It works as clothing for the mountains. On the one hand, it prevents moisture from reaching you, and on the other hand, it allows steam to escape. We are talking about a vapor-permeable foil that forms a so-called additional waterproofing layer on the roof. This is used to catch and drain water precipitation that exceptionally penetrates under the folded covering.
If you are building a house, you should consider the quality of the foil you will get. If you look at units of a thousand crowns when you buy it, it may happen that after twenty years circumstances will force you to remove the roof tiles and replace the foil. Otherwise, your roof will remain functional for fifty years.
“Additional waterproofing film is very important on the roof, but unfortunately sometimes very underestimated. It protects the attic spaces from moisture, which in extreme cases accidentally penetrates through the folded covering. In addition, it also helps against penetrating moisture in the event of a failure of the roof sheathing. However, if this layer is laid inexpertly and the attic space is even poorly ventilated, moisture will begin to accumulate there, which can get into the thermal insulation. A high-quality foil, after being laid correctly, will remove moisture from one side and not let it back in from the other,” says professional roofer Jiří Liška.
According to him, better foils can last up to 50 years, while cheaper ones have a life span of 10 to 20 years according to previous experience. Michal Ponec from the company Betonpres, which manufactures roof tiles and roof accessories, calculates that the share of the price of the foil on a normal roof, including labor, comes to roughly 5 to 10 percent of the total price of laying the roof. “Cheaper foil for an ordinary roof costs around ten thousand crowns. The more expensive one is up to twenty thousand,” says Ponec, according to whom it is worthwhile not to save on this material.
“Some investors, when cutting the cost of a house, out of ignorance save here, but this is not a good solution in our opinion. In any case, this additional waterproofing layer should be laid by a trained roofer or construction company. Attention must be paid to the details, which are often the weakest point of the roof sheathing. For example, the applied film should in most cases be covered on both sides within four weeks in order to avoid degradation caused by UV radiation. After prolonged exposure to the sun, the membrane decomposes and may lose its original properties over time,” points out Ponec.
A highly discussed topic in the field in recent years is how to connect an additional waterproofing layer to the chimney. According to current legislation, there must be no flammable material within five centimeters of the chimney. In order for the foil to be functional, it must be connected waterproof without a gap.
“In that case, there are endless arguments between chimney sweeps and roofers. At the same time, the solution lies with house designers, who have to deal with it and know how to deal with it,” explains Michal Ponec.