Concerns have been raised over the use of plastic skylights as smoke vents in large single story buildings. The plastic skylights provide natural light to the building. There has been an assumption that these plastic panels will melt and provide smoke and heat venting during a fire. The plastics commonly used in New Zealand, polyvinyl chloride (PVC), polycarbonate, acrylic and glass fibre-reinforced polyester (GRP) have been tested in small scale experiments to further the understanding of the behaviour of the materials at elevated temperatures. The behaviour of particular interest is the effectiveness of the skylight as a vent for hot buoyant gases. Small scale experiments have been conducted to find the mass flow rate of an unobstructed vent. Experiments have then been carried out on samples of the four skylight materials for a range of exposure temperatures and times. The mass flow has been measured for the deformed sample and compared to the unobstructed flow. The results of the small scale experiments have been analysed and an empirical relationship has been developed for the PVC and polycarbonate material. The acrylic behaviour does not provide enough data to include it in the empirical relationship. The GRP failed to melt and was therefore also excluded from the empirical relationship. A model has been developed to demonstrate the use of the empirical relationship. The model compares four cases, the vents always open, the vents always closed, the vents opening at a single value and the area when calculated using the empirical relationship.