Vegetated roof systems can act as absorbers to mitigate traffic noise. Various properties of vegetated roofs, such as open porosity, are important for sound absorption and scattering in urban environments. Substratum has been identified as the major contributor to the acoustic absorption of vegetated roofs. However, the coverage of the substratum by plants may have a significant effect on its acoustic absorption. Most existing studies on the determination of the acoustic absorption of vegetated roofs are based either on laboratory experiments or numerical models. So far, little research has focused on the prediction of vegetated roofs’ impedance based on in-situ measurements. This research modified the existing Nordtest Method for measurements at vegetated roofs. It was modified to an extended reacting double-layer medium with a variable reference surface, surface roughness was included and the three-parameter Miki impedance model was used. Using measurements on four different in-situ green roofs, a good agreement between measured and predicted results was obtained with the modified Nordtest Method. Finally, some simplifications in the modified Nordtest Method were found to reduce the complexity of the prediction without sacrificing accuracy too much.