Due to leakϊness of a roof covering a watertight foil is often suspended at a distance below the roof covering when, due to inter¬ nal isolation, no possibility exists for making the roof covering tight from the inside. At locations of the roof where trimmings are posi¬ tioned for e.g. roof windows or chimneys, and where the passage of the air from the air layer below the foil and to the roof covering accordingly is prevented, a valve must be arranged in order to avoid condensation of water which may seep through the layers in the roof. Accordingly, the invention relates to a valve as indicated in the opening clause of the claim and the features characteristic for said valve are defined in the characterizing clause of the claim.

The valve is being mounted at the required location by adhering the valve to the upper surface of the foil and then by cutting away the covered area of the foil within the valve. Besides creating in this way circulation from the lower side of the foil to the roof covering through the leakages of which the air then can escape into the atmosphere ^" , the valve will due to the construction ^' and the way of arrangement thereof, effectively obstruct rainwater, which streams downwardly along the foil, where the valve is arranged. Accordingly, the water will only flow past the valve. A valve according to the invention will be explained with reference to the drawing, on which

Fig. 1 shows an embodiment of the valve, and wherein the valve shown perspectively consists of an obliquely cut weather- resistant plastic tube (10) having a collar (11 ) which at the side facing away from the tube has been provided with a weather-resistant adhesive. Fig. 2 illustrates the valve arranged in a roof covering where num¬ bers (10-11) for the sake of easiness have been united to number (4). The valve (4) is mounted on the foil (3) below the roof covering (6), but above the isolation layer (2) membrane (7) and internal covering (1 ).

The valve may be produced in several sizes, the one here shown being suitable, seeing that more valves may be mounted ac- cording to need. For instance, two valves mounted respectively above and below a skylight, chimney of the like breach will be sufficient for maintaining the structure free of condensed water and to provide a natural pressure equalizing .