TS EN 13859-1

TS EN 13859-1 is the standard that addresses the characteristics of flexible sheets used as underlays beneath discontinuous roofing. In practical roofing terms, this underlay forms a secondary weather-control layer below the outer covering, helping the roof system manage wind-driven rain and incidental water that may pass through joints or overlaps in the primary covering. The standard’s focus is the definition of the relevant characteristics for that specific product category, so the underlay can be described and compared in a consistent technical way.

The role of an underlay in discontinuous roofs

Discontinuous roofing covers systems made from separate units such as tiles, slates, or similar elements, where the outer layer is not a continuous watertight membrane. In that context, the underlay is a supporting component of the roof build-up rather than the main roof covering. General technical practice treats it as part of the waterproofing family because it contributes to moisture management, temporary resistance to water ingress, and protection of the roof structure beneath the visible covering.

What this standard frames technically

• It defines the characteristics of flexible sheets intended for use as underlays.
• Its scope is limited to products placed under roof coverings on discontinuous roofs.
• It helps align terminology and performance description for roof underlay products.
• It references related test standards that are used to assess the relevant characteristics.

Standards of this kind are generally used to organise how a material is specified rather than to describe one single roof build-up. For underlays, the important technical ideas include resistance to water penetration, mechanical strength, ageing behaviour, and how the sheet performs as part of a layered roof system. Although this TS EN 13859-1 edition has been withdrawn, it remains a useful reference for understanding the technical category of flexible underlay sheets and the distinction between the underlay and the external roof covering that it supports.