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Solar stills are convenient, cheap and ambient friendly but has the drawback of low production of potable fresh water. To explore better the concept and make more attractive for use some more research is necessary to improve the productivity and thermal performance of these units. In this paper a numerical study on an inclined type solar still is performed to investigate the influence of the water film thickness on the heat transfer and fluid flow inside the device cavity. The solar still system temperatures are calculated for four film thicknesses, 0.14, 0.5, 1.0 and 3.0 mm, through the Runge-Kutta method for the ambient conditions of Campinas, southeast of Brazil. The temperatures of the film and the glass are input to the computational fluid dynamics investigation. The radiative transfer and double-diffusive convection of air and vapor mixture are simulated considering the nongray Spectral Line Based Weighted-Sum-of-Gray-Gases method. The results indicate that the use of film thickness δ w = 3.0 mm increased the temperature difference between the water film and the glass from 2.14 °C to 9.15 °C when compared to the film flowing over the bare plate, δ w = 0.14 mm, and increased the rate of fresh water yield more than 2.0 times, for summer conditions and peak insolation hours. The use of transparent gas approach slightly overpredicted radiative and convective heat transfer inside the solar still cavity. • An inclined solar still is numerically simulated considering nongray gas approach • Thick water films increase temperature and vapor concentration in the cavity core. • A film thickness of 3 mm is adequate for increased fresh water yield. • The moving water film does not induce a clockwise moving mixture. • The transparent gas approach results in errors up to 39% in temperature and vapor profiles. [ABSTRACT FROM AUTHOR]