A shortfall is an additional measure for rain protection under roof coverings. It is the second water-bearing layer under the roof covering. It can be made of roofing membranes or roofing panels. Roofing membranes are water-impermeable, vapour-permeable membranes that are laid on a flat base made of wooden formwork, wood-based panels or dimensionally stable insulation. Under-roof panels are water-impermeable, permeable wood fibre or fibre cement boards and are laid freely spanned over the rafters or on dimensionally stable insulation. In the case of grooved panels, further sealing of the joints can be dispensed with. More precise properties and requirements of a shortfall are specified in the ZVDH's "Leaflet for Roofs, Underlays and Undervoltages".
Source: bauwion
