Onduline under roofing system is a unique roof underlay system utilizing Onduline BT range and Onduline Classic products, below mechanical tiles, barrel tiles or slates. It creates an effective independent secondary weatherproof roof that allows to install tiles and slates safely and rapidly with a substantial cost reduction. In other words, this revolutionary system allows the construction of a proper waterproof and ventilated roof in conditions not usually permitted to tiles.The under roofing system is suitable for most types of clay or concrete tiles, natural and fibre cement slates.