Roofing Underlayments

Roofing underlayments are membranes that are installed directly on a roof deck, underneath the shingles.  Their primary purpose is to provide a second layer of protection against water penetration through the roof shingles.  Shingle materials are effective at shedding water from a sloped roof, but there are circumstances where water can get underneath the shingles. In strong winds, rain can be pushed upwards on a roof and be driven underneath the bottom edge of a shingles.  The tendency for water to get underneath the shingles depends on the slope of a roof, with shallower sloped roofs much more susceptible to water penetration from wind driven rain.

Manufacturers recommend the use of roofing underlayments on roofs with a slopes in the range from 4:12 to 8:12.  Roofs with slopes less than 4:12 require special attention due to the slow rate of water run-off.  With a low slope, water has a greater tendency to flow sideways, and penetrate under the shingles through key-ways or the edges of shingles that are butted against each other.  Capillary action also helps to draw water underneath the shingles.  On slopes from 2:12 to 4:12, ice shield is used as an underlayment due to its superior waterproofing properties.  A double layer of saturated felt underlayment can also be used, however ice shield is recommended because it self seals around nails that are driven through it to fasten shingles.

Roofing underlayments offer several other advantages:

  • They provide protection from the weather during the roofing installation process.
  • They provide water protection in the event of shingle blow off in high winds, or deterioration of the shingles as they near the end of their life.
  • They protect shingles from resins from a wood roof deck that can interact with the asphalt shingle material
  • They help to smooth out bumps from unequal thicknesses of roof deck material, that can leave a tiled pattern on plywood decks, or a striped pattern on board decks.
  • They are a required component to achieve a Class A fire rating for a roofing system when installed in conjunction with most current asphalt shingles.

Types of underlayment

Asphalt Saturated Organic Felt (otherwise known as tar paper)

Asphalt saturated felt is made from a thick construction paper (felt) that is impregnated with an asphaltic substance to provide water resistant properties.  It comes in 36” wide rolls, usually with one side marked with lines that installers use like chalk lines to aid in ensuring shingles are installed in straight rows.

All tar papers are not the same!  When choosing a tar paper, you want a product that will perform well during installation and over the life of the roofing system.  Cheap tar papers are available that may not provide the desired functionality and protection.  Because they are made with a felt comprised of paper and sawdust, tar papers have a tendency to wrinkle, pucker and shrink when exposed to moisture or the sun.  These effects can cause bumps that can transfer through the shingles and cause an uneven appearance in the finished roof.

Good quality tar papers minimize these effects and meet industry standards for tear strength, pliability, material loss over time (through evaporation), and liquid water transmission.  They also provide uniform properties over an entire roll of material.  To ensure a minimum acceptable level of performance, you choice of tar paper should meet the ASTM D4869 Type I standard.

Inorganic Fiber-Reinforced Organic Felt (otherwise known as premium quality tar paper)

Tar papers reinforced with inorganic fibers are designed to significantly reduce wrinkling and distortion that can occur with regular organic tar papers.  These underlayments provide superior physical properties such as increased tensile strength, tear strength, and dimensional stability. As a performance guarantee, products in this category should meet the ASTM D6757 standard.

Synthetic Underlayments

Synthetic underlayments perform the same function as tar paper, but offer superior performance in every category.  They are made from a polymer based top layer, applied to a reinforced scrim fabric. They are stronger, more water resistant, and virtually wrinkle free when compared to tar paper alternatives.  Synthetic underlayments offer additional benefits:

  • They provide UV protection for up to 6 months.  A roof can be covered with underlayment and shingled later when production schedules allow.  This is particularly useful with new construction projects, or in emergency repairs from high wind blow-offs.
  • Many products are made with a slip resistant surface that enables installers to work faster and safer, even in wet conditions.
  • They have a light coloured surface which keeps the working surface cooler during installation on hot summer days.  A cooler crew is a happier crew, with less likelihood of installation errors brought on by fatigue.
  • They are environmentally friendly.  Many suppliers use 20% recycled material in the manufacture of these products.  Due to their lighter weight and volume, one truck load of synthetic underlayment replaces six truck loads of tar paper resulting in fuel savings and dramatically reduced pollution.  Also, synthetic underlayments are 100% recyclable.

Ice and Water Barrier Membranes

Ice and water barrier membranes provide heavy duty waterproofing and protection against leaks.  They are commonly used at roof edges, where ice can build up creating a dam that causes water to pool and travel under shingles during thaw conditions.   is also commonly used in valleys created by two intersecting roof surfaces, and around chimneys and skylights, and at dormer edges

Ice and water membranes are highly impervious to water.  They are pliable so they don’t crack, and are designed to create a water tight seal around nails that are driven through the membrane.

Ice and water barrier membranes are composed of a reinforcement layer that is impregnated and coated with polymer modified bituminous material (rubberized asphalt).  They have an adhesive coating on the under-surface that sticks to the roof deck, and a surface material that consists of sand, mineral granules, or non-slip polymer surface.  They are available in 36” wide rolls, in lengths up to 100′.

How ice damming causes leaks:

  1. Heat from the home enters the attic space, warms the roof deck, and melts snow on the roof.
  2. Water from the melted snow runs down the roof toward the eave.
  3. When the water travels past the outside wall of the home, beyond the warm living space it freezes creating an ice dam at the outside edge of the roof
  4. Over the course of a winter, the ice dam grows and extends back toward the heated living space.
  5. As more snow melts, water pools at the edge of the ice dam and travels under the shingles and leaks into the home.

Ice shield prevents leakage due to ice damming only if it covers the area from the eave upward to a point well past the transition line that separates the warm and cold regions of the roof.

An eave that is adjacent to an outside wall of a heated living space is called a “heated eave.”  The Ontario Building Code requires that for roofs with a slope less than 8:12, underlayments be installed at heated eaves  to 300 mm (11.81”) past  the intersection of the roof deck and  imaginary line drawn from the inside wall surface.  This requirement is insufficient to protect against leakage due to ice damming in the Canadian climate as evidenced by the countless “building code” roofs that  leak due to ice damming.

Experience dictates that to provide adequate protection against leakage due to ice damming, ice shield be installed at least 18” past the transition line that separates the heated and unheated regions of the roof, on roofs with slopes less than 8:12.