Because a steep flat roof is exposed to moisture from the outside due to rainfall, and from the inside by the water vapor flowing through the partition to the outside, therefore, over the living quarters of the attic, ventilated slotted flat roofs should be built instead of solid flat roofs. Ventilation of the flat roof is possible after creating an air gap between the roofing and the upper surface of the thermal insulation.

According to the DIN standard, the minimum areas of air gaps in ventilated slot roofs, with a slope of up to 50 °, converted to the unit area of ​​the FD roof slope [cm2] are:
• on the air inlet at the hood FE = 0,002 FD

• at the air outlet at the ridge FA = 0,0005 FD

where:

FD = L [cm] x 100 [cm] – FD [cm2],

L – distance between the eaves and the ridge.

The symbol F denotes the total area of ​​the outlet slots, in the ridge in both roof slopes. The cross-section of the joint F over the entire area of ​​the roof slope should meet the condition FL ≥ FE. In addition, the minimum thickness of the FL gap between the eaves and the ridge should be 2,0 cm and min. FL = 200 cm²/mb.

Gap areas are given (in cross section) are minimum sizes. the use of which will allow the complete discharge of water vapor through the air stream from the roof gap to the outside, except for the roof area above wet rooms, e.g. bathrooms, kitchens.