A demonstration project with a large roof integrated solar collector for domestic hot water (DHW) was built in 1984, for the KAB Building Society, in Ballerup near Copenhagen, as part of a Nordic cooperation on solar energy. The project had received funding from the Nordic Building Research Cooperation (NBS), the Building Research Council in Sweden, and from the Energy Agency in Denmark. The 156 m[sup 2] solar collector used, replacing an ordinary roof, was developed as an improvement of a Swedish solar collector. Projecting the system was performed by Danish and Swedish consultant engineers, and the Thermal Insulation Laboratory at the Technical University of Denmark was responsible for realizing the project and the following monitoring programme. Two different selective absorber types of equal size were used for the solar collector construction. One was a Swedish pipe and fin absorber of aluminium and copper, and the other was a Danish channel plate absorber of stainless steel. For the mentioned solar heating system with approximately 1 m[sup 2] of solar collector per appartment, a 40% system efficiency of the insolated solar energy has been documented, during a one year monitoring period. Monitored results on a monthly basis, together with a discussion onmore » difference in yield between the two different absorber types used, and operation experience, are presented. 10 refs., 9 figs., 2 tabs.« less