The precise geometries of three reinforced concrete shell roofs have been measured with a laser scanner. The resulting point cloud has been modelled by NURBS surfaces. Two methods have been developed for determining the shape imperfections with lengths between 0.5 and 5.5 m. The largest observed imperfection amplitude is 80 mm with length of 5 m. The imperfections are represented by a variance spectrum and an extreme value distribution. From this are derived a formula for the characteristic imperfection amplitude and the partial safety factor.