Abstract: Modular building panels having central insulating cores and protected by bonded sheet material are joined in a roof, wall, or other building system by way of an interlock feature that also provides a deep water drainage channel. This interlock feature utilizes an outwardly facing channel formed of the end of one of the bonded sheets which is mated with an inwardly extending locking member formed of the opposite end an identical bonding sheet from an adjacent panel. The inwardly extended locking member is inserted into the outwardly facing channel and rides over a camming surface which locks the inwardly extending locking member into the outwardly facing channel. Once two adjacent panels are joined together, the interlock feature forms a deep water drainage channel to catch and carry water and moisture that penetrates seams between the panels due to imperfections of caulking seal.