An apparatus and method for sealing an underground junction between a lateral service line and a main pipeline from the inside-out. One embodiment provides a substantially rigid patching apparatus which may be positioned and installed with a robotic device within a main pipeline. The patching apparatus has a flange shaped to fit the internal diameter of the main pipeline and a tubular stem or sleeve which penetrates into the lateral service line. The patching apparatus contains an interconnection substrate such as felt, sponge or other similar material. The interconnection substrate may be impregnated with a bonding agent which sealingly interconnects the patching apparatus to the main pipeline and lateral service line. The seal provided by the inverted-tee patch effectively eliminates most water intrusion or the influx of plant roots and other matter which can seriously damage or obstruct the main pipeline. A remote control robotic arm is used in conjunction with a support cage to apply the patch to the underground junction between the main pipeline and lateral service line from within the main pipeline.