Calculation of roof drainage systems in Poland. Selection of rainfall intensities

Abstract

The design workshop for roof drainage systems in Poland has significant gaps, the main one being the lack of guidance on the adoption of rainfall intensities for calculating roof runoff. Very often in practice, subjective and underestimated values of the representative rainfall intensities are adopted for drainage calculations. This is very dangerous, especially when designing drainage systems for flat roofs. According to the recommendations of the European standard, these intensities should be determined on the basis of local rainfall models. However, for this it is necessary to adopt standards as to the required rainfall frequencies and durations. As part of the implementation of the WaterFolder Connect project and the construction of roof drainage systems’ calculators, it was proposed that local rain intensities should be adopted for a duration of 5 minutes and a frequency of occurrence ranging from C = 5 years (probability p = 20%) to C = 100 years (probability p = 1%) depending on the type of installation and the function of the building. In the final part of the paper, the results of the application of the proposed standards are discussed on the basis of nationwide readings of the design rainfall intensities from the Polish Atlas of Rainfall Intensities (PANDa) in relation to the safety of roof drainage systems in view of projected climate change.