Waste materials are composted in an elongate pit which is maintained covered to retain odors. The pit includes two rails each arranged along a respective side of the opening for receiving operating machines and a carriage for supplying a covering sheet. Each side edge of the sheet has attached thereto a channel shaped support member arranged for engaging over the respective rail so as to attach the side edge to the rail. A reel is mounted on the carriage and is arranged to roll the sheet and the two support members onto the reel and to un-roll the sheet and the two support members therefrom for deployment. An arch support bridges the support members and can flex to a flat condition from rolling on the reel. The carriage includes guides for moving the support members apart to a wider spacing for rolling on to the reel with the sheet flat and for guiding the support members onto the rails and for bowing the arch upwardly in the deployed position.