1. Introduction

Building envelopes deteriorate due to many factors. The exposure of building envelopes to the in- and outdoor climate is the inevitable factor, while the quality of their design, building and maintenance determines the durability as well. To which extent deterioration should be prevented, depends on requirements such as safety, aesthetics, use, life time, building costs and maintenance costs. Anyhow, the first step in minimising the deterioration is better design, for which knowledge of the local outdoor climate is primordial. In the present thesis, one facet of the local climate is studied, namely driving rain. Driving rain is rain that is carried by wind and driven onto building envelopes.

Deterioration problems related to driving rain are many and often linked to each other. Moisture in an exterior wall due to driving rain may cause for example reduction of the thermal insulation, corrosion to metal fixtures, cracks in porous masonry during freezing, efflorescence of salts, expansion or shrinkage (and thus damaging stresses), or it may cause fungal growth on the façade or on the indoor side. Direct rain water leakage into a building can also make the building less useful. Other examples for which driving rain directly determines the functionality of a building or building parts, are canopies over entrances and bus shelters. Together with pollutants (by wet and dry deposition), driving rain causes staining and weathering of the surface of building envelopes.

Deterioration of façades was described comprehensively in e.g. [Schaffer 1972] ([Schaffer 1972], reproduction of the 1931 edition), [Simpson and Horrobin 1970], [Grunau 1973] ([Grunau 1973], [Grunau 1975]), and [Winkler 1997]. These references are also instructive because they contain many pictures of weathered building parts. In literature one finds also more specialised studies relating to the issues mentioned in the previous paragraph, for example: [Couper 1974] (splashing and run-off from differently shaped projections), [El-Shimi et al. 1980] (weathering on precast concrete panel façades), [Newman et al. 1982] (rain penetration through cavity fills in masonry walls), [Flori 1990] (wetting and drying of a façade), [Mulvin and Lewis 1994] (weathering of a classical 18th-century building), and [Brocken 1998] (moisture transport in masonry).

Although many decisions at designing buiding envelopes imply knowledge of (driving) rain and its statistics, much is still unclear (see also the survey on driving rain research in section 1.3). The general motivation of this research is to develop tools to estimate driving rain amounts on building envelopes, which are useful for (1) the design of façades and roofs, (2) the testing of building materials and elements in laboratories, and (3) research on moisture transfer in building envelopes.