Ultra-thin silicon wafer have to withstand forces and stresses during handling procedures without breakage. Here we investigate the failure stresses of ?30 ?m thick monocrystalline silicon films produced with the porous silicon process by use of a three line bending setup. We use a finite element simulation in order to evaluate the experiments and conclude that the porous silicon layers break at stresses comparable to those of silicon wafers with standard thickness. The edge preparation has a large impact on the failure stress. For samples with manually cleaved edges the failure stress surpasses 600 MPa, which is the largest stress that is accessible with our testing setup.