The flat roofs can be equipped with two types of drainage:
• Odwodnienie zewnętrzne – Water is discharged through gutters located on the edge of the cornice or eaves and downspouts located outside the building.
• Odwodnienie wewnętrzne -Odprowadzanie wody odbywa się korytami ściekowymi utworzonymi na powierzchni stropodachu do wpustów dachowych i rur spustowych umieszczonych wewnątrz budynku.
On flat roofs with external drainage, there is a risk of re-freezing in the drain pipes of melted water from snow lying on the surface. To avoid this phenomenon, it is more advantageous to locate downpipes inside the building. This is especially important in the case of flat roofs with the smallest slopes of the drainage layer.
Conditions for locating drain holes:
• Droga spływu wody jest możliwie krótka i równa na całej powierzchni dachu.
• Spadki poszczególnych połaci są zbliżone i możliwie niewielkie.
• Przebieg rur spustowych wewnątrz budynku odbywa się bez odcinków poziomych.
Nominal diameter of drains and downspouts in flat roofs depends on the area of the drained roof and the amount of rainfall.
For example, during rainfall 300 l / h these diameters are:
o 70 mm for a roof area of not more than 57 m2
o 100 mm for a roof area of not more than 150 m2
o 125 mm for a roof area of not more than 270 m2
o 150 mm for a roof area of not more than 443 m2.
Each flat roof should be equipped with at least two rainwater outlets or with one outflow and overflow. With large slopes of flat roofs, the number of outflows is increased, due to the keeping within reasonable limits the length of the water drainage routes and the height differences related to the required slope of the roof.
In order to avoid leading the gutter channels through the slot, the roof surfaces adjacent to the expansion joint are drained separately.
For internal drainage, Drain troughs should be moved apart at least 1,0 m from the walls adjacent to the flat roof. A diameter flattening is necessary for the inlet funnel 1,0 m.
Waterproofing should be installed under the terrace surface, which protects the lower layers from leakage of surface water. The waterproofing should be rolled up onto the walls 15 cm. The eversion should be gentle, without kinking the insulation at right angles – a bumper in the form of a triangular wedge is used for this, placed in contact with the wall.
The waterproofing layer should have a slope of min. 1,5 – 2,0 % towards the inlets. On the waterproofing layer, it is recommended to use an additional concrete screed or mats to protect against mechanical damage.