One construction for a pitched roof is to use tiles on battens, with a liner attached below the battens. The shape of some types of tiles is such that, at each corner where four tiles overlap, a small gap is formed. At certain positions of the sun solar radiation can penetrate through these gaps and strike the liner, accelerating its degradation. The purpose of this study is to estimate the extent and duration of the exposure. A typical gap is modelled in a ray-tracing program and the size and position of the illuminated area on the liner is calculated for given directions of the light beam relative to the roof. Analytic expressions for the size of the illuminated area are obtained using a thick slit model. The accuracy of the model was assessed by some experimental measurements. The exposure over one year of the roof liner was calculated using the Design Reference Year for Copenhagen, Denmark. Simulations were carried out for a roof tilted at 25degrees, 35degrees or 45degrees, facing SE, S, SW or W. For the particular roof construction and gap studied, the maximum annual exposure of a 25 mm(2) piece of the liner placed 150 mm below the gap (corresponding to about 100 mm below the base of the tiles) is about 140 U m(-2) to UV-B and 4.2 MJ m(-2) to UV-A radiation. Variations in tile size and misalignment of tiles could lead to other gap shapes and sizes, possibly leading to greater exposure. Constructions with the liner placed closer to the tiles would also lead to greater exposures. However the method developed in this study could form the basis of a standard method of assessment.
Solar Energy, 2005, Vol 78, Issue 2, p. 199-209
roof finer; degradation; building materials; ultraviolet radiation