Onduline sheets are manufactured by impregnating organic cellulose fibres with bitumen under high temperature and pressure. The sheets are pre pigmented with special purpose resins. The impregnation of colour into the sheets is done using patented techniques that enable the sheets to retain their colour over a longer period of time even under harsh conditions. The patented manufacturing process ensures uniform bitumen penetration into the sheets providing an excellent dimensional stability to the product and thereby preventing delamination.