Roof underlayment acts as water barrier installed underneath a metal roofing system. Essentially, it is a roof underneath the roof. It protects your roof from condensation that may form underneath the metal due to the differences in temperatures in the attic and outside.
Synthetic underlayment became popular for use in metal roofing systems because of its strength and water barrier properties. However, there was one major fault with many synthetic underlayment systems used in metal roofing. Most of the systems acted not only as a water barrier, but also as a vapor barrier, which would result in moisture being trapped underneath the underlayment. The moisture would cause the roof deck to rot eventually destroying your roofing deck.
Exposed Roof deck with rotted boards that have just been replaced
The roofing deck on this shed was originally covered with TAR paper, an inexpensive, but not very efficient roof underlayment commonly used in conventional roofing. The roofing deck became damaged and rotted due to the use of inferior roofing underlayment that did not provide adequate water barrier protection nor allowed the roofing deck to breathe. The trapped moisture had no way of escaping and it caused the rotting of the roof deck.
The Roof deck damage on this roof required some very involved carpentry work
In order to combat the problem of rotted roof decks due to the use of vapor barrier underlayments, Roofing Manufacturers have introduced a new product, which offered the same strength found in the conventional roofing underlayments, yet it would not act as a vapor barrier. Therefore, the roof deck could now breathe because excess moisture would no longer be trapped underneath the underlayment.
Breathable Metal Roofing underlayment enables roof deck to breathe
If you are involved in any type of DIY roofing project, then we recommend you spend the extra money and install the breathable roof under layment for your metal roofing project. We make this recommendation based on the fact that any roof deck that is not properly ventilated can fail even with the best roof protecting it. Choosing a reliable breathable roof under layment for your project will make it easy for you to walk on your roof. It will prevent the tearing associated with lesser grade underlayments, and it will help your roofing deck breath once the roof is installed. The use of the correct roofing underlayment is no substitute for the proper roof ventilation, but it is the first important step in preserving the dryness of your roof deck.