You can’t judge a book by looking at the cover and you can’t judge a roofing material just because it is sold in a roll. At first glance roll roofing and saturated felt roof underlayment look very similar. But the similarity ends there.
Roll roofing is a common product chosen to cover low slope roofs. It is a rather inexpensive roofing material often installed over #15 felt underlayment by DIY homeowners. While it can, and has been, used for residential roofs, it is primarily used on utility structures. It is usually sold in 36-inch wide by 36-foot long rolls. Common roll roofing colors are white, brown, and black. Some roofers refer to roll roofing as "90-pound felt" since a standard roll weighs close to ninety pounds.
The earliest forms of roll roofing consisted of sheets of felt canvas, or cloth that were saturated with asphalt, then rolled flat along a low-slope with the material's long edge parallel to the building eaves. Modern 90-pound asphalt roll roofing is built of either mineral-coated fiberglass-reinforced mat or an organic-mat material very similar to asphalt roof shingles. Roll roofing is coated on both sides with asphalt and its exposed side is protected with mineral granules.
Single-coverage roll roofing usually has a 2-inch lap with exposed nails. With two layers of protection, double-coverage roll roofing is acceptable for small roof areas and can be used on roofs as shallow as 1:12. When double-coverage roll roofing is installed, nails are concealed under the lap joints that are then sealed with asphalt lap cement.
DIY homeowners, who plan to self-install a roll roof, should know the expected life span of this type of roof. Because asphalt roll roofing is generally used on low slope and nearly flat roofs, it has an anticipated wear life of less than ten years, often just five years. Granule loss and exposure of "bald spots" or cracking are common signs that roll roofing is at the end of its life. Roll roofing can have a longer life if installed according to the product instructions, nailed at correct intervals, and the seams are properly sealed. It's also important to install roll roofing over a smooth, solid roof deck. If the deck sags, rain ponding on the roof surface will reduce its life.
Between the shingles selected to cover the roof and the bare roof deck is the underlayment— a very important component of a roof system. Underlayment is an essential component to a roof’s ability to stand up to the elements, protect a home’s interior, and prolong its service life. There are many underlayment products available on the market and it is crucial to know which one will be best for the specific roof system being installed.
For many years the most commonly used product has been asphalt-saturated felt underlayment. Although it is a reliable product, saturated felt underlayment may be used less in the future, eventually giving way to the newer synthetics. Asphalt is a product of the process of refining crude oil. Less asphalt is being produced because an allocation program was established to regulate the amount of asphalt available for use to manufacturer shingles and underlayment.
Various types of rubber-like materials are also used as underlayment and are referred to as “rubberized asphalt.” These typically have adhesive on one side, which is protected by a peel-off membrane, making them self-adhering. The rubber-like qualities of this underlayment make them self-sealing, meaning that they seal well around fasteners, such as staples and nails.
Polymer-modified bitumen, or “mod-bit”, is a generic name applied to various products. To improve characteristics such as strength and elasticity, bitumen is modified using polymers, which give it plastic, or rubber-like properties. Polymers are materials made of molecules that are custom-designed to give the underlayment properties to increase resistance to damage and deterioration.
Synthetic underlayment is becoming very popular because it offers resistance to fungal growth and, since it doesn’t absorb moisture, is wrinkle-free. This product is lightweight, high strength and typically non-skid. It is very resistant to UV damage and some can be left exposed to weather from six months to a year. Fastening is generally done with plastic caps or roofing nails. The use of staples is discouraged because synthetics are not self-sealing.
Although underlayment is the unseen component of a roof system, the protection it provides is essential to maintaining the integrity of a roof. Choosing the right roof underlayment is just as important as the type of shingle selected.
Yes, they both come in rolls and look very similar, but can roll roofing be used as underlayment? Can the strongest underlayment be used as a roof covering? The answer to both questions is a resounding “NO.” The more homeowners understand roof components, the better they can make an informed decision.
Atlas Roofing has long been the world’s largest producer of roll roofing and saturated felt underlayment. To get the most details, do your research at www.atlasroofingcom.