An underlay is required under slates and tiles on boarded roofs, whether the sarking boards are in sheet or plank form to carry any moisture from condensation or wind driven rain down to the eaves to drain safely away. Vapour permeable underlays are now just as cost effective as non-permeable or High Resistance (HR) types.
In sheet forms of sarking, the vapour permeability is not considered as the roof cannot ‘breathe’ through the boards. In planked applications, providing that there are sufficient and regular gaps between the planks, then the permeability of the underlay is still useful. Leaving these gaps is generally not recommended where slates or tiles are being nailed directly to the sarking boards because of missing the board or splitting the board edge with the nail.
