with it that we wish to solve. The first is the more immediate as there is a leak where this roof adjoins the main part of the building and is causing damp in the two walls, one of which is cob. Although we have had a short term fix applied we have no idea yet if this has stopped the problem as there has been no rain but either way a long-term solution is required and this is probably best tackled as part of solving the second problem. We also wish to insulate this roof and have had conflicting advice on whether this is possible to do on the outside (if so we would then want the new structure to be covered in a more resilient coating such as EPDM). There is an small lip of approx 70mm depth between the felt and the raised edge of the roof's perimeter. Should it be insulated on the inside or outside?