What does the watertightness of the roof surface have to do with the slope? Whether there is a slope or not, a waterproofing should always be tight. The slope is there so that no water stands on the roof surface, and that is only achieved from a 5% slope onwards. Water is allowed to stand on the roof. It is only an indication that standing water on roof surfaces with less than 5% slope cannot drain and can lead to puddling.
When planning a proper slope, it must be distinguished whether this should be executed according to the rules for waterproofing (Flat Roof Guideline) of the German roofing trade or according to the waterproofing standard DIN 18531: Waterproofing of roofs as well as balconies, loggias, and corridors. Due to differences in the regulations, planners should determine or agree in advance according to which specifications the service will be provided.
Due to deflection and/or permissible tolerances in the flatness of the substrate, the thickness of materials, overlaps, and reinforcements, puddling can occur even on surfaces with a slope of up to 5% (approx. 3°). If standing water can damage the following build-ups (e.g., slab coverings in mortar bed), adequate water drainage must be ensured (i.e., planning for more than 5% slope).
In the waterproofing standard DIN 18531 it says about the point roof pitch / slope:
"The waterproofing should, except for intensively greened roofs with retention irrigation, be planned and executed in such a way that precipitation water cannot stand for a long time on the waterproofing layer. For this purpose, a minimum slope of 2% should be planned."
"For roof areas with a pitch of up to approximately 5% (3°), puddling can occur due to permissible flatness tolerances, the deflection of the supporting structure, existing counter-slopes, and due to unevenness at membrane overlaps and reinforcements. If puddle-free conditions are to be achieved, a slope of more than 5% must be planned."
Furthermore, DIN differentiates between two application classes (standard execution [K1] and higher quality execution [K2]):
Application class K1: The minimum slope is 2% – with the possibility of deviating from this recommendation in justified cases: "Roofs of application class K1 can also be planned without slope if the choice of waterproofing meets the requirements of application class K2."
Application class K2: The minimum slope is 2%, at valleys at least 1%. For used roofs with plastic membranes (minimum thickness 1.5 mm), e.g., the type of protective layer must be adjusted. In addition, for intensively greened K2 roofs with retention irrigation up to 100 mm, a lower planned slope is permissible provided that this is taken into account in the choice of materials and execution method.