MANUAL

Durable roof is a dry one. In order to ensure security, effective hydro-insulation is not enough. So that to ensure proper carrying of excess moisture and water vapour to the outside and consequently prevent from even small dampness one shall ensure proper roof ventilation.

On sloping insulated roof with functional attic the most popular solutions are:

  • Boarded roof with initial covering
    Double roof ventilation: beneath boarding and above initial covering. Initial covering and boarding should be open in the ridge area so that to ensure proper air circulation under the boarding and consequently carrying the moisture outside the thermo-insulation area. Between boarding and thermal insulation there is the necessity of forming the venting slot.
  • Roof without boarding with initial covering
    Double roof ventilation: beneath and above the initial covering. Initial covering should be open in the ridge area so that to ensure air circulation above thermal insulation and consequently proper carrying the moisture outside its area. Between foil and thermal insulation there is the necessity of forming the venting slot.
  • Roof without boarding with initial covering of highly vapour-permeable membrane
    Single roof ventilation. Low diffusive resistance of roof membrane results in unconstrained carrying the moisture by initial covering to the outside. The initial covering in ridge area doesn’t need to be open owing to what we don’t unseal the ridge preventing it from atmospheric precipitation. The lack of additional venting slot has a beneficial influence on energy saving (used for attic heating) – thermal insulation isn’t cooled by circulating air.

Asphalt-impregnated roof paper

Very popular method for initial covering was and still is bituminous roofing paper laid on boarded roof. Considering its high diffusive resistance there is the necessity of proper roof ventilation that will prevent from accumulation of moisture under the boarding. The excess dampness beneath boarding may result in vapour condensate flowing down on thermal insulation. For this reason it is so important to consider the venting slot. Moreover, its size shall correspond to roof specificity. It means that the size of venting slot mainly depends on the length of roof sweep that is the length and width of rafter span. Often the ventilation itself as well as its quality will be reinforced by accessories used for roof ventilation. The data concerning the slot size and its calculation method is often presented in assembly handbooks of roofing producers. Unfortunately, in practice such bookish slot design is rather rare.

When the venting slot is properly designed there is still the case of its proper realisation. Again the practice shows that the use of strings, threads etc. which forms the distance between wool and boarding is rather a short-lived method. With time such distance undergoes ageing which effects we can see after few years – trusses don’t keep the distance and untie flexible wool. The blocking of ventilation area takes place, moisture and water vapour have impeded outlet possibility. Such situation may soon result in roof decay. It should be stated that forming of venting slot is rather toilsome, extends the work duration and increases its cost. Moreover, boarding carries with itself the increase of cost of materials used. To sum up, such solution based on boarding and roofing paper is costly and toilsome. Moreover, for its assembly specificity it increases the risk of faults in assembly. Taking abovementioned aspects into account as well as roofing paper price, weight and comfort in use, one can see gradual departing from boarding and roofing paper in sloping roofs and increase of importance of alternative roofing forms, that diminish diffusive resistance of system.


Low vapour-permeable roof foils

Low vapour-permeable roof foils have revolutionised initial covering market in the sixties of last century, mainly because of their relatively low price in comparison to roofing paper and low weight that resulted in improvement of working conditions. Those were relatively modern products that have ensured the proper realisation of works. For a long time they were one of few alternatives to roofing paper. At the same time other option were foils and highly vapour-permeable membranes that needed more time to become a standard due to their high price.

One of the reasons why low vapour-permeable foils become less popular for initial covering was the fact that faults in assembly had often taken place. These faults connected with the lack or completely incorrect venting slot realisation have influenced those materials’ reputation. The increase of popularity of lofts’ conversion was also important. Long ago when attic conversion wasn’t so common regardless of covering type the moisture in attic was of no importance. In times when loft conversion became popular, the lofts with initial covering with classic method (roofing paper, low vapour-permeable roof foil etc.) turned out to have the roof barrier infested with wet rot and advanced decay. In such cases the intervention in roof structure is necessary. The most common evaluated reason of attic and roof dampness was improper ventilation or even its lack. It also proved the importance of work dedicated to design and forming of correct roof ventilation along with significance of products used. Today it is commonly known that every type of roofing functions well provided that it is properly designed, realised and used (e.g. it is protected from elements coming under roofing that could impede proper ventilation). Factors that differ the roofing nowadays are: man-hour, weight, price (the cost of initial covering or money saving when there is no initial covering) and their heat effectiveness dependant on ventilation type (double ventilation above and under the initial covering will cause the attic to be cooled more than in case of single ventilation placed above initial covering).


Highly vapour-permeable roof membranes

The materials that have undoubtedly revolutionised the market are highly vapour-permeable roof membranes also called diffusive open membranes, initial covering membranes and sometimes highly vapour-permeable roof foils. Thanks to low diffusive resistance those membranes perfectly enable vapour diffusion and protect the attic from water getting in.

They don’t require venting slot because they virtually don’t form resistance to water vapour. Owing to this fact, the surface of thermal insulation isn’t cooled by slot previously formed beneath initial covering. Moreover, the membranes are relatively light, they facilitate assembly, decrease work costs and don’t require boarding. Depending on type, they ensure even extreme resistance to column of water and proper durability. These are the products that at relatively affordable price enable attic and roof protection for long years, ensuring the durability of loft along with its interior.

These products are diversified depending on: production technology, the type of fibre used, reinforcing additive (e.g. resistance to UV), the type of water-proof coating, layer quantity, lamination type. All these factors diversify the quality of final products, their price, durability and functionality for roofer and investor. Both of them wish that the work proceeds fast and smoothly and that its effects last for long years without damage. Now lets look closer at roof membranes.


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