A pagewidth thermal ink jet printhead for an ink jet printer is disclosed. The printhead is the type assembled from fully functional roofshooter type printhead subunits fixedly mounted on the surface of one side of a structural bar. A passageway is formed adjacent the bar side surface containing the printhead subunits with openings provided between the passageway and the ink inlets of the printhead subunits, mounted thereon so that ink supplied to the passageway in the bar will maintain the individual subunits full of ink. The size of the printing zone for color printing is minimized because the roofshooter printhead subunits are mounted on one edge of the structural bar and may be stacked one on top of the other without need to provide space for the printhead subunits and/or ink supply lines. In addition, the structural bar thickness enables the bar to be massive enough to prevent warping because of printhead operating temperatures.