In this paper we discuss the requirements of the generic international standard (ISO, BS EN ISO 10140) for testing of sound transmission through sample roofs exposed to simulated rainfall and of lessons learned during a recent test program. The test data forms the basis for calculating in-situ sound levels in rooms beneath the roof and we discuss the differences in sound produced by simulated rain to that of natural rain. The differences in impact velocity and raindrop distribution between simulated and natural rain are key factors that are not addressed by the Standard. In addition, an optional normalization test using a pane of glass is included, for the explicit comparison of products tested and as quality control for test laboratories, and its results have been incorrectly shown in some manufacturer’s publicity material as the basis for calculating room sound levels. The Standard does not specify whether the normalization test should be carried out as a skylight or as glazing but the two tests have different requirements. Being optional and intended for inter-lab comparison suggests that the normalization data should not be released to clients as it is misleading and thus should be excluded from reporting.