Ponding of flat roofs is the phenomenon that a roof deflects under rainwater loading, which means that additional rainwater runs to the deflected spot, giving more loading and thus more deflection, etc. For well-constructed roofs, equilibrium is reached and the roof is able to carry the loading. Collapse may occur for roof structures which are too flexible, have too high emergency overflows or insufficient slope. Collapse of flat roof structures under heavy rainfall and subsequent rainwater loading due to ponding of the roof has frequently happened in The Netherlands. From 1990 till now the average number of collapses of major roofs due to ponding is estimated at approximately 20 per year. One of the causes is the fact that ponding, especially on light roof structures, is underestimated as a severe load case. Moreover, the calculations necessary to design the roof to resist ponding are complex due to their non-linear and iterative nature and therefore not very popular among structural engineers. Also, roof structures may be extremely sensitive to construction inaccuracies of roof slope and sill height of emergency overflows of flat roofs. All this led to further investigations, additional regulations and a number of national publications in The Netherlands. Part of the work carried out within the framework of ponding prevention is reported in this special issue on "Ponding of Roof Structures".
In the first article, the size of the problem of ponding of roof structures and the number of collapses is investigated. Also, the actions taken by the government are introduced and the inspections carried out are reported on. Failure causes are classified and it is concluded that the problem of ponding does not stand alone in current building practice. The second article treats the analysis of ponding making use of the so-called "water raising capacity" which is the highest water level that can be achieved on a structure and which can be used to explain collapse as well as to safely design against ponding. In the third article, simple analytical design methods for ponding are summarized and developed, also for roof structures consisting of beams and purlins. These methods are subsequently applied in a sensitivity analysis to show that construction inaccuracies have a huge effect on ponding. The fourth article treats ponding in a different way using two simplified methods, one for strength governed cases of ponding and one for stability governed cases. This article also includes the effect of profiled steel sheeting on ponding in the analysis in addition to the effects of beams and purlins. The series of articles on ponding closes with an article on the evaluation of ponding analysis methods, demonstrating the importance of 3D Finite Element modelling to evaluate complete realistic roof structures.
Though not all issues related to ponding are covered in this special issue, the articles presented are believed to be representative for "Ponding of Roof Structures". The authors of the articles in this special issue are acknowledged for their contributions, which hopefully will help preventing roof collapses due to ponding.
Professor of Structural Design (Steel Structures)
Eindhoven University of Technology
Department for Structural Design and Construction Technology
Eindhoven, The Netherlands