The initial covering layer prevents dampness of the thermal insulation and the roof truss during strong winds combined with rainfall and protects against the formation of condensation of water vapor contained in the air under the main covering.
The initial covering layer determines the degree of insulation of the entire roof covering. This degree is based on the requirements, which the roof must meet. The point of reference for determining these requirements is a non-habitable attic roof, with a simple design, located in an area with low wind loads.
The initial roof covering is made depending on the needs, with various degrees of insulation. Lowest, the basic step is obtained by laying the initial covering foil on the rafters with loose or light tension (depending on the type of material), without any stiffening elements underneath and the individual foil strips are joined in a simple overlap. Increasing the insulating power of the initial covering layer is achieved by treatments gradually increasing its tightness, until the layer is fully sealed in the form of the so-called. "Bottom roof", which consists of heat-welded roofing felt placed on full boarding. Intermediate degrees are obtained by: sealing of plants, stiffening of the initial covering foil (sheathing or thermal insulation) and by sealing the counter battens. For all treatments, the rules of ventilation must be followed as well as the recommendations of the manufacturers of materials used for the construction of the initial covering (e.g. under the "sub-roof" - there must be a ventilation gap under the boarding; stiffening the foil with thermal insulation is possible only then, when it is made of highly vapor-permeable materials).
Increased insulation requirements (tightness) undercover layers occur when:
• Nachylenie połaci dachu jest mniejsze od najmniejszego zalecanego pochylenia połaci.
• Poddasze wykorzystywane jest do celów mieszkalnych.
• Dach ma skomplikowaną konstrukcję – long rafters, additional installations (e.g. heat exchangers or heat collectors), complicated shape (e.g. eye crop or a large number of stretches).
• Budynek znajduje się w strefie o trudnych warunkach klimatycznych, that is, with increased wind strength or the amount of rainfall.
• Inwestor lub lokalne prawo narzucają specjalne warunki dla budynku.
The minimum size of the ventilation gap in the pitched roof is determined by its cross-sectional area to the direction of the ventilation air flow. This area is proportional to the area of the ventilated roof slope. For the free movement of air, it is important to precisely define both the inlet (airflow) as well as the outlet (exhaust) slots.
• Wielkość nawiewnego otworu wentylacyjnego przy okapie musi stanowić co najmniej 0,2% associated roof area, however not less than 200 cm2 on 1 meter of the width of the roof.
• Przekroje wentylacyjne otworów wylotowych szczeliny wentylacyjnej na kalenicy lub na narożu dachu muszą stanowić 0,05% the area of the entire roof.
• W pozostałych miejscach połaci dachowej wielkość przekroju wentylacyjnego musi wynosić co najmniej 0,2% roof surface, however not less than 200 cm2 on 1 meter of the width of the roof.