Using of subdiagonals is one of the most common methods to decrease the materials consumption of the beams in the case of increased spans. Two variants of the trussed steel beam with the span in 12 m were considered. The considered beams are the main load bearing elements of the roof structure. The trussed beams consist of the top and bottom chords, struts and pillars. Rational from the point of view of materials consumption distances from the support to the joint of the edge strut with the top and bottom chords, height of trussed beam and vertical dimension of the edge strut were evaluated by the response surface and inspection methods. The dependences of materials consumption and distances from the support to the joint of the edge strut with the top and bottom chords, height of trussed beam and vertical dimension of the edge strut were obtained as the second order polynomial equation on the base of numerical experiment. Rational from the point of view of materials consumption structure of the trussed beam was suggested.