Based on ab-initio calculations we find that Fe_1-xCo_xSi alloys in the Fe-rich region behave as disordered ferromagnetic half metal. Half metallicity can explain the recently found electronic, magnetic and transport properties. We can also determine a sharp transition from the paramagnetic metallic behavior to the ferromagnetic half metallic state as a functions of Co concentration. At concentrations higher than 0.25 the system starts to segregate Co from Fe atoms, this giving rise to the disappearance of half metallicity and to reflect it in the decreasing magnetic moment.