The compression force during traditional roofwork and improved roofwork was calculated by means of a two-dimensional static biochemical model. The improvement resulted in a 30% reduction in compression force, but also influenced factors which cannot be incorporated into the model but may cause back injuries. The frequency and velocity of movements were reduced and the trunk torsion increased. It is recommended that in further development this trunk torsion should be avoided. For valid application of biomechanical models in ergonomics, further research is needed. Priority should be given to epidemiologic research to establish the influence of compression force, trunk torsion and latero-flexion, frequency and velocity of movement on the development of lumbar injuries.