The plastic covers are usually 220 μm thick (5.6 mil) and last for 3 years. The fi lm is fabricated in Morocco from imported resin. Present costs per square meter (1988) are 4 to 5 DH ($0.050to $0.60). The plastic covers usually are stretched over the houses in overlapping strips and areburied at the ends. In some types, the plastic is woven through the steel pipe frames (Fig. 1D),leaving a slight gap between the strips that offers ventilation but causes low temperature problemsin the winter. Ventilation is achieved by separating the strips or by lifting the sides, or, in somecases, by dropping the side walls (Fig. 1C).