A sarking membrane is a crucial detail in a sheet metal roof

A sheet metal roof is airtight. Therefore no moisture can escape. In the substructure, it must be ensured that the moisture finds a way out. At the same time, the influx should be reduced to warm air and moisture, since the air cools up on sheet metal and creates condensation water.

Sheet metal must be shielded in the substructure

It does not matter if a sheet is mounted without or with insulation when it comes to moisture transposition. In case of incorrect design, almost always a problem with condensation arises. This is due to the air impermeability of the sheet and to the other on the temperature difference of the sheet for air temperature.

For both causes, which lead to a wet and dripping tin roof, there are work-based solutions. In the substructure of the sheet metal roof, a underlying ventilated underlying lines is attached between the rafters. It takes over the following tasks:

  • Directs the moisture and warm air to the roof ventilation points
  • Prevents rising warm air from hitting cool sheet metal and condensing

Sheet metal reacts extremely to temperatures

More than with other materials, the humidity in the attic and warm air must be kept away from the roofing.

In addition to being airtight, sheet metal has the property of cooling or heating up significantly, depending on the external conditions. This often results in fluctuating and very large temperature differences, which physically and thermally lead to "extreme" conditions.
The roof membrane should not be confused with the vapor barrier or barrier that is attached to the inside of the insulation facing the attic. The task of the sarking membrane is to block the direct circulation between the loft insulation and the underside of the sheet metal roof directly under the sheet metal (with ventilation duct spacing) and on the side of the insulation facing away from the attic (also with ventilation duct spacing).

A problem when installing the roof membrane can be a very large distance between the rafters. Moisture only flows off the film when it is permanently tensioned. When water pockets and bumps form, the membrane becomes susceptible to wind and, in the worst case, can tear sooner or later.

Tips & Tricks Sheet metal coated with non-woven fabric is now available in stores. With these you can do without an underlay if necessary.