Getting on top of underlayments

For much of the U.S., the possibility of ice dams forming along steep-slope roofs' eaves is enough to require an ice-dam protection membrane in addition to any other roof underlayment local building codes may require. In such cases, NRCA recommends using a self-adhering polymer-modified bitumen sheet underlayment.

Applications

In areas where ice-dam protection is required (based on temperature, history and/or local building codes), an ice-dam protection membrane should extend from an eave's edge to a point 24 inches inside the interior face of a building's exterior wall. The NRCA Roofing and Waterproofing Manual, Fifth Edition recommends extending an ice-dam protection membrane 24 inches beyond the interior face of a building's exterior wall if a roof's slope is greater than or equal to 4-in-12 (18 degrees). If a roof's slope is less than 4-in-12 (18 degrees), the ice-dam protection membrane should extend 36 inches beyond the exterior wall.

Materials

Assume the decision has been made to use a self-adhering polymer-modified bitumen sheet product to make ice-dam barriers and line any valleys. The next question is what specific product to use.

The standard for self-adhering sheet materials is ASTM D1970, "Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection." This standard tests self-adhering sheet products against several criteria, including thickness, adhesion to plywood at 40 F and 75 F, and fastener sealability. Generally, products will be labeled as meeting or exceeding ASTM D1970.

NRCA has tested several materials for ASTM D1970 compliance (see "Analyzing self-adhering underlayments," November 2006 issue, page 30), but such testing is not within roofing contractors' scope of work; it is manufacturers' responsibility to verify compliance. The most practical advice I can offer is to check product labels or specification sheets to verify a material is labeled as complying with ASTM D1970.

The next consideration is what type of roof covering should be used. Most self-adhering polymer-modified bitumen sheet products are appropriate for use under asphalt shingles, wood shakes and wood shingles. But things get interesting when a roof is going to be covered with metal panels, metal shingles, concrete tiles or clay tiles. Such products can get hot under sunlight, and the underlayment will be subjected to higher temperatures during summer.

Additionally, metal shingles expand and contract based on temperature. Some self-adhering sheet products have mineral surfacings that may abrade the underside of metal shingles. This can wear away coatings on the underside of the shingles, exposing bare metal and increasing the risk of corrosion and premature failure. Most manufacturers offer a high-temperature product, and most offer self-adhering sheet products that do not have mineral surfaces.

Climate

As I mentioned, many codes require self-adhering polymer-modified bitumen sheet products in areas where average January temperatures are below freezing.

At least one manufacturer, W.R. Grace & Co., Columbia, Md., also bases some of its product recommendations on whether a building is in an "alpine/high-altitude" area. The reasons for this have to do with mountain climate and conditions.

Because mountain air is thinner than air at lower elevations, more ultraviolet (UV) radiation can strike a roof's surface and degrade the product, reducing the UV exposure time recommendations. Also, metal roof systems are common in mountain areas because of the risk of forest fires.

UV exposure time

Although the intent is to get a roof deck covered as soon as possible, there may be delays between installing a self-adhering polymer-modified bitumen sheet and a roof covering.

All self-adhering products have limited periods during which they can withstand UV exposure. Some, such as Carlisle, Pa.-based Carlisle Coatings & Waterproofing Inc.'s WIP® 100, should not be exposed to sunlight for more than 14 days; in contrast, the company's WIP 403HR can be left exposed for up to 30 days. Other manufacturers provide different recommendations.

Application temperatures

All self-adhering polymer-modified bitumen sheet products need to be applied to clean, dry roof decks. However, some manufacturers also specify an ambient temperature at the time of application.

For example, Morrilton, Ark.-based BITEC Inc.'s MAT-40™ product (for use where roof temperatures can exceed 260 F, such as under metal) should be installed when the outside temperature is 55 F or higher. The company's DEFEND™ product, which is intended for use under shingles, can be applied when the weather is cooler, but the sheet will take longer to bond fully to decks.

Product comparisons

The table that appears on pages 36-43 provides information about various self-adhering polymer-modified bitumen sheet products from various manufacturers. The table shows the manufacturer, product name and product characteristics.

The information in the table was obtained from NRCA's 2007-08 Steep-slope Roofing Materials Guide, manufacturers' Web sites and a telephone survey. The manufacturers and products listed are not intended to be all-inclusive; not all available products are listed.

Click on the image above to view the table in its entirety.

The table addresses underlayments' top surfaces. Some products have granular surfaces encompassing sand or minerals. These products provide anti-slip protection for workers walking on them. However, gritty surfaces may not be appropriate for all roof coverings.

Most manufacturers also make a smooth underlayment. These smooth underlayments usually have some sort of anti-slip surface. Some patterns are quite pronounced, while others are almost smooth. The surface texture should not affect underlayment performance, but it can make an underlayment safer to walk on, especially when the underlayment is damp or frost-covered. Because there are so many nongranular surfaces, the table uses the term "smooth" as a generic description.

The "minimum application temperature" noted in the table represents the lowest ambient temperature for application. Almost all manufacturers state the outdoor temperature should be rising.

For instance, if the temperature is 40 F and falling, you should not apply a product with a minimum application temperature of 40 F. Similarly, if a product's minimum application temperature is 45 F and the temperature is falling, you may consider not applying the product. Some products have lower application temperatures; Carlisle Coatings & Waterproofing's WIP 401LT is designed for application during colder weather. As temperature increases, the ice-dam protection membrane's adhesion also increases.

Similarly, the "maximum exposure time" represents the longest time frame for leaving a membrane uncovered. Several factors affect the length of time a self-adhering sheet can be left exposed, including weather, whether there is shade over the roof deck, etc.

All manufacturers recommend installing the primary roof covering (shingles, tile, shakes, etc.) onto a roof deck as quickly as possible. Some products have fairly short exposure periods; Huntington Park, Calif.-based Henry Co.'s Eaveguard® and Carlisle Coatings & Waterproofing's WIP 100 both have 14-day exposure periods.

At the other extreme, Valley Forge, Pa.-based CertainTeed Corp.'s WinterGuard™ has a six-month exposure period; Coshocton, Ohio-based MFM Building Products Corp.'s IB-4™ can be left exposed for a year; and Moores­town, N.J.-based Andek Corp.'s Flashband has an "indefinite" exposure period. NRCA recommends contacting underlayment manufacturers if you anticipate needing a longer exposure time.

Also, note certain roof coverings create special circumstances. For example, zinc, copper and COR-TEN™ steel all result in higher roof temperatures, especially in desert or alpine climates.

And product lines change. For example, Fernley, Nev.-based Polyglass® USA Inc. lists Polystick™ P on its Web site; NRCA was told the product is no longer being produced, so the table does not list it. On the other hand, Cleveland-based The Garland® Co. Inc.'s new product, HPR Aqua Shield, is not currently listed on the company's Web site. In addition, GAF Materials Corp. and Elk Corp., both based in Wayne, N.J., are reviewing their product lines to eliminate duplication following the companies' merger in 2007.

I recommend you contact underlayment manufacturers if you have specific questions about individual products or applications.

Choosing a product

The information in the table should help you make an informed choice among underlayment products. In some cases, the choice hinges on the type of roof covering being used; tile or metal roof coverings may require different underlayment products than asphalt shingles or wood shakes. In other cases, the choice depends on your preference for a smooth or granular surface.

Barring other mitigating circumstances, there does not appear to be a need to use a more highly engineered product than is recommended. For example, there is no reason to use a product under asphalt shingles if it is primarily intended for use under metal.

For more information about NRCA's 2007-08 Steep-slope Roofing Materials Guide, log on to shop.nrca.net or call NRCA's Customer Service Department at (866) ASK-NRCA (275-6722).

James Munro is NRCA's technical writer.

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