The present disclosure concerns a system, methods and compositions for attaching architectural panels to a building surface, such as a wall. In certain embodiments, the system comprises panels, each with upper, lower and side interlocking members. In a preferred embodiment, the upper and lower interlocking members form a modified tongue and groove joint, with a 45 degree angle interface, while the side interlocking members form a ship lap joint. The upper and lower interlocking members may be designed to press the panels against the building surface and to hide the locations of screws or other attachment devices when the interlocking members are engaged. The system provides a stronger and more secure attachment of panels to a building surface, while eliminating the problem of having to disguise the location of the attachment devices where such devices are inserted directly through the surface of the panel.