When designing the ventilation gap, calculations are made for a roof strip with a width 1 meters. The cross-sectional area of the slot is equal to the length of the rafters (expressed in cm) multiplied by 0,2. For example, for rafters in length 8 mb, the cross-sectional area of the ventilation gap will be 160 cm2. However, since the minimum area must not be less than 200 cm2 on 1 meter of the width of the roof, should be taken as the cross-section of the ventilation gap 200 cm². It follows from here, that the height of the ventilation gap above the insulation at its inlet must be at least 2 cm. This condition dictates the dimensions of the gap formed between the rafters with a length to 10 mb. With rafters longer than 10 m, the ventilation gap should have a greater height, e.g.. for 11 meter rafters will be 2,2 cm.
• The upper ventilation gap is created by the use of counter battens. When calculating the area of the ventilation gap, the loss of surface caused by the thickness and width of the counter-battens should be taken into account. In roofs with a rafter length up to 10 m, it is recommended to use an upper ventilation gap with a height of approx 2,5 cm. The area of the outlet openings of the ventilation gap under the ridge for an exemplary roof with rafters in length 8 mb will be 40 cm2 on one side of the ridge na 1 meter of the width of the roof.
• If a second is designed in a pitched roof – lower ventilation gap under the pre-mating layer, its size must take into account the dimensions of the rafters, between which it is conducted. Assuming, that wooden structures take an average 16% cross-sectional area, easy to calculate the minimum height of the gap – it will amount 2,32 cm and rounded 2,4 cm. A greater height of the gap is required only for rafters longer than 10 mb. In construction practice, keeping the exact dimension 2,4 cm is very difficult. In roofs with a rafter length up to 10 m, it is recommended to use a lower ventilation gap with a height of approx 3 cm.
The roof is ventilated in the place of the valley rafter through a ventilation duct under the basket. The roof foil provides a tight connection of adjacent roof slopes thanks to an additional trough-shaped strip. First, the lowest strip of roofing foil, it is placed under tension on the strip of the eaves plate. At the gable wall, the roof foil extends beyond the last rafter, up to the outer edge of the roof. In the ridge, the foil is finished with approx. 5 cm below the vertex. For small slopes, approx. 10 cm overlap of subsequent rolls of roofing foil.