What Lies Beneath: Roof Underlayment and Ice & Water Shield

When homeowners look at their roofs, they focus on the shingles. Shingles provide color, curb appeal, and the primary layer of defense against direct weather. However, the true waterproofing performance of any steep-slope roof depends almost entirely on the layers beneath the shingles.

In Utah, our homes face extreme seasonal shifts. From winter blizzards and prolonged snowpack along the Wasatch Front to 100-degree summer heat in Utah Valley, the sub-shingle system endures severe stresses. If the materials beneath your shingles are substandard or installed incorrectly, water will eventually find a path into your home, regardless of how new your shingles look.

The Roof Deck: The Structural Foundation

Everything on your roof rests on the roof deck, typically composed of oriented strand board (OSB) or plywood. Before any underlayment is unrolled, the deck must be clean, dry, and structurally sound.

During a roof replacement, stripping off the old shingles down to the bare wood is essential. This step allows roofers to inspect the decking for rotted panels, delamination, sagging between trusses, and loose fasteners. In older Utah neighborhoods, we occasionally find spaced board sheathing (common beneath old cedar shake roofs). Modern asphalt shingles require a solid nailing surface, which means spaced decking must be re-sheeted with new plywood or OSB before proceeding. Nailing into compromised decking leads to loose shingles during canyon wind events.

Ice and Water Shield: Critical Protection Against Ice Dams

Utah building codes require an ice barrier along the eaves, and for good reason. During our winters, snow accumulates on the roof, melts over warm attic spaces, and runs down to the colder overhangs at the eaves. There, the water refreezes, creating an ice dam. As water pools behind the ice dam, it can back up under traditional shingles.

An ice-and-water shield is a self-adhering, rubberized asphalt membrane. Unlike standard underlayment, it bonds directly to the wood decking and creates a watertight seal around the shingle nails that puncture it.

Per code throughout most of Utah, this membrane must extend from the roof edge to a point at least 24 inches inside the exterior wall line of the building. On shallow-pitched roofs or homes with deep overhangs, this often requires two full courses of membrane to meet the 24-inch threshold. Skipping this step or skimping on the width is one of the most common causes of interior winter water leaks.

Valleys, Roof Penetrations, and Low-Slope Transitions

Ice-and-water shield is not just for eaves. It serves as essential protection in high-risk zones across the entire roof structure:

• Valleys: Where two roof planes meet, water concentrates in high volume. Lining every valley with a full layer of self-adhering membrane before installing valley metal or shingles prevents erosion-related leaks.
• Roof Penetrations: Plumbing vent pipes, furnace flues, and skylights interrupt the water flow. Flashing boots and collars should always be backed by self-adhering membrane sealed tight to the deck and pipe base.
• Rake Edges and Sidewalls: Along gable edges and where roof planes meet exterior siding, peeling back the weather wrap and installing membrane prevents wind-driven rain from penetrating the seam.

Synthetic Underlayment vs. Traditional Felt

Beyond the areas protected by ice-and-water shield, the remainder of the roof deck is covered with underlayment. For decades, asphalt-saturated organic felt (commonly known as 15-pound or 30-pound felt paper) was the standard. While functional, paper felt has significant weaknesses in Utah's climate.

Traditional felt absorbs moisture, which can cause it to wrinkle and buckle before or after shingles are laid, leading to an uneven surface. In high summer heat, asphalt felt becomes brittle, and during installation, it tears easily under foot traffic.

Today, modern synthetic underlayment—woven from polypropylene or polyethylene—is the superior choice. Synthetic materials offer several distinct advantages:

• High Tear Resistance: Synthetic underlayment resists tearing even during sudden Wasatch wind gusts during installation.
• Moisture Resistance: It does not absorb water or rot, keeping the wood deck dry even if rain interrupts the installation process.
• Heat Stability: Synthetic sheets do not dry out or crack when subjected to elevated attic temperatures in midsummer.
• Better Traction: The textured walking surface provides safer footing for installation crews, reducing safety hazards on steep pitches.

Why Sub-Layer Installation Determines Roof Longevity

Having high-grade materials on the job site is only half the battle. How those materials are layered matters just as much. Roofing is based on a gravity-shedding system: each layer must overlap the one below it so water flows naturally downward without encountering an upward-facing seam.

Drip edge metal must be integrated properly with the underlayment. At the eaves, drip edge goes down first, and the ice-and-water shield adheres over the top of the metal flange. Along the rakes (the sloped sides of the roof), the underlayment goes down first, and the drip edge is installed over it. Reversing this sequence allows driven wind and rain to push under the edge of the metal.

Because these critical materials are completely hidden once the shingles are installed, quality control during the teardown and underlayment phase is paramount. Having dedicated, in-house trained crews and an on-site project manager ensures that decking repairs, nail spacing, and overlap tolerances are verified before a single shingle is nailed into place.

If you want to know what condition your roof's underlying layers are in, reach out to DFNDR Roofing. You can call or text DFNDR to schedule a free, thorough roof inspection across Utah Valley and the Wasatch Front.