The Marangoni flows are studied by numerical simulation in a liquid layer with an undeformable free upper surface heated by a constant flux and with a rigid bottom at constant temperature. Multirolls structures with different symmetry properties are exhibited in the case of a one-component fluid. More insight is given to the situation with an aspect ratio A = 4 with the determination of bifurcations up to a time dependent flow. The focus of the study is on the influence of the Soret effect on the flow structure when the fluid is a non-reactive binary mixture. Stationary results are compared to the expected behaviour given by previous linear stability analyses. Situations where time dependent flows occur as first bifurcation are also computed : an interesting behaviour corresponding to the successive apparition of new rolls in the middle and on the sides of the cavity is obtained. for which a physical interpretation is proposed.