Ponding on flat roofs: A different perspective

J. Blaauwendraad
Faculty of Civil Engineering and Geosciences, Delft University of Technology, the Netherlands

Rain water ponding in flat roofs is studied with aid of two simplified models. Distinction is made to two categories of rainwater ponding on flat light-weight roofs. In the first category strength is governing failure of roofs and in the other one stability is governing.

For the first category the nonlinear strength problem is discussed in a way which closely leans towards the 1/8ql²-method for a homogeneously distributed load q, familiar to structural engineers. The approach applies for horizontal roofs, sloped roofs, roofs with initial deflection due to permanent load and camber and for roofs which are composed of primary and secondary members and profiled steel sheeting. The deflection of all these components is accounted for. Application examples demonstrate that the model is an easy-to-use design tool. The design method clarifies the effect of the profiled steel sheeting on the safety. Also other practical hints are included in the paper.

For the second category of ponding, in which stability governs the problem, a separate check on the ultimate capacity is available. The paper is a designer-oriented version of a research paper published in Engineering Structures [1]. Here, designer-oriented features are summarized and the discussion is extended to aspects of interest to structural engineers and code developers.

Key words: Flat roof collapse, rainwater ponding, structural safety, method of analysis, design method