5.1 Underlayment Types & Ice/Water Shield (Illinois)

5.1 Underlayment Types & Ice/Water Shield (Illinois)

Quick Answer: On Illinois steep-slope roofs, underlayment is the secondary weather barrier beneath the finished covering. Asphalt-saturated felt and synthetic sheets both work when installed to manufacturer and code rules, but Illinois winters make self-adhering ice and water shield at eaves and valleys a high-stakes detail—typically extending at least 24 inches past the interior wall line so meltwater from ice dams cannot soak the deck and wall top plate.

Underlayment is not decorative. When wind-driven rain or ice-dam backup gets under asphalt shingles, metal panels, or other steep-slope coverings, the underlayment is what keeps the sheathing dry long enough for the roof to shed water again. Illinois licensing and inspection questions often test whether you know which product goes where, how far ice barrier must run, and how laps and fasteners affect leak resistance.

Why Illinois Treats Underlayment Seriously

Illinois sits in a cold, humid continental climate. Winters bring repeated freeze–thaw cycles, lake-effect and frontal snow, and long stretches where attic heat melts snow on the upper roof while eaves stay below freezing. That pattern creates ice dams: ridges of ice at the eave that trap meltwater upslope. Water then works under shingle tabs and side laps. Without a sealed membrane at the eave, that water reaches the deck, fascia, and interior wall line—exactly where Illinois homeowners see stained drywall and rotten soffits each spring.

Building officials and exam writers therefore emphasize:

1. A continuous underlayment layer over the roof deck on steep-slope assemblies
2. An ice barrier (self-adhering polymer-modified bituminous membrane or approved equivalent) in ice-dam regions
3. Extra protection in valleys, where two planes concentrate flow

Treat underlayment and ice shield as part of the weather-resistive system, not as optional “felt paper.”

Felt Underlayment

Traditional asphalt-saturated felt (often called #15 or #30 felt by trade habit) remains common on Illinois residential reroofs when the manufacturer of the finished covering allows it.

Felt is still exam-relevant because many older Illinois homes and many manufacturer instructions still list it. Key installation ideas:

• Install over a clean, dry deck free of protrusions that would puncture the sheet
• Horizontal courses start at the eave and work toward the ridge
• Maintain manufacturer or code end and side laps so water running downslope cannot find an open seam
• Avoid leaving felt exposed longer than the product rating—UV and wind degrade it quickly on unfinished Illinois jobs

Felt does not replace ice and water shield at eaves in cold-climate practice. It is a secondary barrier under the finished covering, not a sealed ice-dam membrane.

Synthetic Underlayment

Synthetic underlayments (polyolefin or similar woven/nonwoven sheets with polymer coatings) have largely displaced felt on many Illinois steep-slope jobs.

Advantages Illinois contractors cite—and exams may probe—include:

• Higher tensile and tear strength, so sheets survive windy Midwest staging days
• Better walkability and printed nail lines on many products
• Lighter rolls and wider coverage per roll, which speeds large ranch and two-story footprints common in Illinois suburbs
• Improved resistance to wrinkling compared with wet felt

Limitations still matter:

• Not every synthetic is rated as an ice barrier; most still need a separate self-adhering membrane at eaves and valleys when ice-dam protection is required
• Some synthetics are slippery when frost-covered—treat winter mornings as a safety issue
• Always match the synthetic to the finished covering warranty; asphalt-shingle makers often specify approved underlayments by brand or type

Whether felt or synthetic, underlayment remains a water-shedding layer. It is not a substitute for flashing, ice shield, or a sound deck.

Self-Adhering Ice and Water Shield

Ice and water shield (self-adhering modified-bitumen membrane) bonds to the roof deck and seals around fasteners that penetrate it. That sealing behavior is why Illinois cold-climate practice relies on it where ice dams and valley overflow are expected.

Where it goes on Illinois roofs

Typical exam-relevant locations:

• Eaves from the edge upslope to a line measured from the warm side of the exterior wall
• Valleys full length or as required by the covering manufacturer and local amendments
• Often rakes, around skylights, and at low-slope transitions or roof-to-wall intersections when manufacturer instructions call for it

The 24-inch interior-wall-line rule (IRC-style cold climate)

IRC-style cold-climate practice—widely taught as exam-relevant for Illinois—requires an ice barrier that extends from the eave upward to a point at least 24 inches inside the exterior wall line of the building. In plain language: measure past the wall into the heated floor plan, not merely 24 inches up the roof slope from the fascia.

Why the interior wall line matters:

• Ice dams form near the cold overhang
• Meltwater can travel upslope under shingles before it finds a path into the attic or wall top plate
• Extending the sealed membrane past the wall line protects the structural wall and interior ceiling junction where leaks do the most damage

On steep pitches the roof distance needed to reach 24 inches horizontally inside the wall is longer than on low pitches. Candidates should think in terms of horizontal projection past the interior face of the exterior wall, then convert that to slope distance when laying membrane.

Valleys deserve equal attention. Two roof planes dump water into a channel that can back up under coverings during freeze–thaw. Self-adhering membrane in valleys, combined with proper valley metal or woven/open valley methods specified for the covering, is standard Illinois storm-ready practice.

Installation Sequence That Exam Questions Target

A clean mental model for steep-slope prep in Illinois:

1. Inspect and repair deck; replace soft plywood/OSB; flush fasteners

• 1. Apply ice and water shield at eaves to the interior-wall-line distance (and valleys as required)

1. Install field underlayment (felt or synthetic) overlapping the upper edge of the ice barrier per manufacturer instructions so water sheds onto the membrane, not under it
2. Integrate underlayment with drip edge: many Illinois details place drip edge over underlayment at rakes and under or as specified at eaves—follow the covering manufacturer and local custom consistently
3. Keep underlayment dry and secured against wind uplift before shingles or panels go on

Common failure modes inspectors flag:

• Ice barrier that stops at the fascia or only covers the overhang, never reaching past the wall line
• Fishmouths and voids in self-adhered membrane that channel water
• Underlayment fasteners left unsealed outside the ice-barrier zone in ways that create chronic drip points
• Synthetic or felt installed upside down or without adequate side/end laps

Product Selection Checklist for Illinois Jobs

Use this list when a scenario question describes climate, pitch, and covering type:

• Confirm the finished roof covering’s underlayment requirements (felt weight, synthetic approval, high-temperature underlayment under metal, and so on)
• In Illinois ice-dam country, plan self-adhering ice barrier at eaves to ≥24 in. past the interior wall line and protect valleys
• Choose synthetic when tear strength and jobsite exposure time matter; choose felt when specifications and budget allow and weather windows are short
• Store rolls dry; winter-warm self-adhering membranes so they bond; summer-avoid wrinkled felt left baking in sun
• Document slope, eave detail, and membrane extent on the job ticket—inspectors ask

Underlayment and ice shield set up every later steep-slope chapter topic: shingle nailing, metal panel clips, flashing, and ventilation all assume a dry deck protected by a correctly detailed secondary barrier. Master the Illinois eave and valley story first, then the fastener and moisture chapters lock in.