Steep-Slope Roof Systems, Part 3

We have previously discussed some of the components of a successful steep-slope roofing system. In addition to minimum slope ratio, sufficient ventilation and proper application of metal flashings, selecting the right underlayment is literally and figuratively “the foundation” of a well-designed and properly installed steep-slope roof system. While organic asphalt felts were popular products for this application in the past, they are no longer the preferred underlayment for steep-slope roof systems.

Over solid wood substrates, a self-adhered modified rubber sheeting generally referred to as an “ice and water shield”, is the proper underlayment. The product is typically manufactured in rolls 36 inches wide by 33 feet long with a plastic “facer” applied to the adhesive-coated underside of the sheet. As the modified sheet is rolled out, the facer is manually peeled away, and the sheet adheres firmly to the substrate.

The modified underlayment offers several advantages over felts. As the underlayment is adhered to the substrate, it provides a secondary waterproofing feature in the event wind storms dislodge a piece of the roof system or any piece of flashing.  And when this modified underlayment is penetrated by a fastener, it “self-seals” around the penetration, maintaining a water-tight condition.

Installation of the underlayment begins at the eave of the roof, prior to installation of any flashing. Succeeding sheets overlap the preceding sheet’s top edge by a minimum of 4 inches. Using a steel roller during installation will help the underlayment fully bond to the substrate.

It’s important to confirm that the underlayment is suitable for the roof system being installed. For example, some modified underlayments are manufactured with a granular surface, which provides slip resistance for installers. Such underlayments are compatible with slate and tile applications. However, these underlayments are not compatible with metal roof panels, as they can prematurely degrade the underside of a metal roof panel. For metal roof systems, a specially modified underlayment is required. And manufacturers of asphalt shingles offer their own version of modified underlayments for shingle applications. The best-modified underlayments carry a “High Temperature” (HT) designation and can be exposed to various weather conditions for up to 60 days before the new roof system is installed.

Proper underlayment is the final element of a successful roof replacement, and—as we’ve shown—is integral to a well-designed, dependable roof. For more information on steep-slope systems or to discuss any concerns about the integrity of your current roof, please contact Standard at

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