We use a combination of transmission electron microscopy, X-Ray and neutron diffraction, SQUID magnetometry and symmetry analysis to determine the nuclear and magnetic structure of an epitaxial LSMO film, deposited on a Si substrate. The film undergoes a magnetic ordering transition at 260K. At 300K, in the paramagnetic phase, the manganite film has a $I4/m$ space group. In the magnetic phase, SQUID and neutron diffraction results lead us to assign a ferromagnetic spin-order to the film, associated to magnetic moments with a slightly out-of-plane component, namely $(3.5, 0, 0.3)\,\mu_B$. Symmetry analysis further shows that only the $P\bar 1'$ group is compatible with such a magnetic ordering.