The surface of a self-adhered roofing underlayment has a big impact on the application of the underlayment, as well as on future re-roofing. Thus, it is critical that contractors take a close look at the two categories of underlayments to determine which one is best for their project.

“Many contractors believe that because a product has met all the requirements of the building code that it is the right choice, but the building code represents the absolute minimum level of performance required,” said GCP Applied Technologies Residential Building Materials Sales Manager Brandon Wulf. “There are significant differences in performance, depending on how the product is constructed.”

Modified Bitumen roofing systems are a very common in low slope commercial applications. SBS (styrene-butadiene-styrene) modified bitumen systems are reinforced with fiberglass or polyester fiber matting to improve the strength of the material. SBS Modified systems use a synthetic rubber based to technology that can be used in many low slope applications, including torch down.

Rubberized asphalt membranes, are typically comprised of just rubberized asphalt and a carrier sheet comprised of a material such as HDPE. These products are typically used in steep slope applications (greater than 2:12) under a wide variety of roof coverings. These are commonly what are referred to as “peel and stick” underlayments.

An important consideration is whether a granular or smooth self-adhered roofing underlayment is a better fit for the job.Here are the features to consider when selecting a self-adhered underlayment:

Granular Underlayments:

Most granular self-adhered underlayments have a sand-like surface. They are heavy with a thickness ranging from 40-60 mils. There are four considerations regarding granular underlayments to be aware of:

  1. After exposure, the granules can become loose and spread over the surface of a sloped roof, creating a slip hazard.
  2. Granular underlayments tend to soften on hot sunny days. This creates the risk that oil and adhesive may be transferred from the surface of the underlayment onto the finished roof covering.
  3. A granular surface is very rough and uneven, making it difficult to adhere to itself at overlaps and to create watertight laps. Frequently, granular underlayments require roofers to use roofing cement to bond these overlaps and end laps to improve water-tightness.
  4. Granular underlayments are double the weight of smooth underlayments. They are usually 65 pounds compared to 30-50 pounds for a smooth underlayment roll with same dimensions. The heavier weight translates to more time and labor to carry rolls to the rooftop.

Smooth Underlayments:

Smooth self-adhered underlayments are also often referred to as film surface underlayments. Four smooth underlayment features to consider are:

  1. They easily adhere to the next membrane layer at overlaps, making it easy to form strong and watertight laps without using roofing cement or sealants. This is critical to preserving the function of the underlayment as a secondary waterproofing layer, which is one of the most important reasons for using a premium underlayment in the first place.
  2. Smooth surface underlayments have multiple levels of performance to choose from. For example, some have a thick layer of aggressive adhesive, offering the ultimate level of protection and is designed to perform even in severe alpine and coastal environments. Other smooth surface underlayments have a thinner, less aggressive adhesive layer. These deliver the protection, economy and ease of installation typical of the granular membranes without its drawbacks.
  3. When it comes time to replace a roof, smooth surface underlayments are re-roofable. Unlike granular underlayments, you don’t need to remove the existing underlayment or cut the area out.
  4. Smooth underlayments are lightweight (30-50 pounds for the same roll size of comparable granular underlayments), making it easier to haul up to a roof. This reduces labor time and costs.

Roofing underlayments serve a critical function, protecting the building from water damage. There is a wide variation in product quality and performance. GRACE ICE & WATER SHIELD fully-adhered roofing underlayment from GCP Applied Technologies pioneered the category over 40 years ago. To ensure quality roof protection, contractors have relied on GCP Applied Technologies’ original ICE & WATER SHIELD underlayment for all these years. It provides premium performance as it seals to itself at overlaps, seals to the roof deck and, most importantly, seals around the fasteners used to attach the shingles. All these features help prevent water from leaking into your home.

Take the time to examine the multiple product types out there, first studying whether a granular or smooth surfaced underlayment is the better fit.

Grace Ice and Water Shield is a registered trademark of GCP.