Measurements of the flow of thick 3He films over a highly polished silver surface, using a high precision torsional oscillator, have found unexpectedly long momentum relaxation times (Casey et al., Phys. Rev. Lett. 92, 255301, 2004). This results in a decoupling of the normal state helium film from the oscillator motion at low temperatures. In the ballistic regime the relaxation rate varies linearly with temperature. This result is explained by a theory in which the effect of surface roughness is incorporated into the transport equation as a disorder potential (Meyerovich and Stepaniants, J. Phys., Condens. Matter 12, 5575, 2000). This potential is fully characterised experimentally by AFM measurements of the surface roughness power spectrum, which we find to have a stretched exponential form. Incorporating the measured spectrum into the theory allows a direct comparison with the observed relaxation rate, with good agreement. The anomalous temperature dependence of the relaxation rate is accounted for by interference between bulk inelastic scattering and this weak surface elastic scattering. Decorating the substrate with scattering centres restores film-substrate coupling at the measurement frequency of 2.5 kHz. These results on the nature of quasiparticle scattering at rough surfaces may have implications in the understanding of the effects of confinement on superfluid 3He. © 2009 Springer Science+Business Media, LLC.