A composite printhead supporting structure for a pagewidth printhead assembly is provided. The assembly has a plurality of printhead modules with a predetermined coefficient of thermal expansion, the modules being disposed along a length of the supporting structure. The structure comprises a composite beam elongated in the direction of the printhead and being at least as long as the printhead. The beam comprises segments bonded together end to end. At one of the segments comprises material that is different from the materials of the other segments. The materials have coefficients of thermal expansion different from that of the printhead modules. The coefficients of thermal expansion and the size of the segments are chosen such that the printhead spacing, or printhead pitch, has an effective coefficient of thermal expansion substantially equal to that of the printhead modules.