Residual water

In principle, residual water can remain on the roof around the drain even with primary roof drains that have no damming element. According to the Flat Roof Guideline, section 2.2 Roof pitch, fall (5), ponding can occur on flat roofs. It is assumed that on roofs with a standard fall the residual water dries off quickly and therefore cannot have a lasting effect on the waterproofing.

This is not to be regarded as a defect, since it can only be avoided to a limited extent due to the design and the corresponding material thicknesses of the flanges. Where necessary, residual water can be avoided by means of a planned fall arrangement, in which the drainage point is levelled, lowered or shaped in such a way that the inlet point of the drain forms the lowest point.

Ponding can be reduced on site by means of suitable measures. Ponding around primary roof drains may have the following causes in particular:

- No fall or insufficient fall towards the primary roof drain.
- Soiling around the primary roof drain.
- Outlet pipe of parapet drains laid with a counter-fall.
- A built-up flange construction that is not recessed (e.g. loose flange).

Improved water discharge, especially in the valleys between sloping surfaces, can be achieved for example by drainage layers, an additional fall layer, a tapered insulation layer (so-called "crickets") or by inclining the load-bearing structure.

- Flat Roof Guideline, section 2.2 Roof pitch, fall: "Even on surfaces with a pitch of up to 5% (approx. 3°), ponding can occur as a result of deflection and/or permissible tolerances in the evenness of the substrate, the thickness of the materials, overlaps and reinforcements."

- DIN 18531-1, 6.3.1: On roofs with a fall of up to 5%, ponding is possible. A fall of 1% in valleys corresponds to application class K2.