A detection control apparatus for the assembly of building components such as rafters, trusses, and the like. A pair of sensors located at the output side of a press for assembling the building components detects metal connecting plates that are intended to be positioned on opposite sides of the building component being assembled. If one of the plates is detected while the other is missing or out of position, the apparatus generates an alarm to signal an operator or automatically shuts down the press so that the missing or out-of-position connecting plate can be corrected.