Roof deck protection is often referred to as an underlayment, and its purpose is to protect your roof deck from exposure to the elements that sneak past your shingles. Additionally, if, or when you find yourself in an emergency, or some shingles happen to blow off your roof, the roof deck protection will provide temporary moisture entry prevention. It differs from leak barrier, in that leak barrier is installed along eaves trough and valley areas whereas roof deck protection is installed on the entire roof surface area from top to bottom.
Underlayment is not a requirement by the Alberta Building Code nor from most asphalt shingle manufactures. For this reason it is not often installed on new home construction, or with entry-level installers. Professional roofers and the most experienced trades people will always recommend installing a good quality roof deck protection. In previous times the most common roof deck protection is a product referred to as #15 felt paper. This product type has been used for many years and is still a common product used today.
There are many different types of roof deck protection to choose from provided by a variety of different manufactures. In general there are two basic types; Felt paper and synthetic. Having anything is better than nothing; however, having a synthetic product is better than having a felt paper. When you spill a glass of water onto a sheet of paper in your office or living room you may notice that the sheet of paper crumples and wrinkles a little bit. This is similar effect occurs when moisture reaches felt paper installed on your roof system. Synthetics are more of a tarp-like fabrication, which provides a more reliable water barrier product.
It is a good practice to run your roof deck protection product across all valley areas on top of your leak barrier protection to provide backup coverage in case ice or water sneak past your valley metal in the area. Remember that if ice can carve valleys through mountains then it sure can rip its way through a roof system like a knife through warm butter.
Choosing a reliable underlayment is an important part of a quality roof system.