If there is no moisture in the roofing systems and none of the previously stated conditions exist that would dictate a complete removal of both systems, the next step is to check with the town building inspector. Does the building code limit the number of roofing systems? Typically no more than 2 roofing systems can be used. There are times when the building inspector may not consider the addition of another waterproofing layer (single ply membrane or coating) to be a 3rd roofing system. Check and make sure. The top roofing system can be removed in its entirety and a new system can be installed on top of the bottom roofing system. If it is possible to remove just the waterproofing layer of the top roofing system and leave the top roofing system’s insulation in place then attempt to do that as it saves on removal and disposal costs as well as future energy costs. Most of the time removing the waterproofing layer will cause some damage to the insulation underneath and something should be placed on top of the old insulation to provide a sound substrate for the waterproofing layer. This usually is another thin layer of insulation or a manufactured board like DensDeck. Check with the manufacturer of the roofing system to be installed as to their requirements (which may change depending on the length of the warranty being requested).