A prefabricated shell for forming a shelter (14) has a groundsheet (30) and a cover (32) having a gas impermeable inner layer (24) and at least one outer layer. The outer layers are each formed by a layer of cloth that has been impregnated with a water-settable material, e.g. cement. The shell is steeped in water to wet the cement and then the cover is pneumatically inflated to form a space between the cover and the groundsheet. The shelter is then left until the cement has set and is able to support the cover. The shelter can easily be constructed to provide a durable shelter, especially in emergency areas.