The sub-roof membrane is located above the insulation level and forms the second water-bearing level. The design of the sub-roof varies depending on the construction, location and roof pitch. SIA 232-1 (pitched roofs) specifies and defines three categories to ensure that the sub-roof is and remains watertight in every case. These categories are explained in this blog post.
The sub-roof
There are various design options for constructing a sub-roof. The second water-bearing layer can be installed in the form of sheets or panels. Depending on the roof pitch, rafter length and, above all, the covering and climatic conditions, there are three different sub-roof categories.
To simplify the decision, SIA 232 has defined three categories:
| Normal load | Increased stress | Extraordinary stress | |
| Reference height | <800m | <800m | >800m |
| Shocks | Overlapping or windproof bonded | Waterproof bonded joints | Homogeneously welded |
| Reservoir water | Water must be able to drain freely | <50mm | >50mm |
| Roof pitch | >15° | >15° | >5° |
| Underroof membrane | SAGER sky light cover sheet | SAGER sky 2 cover sheet | SAGER SL Plus under-roof membrane |
Underroof membrane for exceptional loads
When does my roof need a sub-roof membrane for exceptional loads?
The reference height is a reliable point of reference when determining the sub-roof. From a reference height of 800 metres above sea level, a sub-roof membrane for exceptional loads must always be selected. You can use the map from SIA 261 to determine the reference height. The roof covering is also an important indicator. The roof pitch in combination with the choice of covering determines the load category of the under-roof membrane.
Recommendation SIA Guideline 232/1
Due to the greater load on the sub-roof, a sub-roof for the next higher level, up to extraordinary loads, should be selected for rafter lengths of over 8 metres.
In the case of a sub-roof membrane for exceptional loads, it must be ensured that no screws, nails or other fasteners penetrate the membrane. To prevent this, a nail sealing tape must be installed on the membrane under the counter battens.
Conclusion
The sub-roof membrane is a crucial part of the building envelope. Damage can be counteracted by categorising it into different categories. We have a suitable product for every stress category. If you have any questions, the application technology team and technical sales staff will be happy to help.
