On slopes below 4/12, down to 2/12, the risk of leaks is great, caused by phenomena like wind-driven rain and capillary action that can make water flow uphill, or by the backup of water behind ice dams. To reduce this risk, a redundant system – called shingle underlayment by its manufacturers – is applied beneath the shingles. Shingle underlayment traditionally known as tar paper and roofing felt, is a roll product applied over the roof deck before the shingles are installed.

The term “underlayment” can cause confusion because there are other materials in the construction trade called underlayment. One grade of OSB, for example, used as a flooring sub-base is called underlayment. The OSB on a roof deck is not shingle underlayment. It is properly termed roof sheathing and must be rated by the American Plywood Association as such.

On the other hand, not all shingle underlayment is the same. There are two critically different grades: water-resistant and waterproof.

Water-Resistant Underlayment was invented to keep the roof decking dry until shingles could be applied. Applying this underlayment is called “drying-in the roof”. It was also useful as a separation sheet between the roof sheathing boards and the asphalt shingles before OSB and plywood sheets were used as roof decking. This separation was important because direct contact with resin pockets in the pine planks caused the asphalt to degrade prematurely.

Intact water-resistant underlayment sheds most of the water that falls on it, but its water resistance is temporary. As the sun degrades, the exposed asphalt the material begins to dry out, absorb more moisture, lose its strength and eventually tear. The less asphalt used to saturate the underlayment sheet during manufacture, the shorter its life. Since asphalt is the most expensive component of shingle underlayment, lower-priced materials have less asphalt and a shorter life when exposed to the sun and are also subject to severe wrinkling when wet or even just damp.

Water-resistant shingle underlayment is not warrantied by the manufacturer. It is an expendable material because much of its water resistance is destroyed during the installation of the shingles by driving hundreds of nails through it.
Until recently, only two grades of water-resistant underlayment have been available: Number 15 (standard) and Number 30 (heavy-duty). In recent years new categories have appeared known as premium and high-performance shingle underlayment. These materials are less likely to wrinkle when dampened. The CertainTeed Corporation makes a product in this class called Roofers’ Select. GAF, another manufacturer, produces one called Roof-Mate.

Waterproof Underlayment is an entirely different product that’s used in locations such as eaves and valleys that are most likely to leak under extreme conditions such as high winds, heavy rains and ice dams. This material is known as Waterproofing Shingle Underlayment. It is self-stick modified asphalt on a glass mat reinforcement. The cost is much higher than standard water-resistant underlayment because of its high asphalt content and polymer modifier. These products come with warranties against leaks and are not destroyed when nails are driven through them.

On low slopes where the risk is water running uphill, or in valleys where blockage from storm debris or ice dams can cause trouble, waterproof membrane is reliable insurance against leaks when used according to the manufacturers’ instructions. In all cases the product must be applied to a clean, dry roof deck. Examples of the variety of product types in this class of waterproof membrane:

  • CRC – Storm Tamer
  • NEI – Granular Ice & Storm Seal
  • Grace Ice & Water Shield
  • CertainTeed – Winter Guard
  • ELK – Traction Grip
  • GAF – Weather Watch
  • IKO – Armour Gard
  • Bakor – Eaveguard
  • BP EMCO – Grip Guard
  • Soprema – Lastobond
  • Etc.