The gravel fill on flat roofs is primarily used to protect against wind suction, i.e. protection against wind forces taking off. The following options for wind suction protection can be considered when planning flat roofs:
Mechanical fastening:
If the roof does not receive any additional load, the insulation must be secured against wind suction in addition to the roof waterproofing membrane. This can be secured by mechanical fixation (dowels) in the substructure. However, bonding to the substrate is also possible, either with the help of an approved industrial adhesive or by hot bitumen on bituminous vapour barriers. If a flat roof is designed with an additional load and without mechanical fixation or gluing, care must be taken to ensure that the construction progress is carried out quickly and taking into account the weather conditions and that the additional load is applied immediately after completion. Bonding or fixing the panels is safer.
Gravel fill:
The gravel fill as an additional load is usually applied in a minimum layer thickness of 5cm. The grain size is 16/32mm. In the case of fills, it is possible that drifts occur in areas of strong wind forces, so that the additional laying of slab coverings is recommended in edge and corner areas.
Image: Gravel fill in the edge area (baukobox)
Green roof structure:
A green roof structure (extensive or intensive) against wind suction forces is dimensioned in such a way that the dead weight of the vegetation substrate corresponds to a 5cm thick gravel fill when dry. (approx. 100kg/m2) Especially in the "growth phase" of the greening, additional measures may be necessary to secure the position (erosion control fabric, etc.). It should be noted that roofs made of lightweight constructions may be subject to less stress and that the green roof structure may not be sufficient as an additional load. Additional mechanical fixation may be required here.
The "Central Association of the German Roofing Trade" provides extensive guidelines on this subject:
baukobox.de
