Underlay, Underlay

A roofing membrane (or undertension) or underlay (underlay) is an additional measure for rain protection, against drifting snow and dust below roofing, i.e. the roof covering.

Depending on the location and fastening, a distinction is made between:

• Underlays (USB)
• Underlay membranes (UDB)

More about underlays and roofing membranes

Underlays / Underlay membranes are the second water-bearing layer below the roofing. Underlay membranes / roofing membranes are water-impermeable membranes, they are windproof, but vapour-permeable (as low as possible sd value).

Distinction:

• Underlays (USB) are tensioned without a flat underlay (fastening, e.g. on the top of the rafter) or laid with a planned sag.
On
• the other hand, a roofing membrane (UDB) is when the membrane rests on the insulation or underlay, e.g. on dimensionally stable thermal insulation.

Notes on planning

Underlays must be rear-ventilated on both sides to prevent moisture penetration of the battens and insulation.

Exact properties and requirements of an undervoltage are specified in the ZVDH's "Leaflet for Roofs, Underlays and Undervoltages".

☛ ZVDH rules for shortages and undervoltages

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