In an inverted flat roof, the waterproofing is placed below the thermal insulation made of extruded polystyrene panels. In this arrangement of layers, the waterproofing layer is protected against temperature changes and mechanical damage, while acting as a vapor barrier. The top pressure layer of gravel protects the thermal insulation from being blown up by the wind and protects the extruded polystyrene from UV radiation.
In a flat roof with an inverted layer system, water flows down several levels. A small part leaks at the joints of thermal insulation boards up to the level of waterproofing, the largest part of the water flows down the upper surface of the thermal insulation. During heavy rainfall, water also flows over the top layer of gravel. External drainage is so complicated, that it requires the use of a cover with openings allowing water to flow out of all layers.
In the exemplary flat roof, the ceiling beams are extended above the wall, which is additionally emphasized by the glazing between them. In order to obtain slopes on the flat roof slopes, wooden joists with variable height perpendicular to them were used above the floor beams. lowest – square section of joists, determines the level of the drainage roof outlet.
Sheathing of the structure made of waterproof plywood is the base for the waterproof layer of heat-weldable roofing felt. The roofing felt is rolled up on the edges on the attic surrounding the flat roofs, made of pressure-impregnated wooden square timber joined in a wreath.
Above the waterproof layer there is a thermal insulation layer made of extruded polystyrene, a filter layer with fleece and a pressure layer made of washed gravel. The edge zone of the flat roof is protected against blowing thermal insulation boards with two rows of concrete slabs.