What Goes Under Shingles on a Roof?
What goes under shingles on a residential roof is a sophisticated, multi-layered moisture barrier system engineered to protect the underlying wooden structure from wind-driven rain, ice dams, and condensation. Beneath the visible exterior asphalt shingles lies a sequence of interdependent protective components: the structural roof decking (typically plywood or oriented strand board), metal drip edge flashing along eaves and rakes, self-adhering ice and water protector membranes in high-risk leak zones, and water-resistant roofing underlayment (either synthetic polymers or traditional asphalt-saturated felt) spanning the entire field. Working together, these concealed under-shingle components form a building envelope that keeps homes dry even if outer shingles are torn away in severe storms.
The Anatomy of Sub-Shingle Roof Layers
The structural foundation beneath all roofing materials is the roof decking or sheathing. Fastened directly to roof rafters or prefabricated trusses, roof decking typically consists of 1/2-inch or 5/8-inch CDX exterior plywood or oriented strand board (OSB). The decking provides a solid nailable nailing substrate for shingles while reinforcing the building's lateral shear strength against heavy wind loads and seismic events.
Before any fabric underlayment is rolled out, metal drip edge flashing is nailed along the roof's lowest perimeter edges (eaves) and gable ends (rakes). Fabricated from galvanized steel, aluminum, or copper, the L-shaped drip edge directs water runoff away from the wooden fascia board and straight into gutters, preventing capillary suction from siphoning rainwater backward beneath the bottom edge of the roof decking.
The table below outlines the sequential layers installed beneath shingles from the wooden framing upward to the shingle underside.
| Roof System Layer | Material Composition | Primary Engineering Function | Code / Installation Rule |
|---|---|---|---|
| Roof Decking / Sheathing | 1/2" to 5/8" CDX plywood or OSB panels | Structural nail base supporting all roof dead loads | IRC Section R905.1 (Solid sheathing) |
| Metal Drip Edge Flashing | 26-gauge galvanized steel or aluminum | Guides runoff into gutters; protects fascia edges | IRC Section R905.2.8.5 (Mandatory) |
| Ice & Water Shield | Rubberized asphalt with self-adhesive backing | Waterproof seal against ice dams and wind-driven rain | Mandatory eaves/valleys in freeze zones |
| Roofing Underlayment | Non-woven synthetic polymer or #30 felt | Secondary water-shedding membrane across field | IRC Section R905.1.1 (Underlayment rule) |
| Valley Flashing | W-valley galvanized metal or membrane roll | Handles massive rainwater volume in roof valleys | Open or closed valley flashing code |
| Starter Shingle Strip | Pre-cut asphalt shingles with adhesive strip | Locks down first row of shingles against wind uplift | Manufacturer warranty requirement |
The International Residential Code (IRC) mandates that all roofing layers integrate into a continuous shingle-lap drainage plane.
Synthetic Underlayment vs Traditional Asphalt Felt
For over a century, asphalt-saturated organic felt paper (commonly known as #15 or #30 tar paper) was the standard material installed under shingles. While inexpensive, felt paper is heavy, prone to tearing during installation, wrinkles when exposed to morning dew or rain, and degrades quickly under prolonged ultraviolet sunlight exposure during construction delays.
Modern building science has shifted almost universally toward high-performance synthetic roofing underlayment made from woven polypropylene or polyethylene polymers. Synthetic underlayment is up to twenty times more tear-resistant than felt paper, highly slip-resistant for roofer safety, impervious to rot and fungal breakdown, and can remain exposed to harsh weather for up to six months without losing its waterproof integrity. Furthermore, rubberized self-healing peel-and-stick membranes remain mandatory around chimneys, skylights, plumbing vents, and roof eaves.
The table below compares synthetic polymer roofing underlayment against traditional #30 asphalt-saturated felt.
| Performance Characteristic | Modern Synthetic Underlayment | Traditional #30 Asphalt Felt | Homeowner / Roofer Benefit |
|---|---|---|---|
| Tear Resistance | Exceptional woven tensile strength; resists wind | Brittle; tears easily under foot traffic | Synthetics prevent blow-offs during installation |
| Moisture & Wrinkle Behavior | 100% moisture impervious; lays completely flat | Absorbs water, puckers, and telegraphs wrinkles | Prevents wavy, distorted shingles on roof |
| UV Exposure Rating | 60 to 180 days UV resistance rating | Breaks down within 14 to 30 days of sun | Allows roofs to remain dried-in during delays |
| Weight per Roll | Lightweight (25 to 35 lbs covers 1,000 sq ft) | Heavy (45 to 50 lbs covers only 200 sq ft) | Reduces installer fatigue and roof freight load |
| Slip-Resistant Walking Deck | Embossed traction texture for roofer safety | Slick when dusty; greasy when warm | Enhances contractor safety on steep pitches |
| Material Cost Factor | Modest 10% to 25% premium per square | Lowest initial material purchase price | Extended roof longevity outweighs cost delta |
Ice and water shield should extend at least 24 inches inside the interior heated wall line to stop ice-dam leakage effectively.
How to Install Under-Shingle Roof Layers in 5 Steps
Follow this professional sequence to install a bulletproof waterproof substrate beneath asphalt shingles.
Inspect and Prepare Structural Roof Decking
Inspect plywood or OSB sheets for soft spots, rotted wood, or popped nails, replacing damaged sheets and sweeping debris clean.
Install Metal Drip Edge Flashing Along Eaves
Nail metal drip edge directly to the wood decking along bottom eaves using roofing nails spaced every 12 inches.
Apply Self-Adhesive Ice and Water Shield
Peel the protective backing and stick rubberized membrane along eaves, in roof valleys, and around chimneys and skylights.
Roll Out Synthetic Underlayment Across Roof Field
Roll synthetic underlayment horizontally starting at the bottom, overlapping courses by 4 inches and fastening with plastic cap nails.
Install Rake Drip Edge and Starter Shingles
Install rake drip edge over the underlayment along gable edges, then secure adhesive starter strips along all eaves.
Frequently Asked Questions (7 Questions Answered)
Q1: Can you put new shingles over old shingles without underlayment?
While some local codes allow a single shingle overlay (re-roofing), building science strongly recommends tearing off old shingles down to the decking to replace underlayment and inspect for hidden wood rot.
Q2: What is the best underlayment for an asphalt shingle roof?
High-performance synthetic woven polypropylene underlayment paired with self-adhering rubberized asphalt ice-and-water shield is considered the gold standard.
Q3: Does underlayment go over or under the drip edge?
At the bottom eaves, underlayment goes OVER the drip edge; on the angled rake edges (gables), underlayment goes UNDER the drip edge.
Q4: What happens if you do not put underlayment under shingles?
Without underlayment, wind-driven moisture and ice dam leaks penetrate directly into roof plywood, causing structural wood rot, interior drywall leaks, and voided shingle warranties.
Q5: Is synthetic underlayment waterproof or water-resistant?
Synthetic underlayment is water-resistant and sheds liquid water efficiently, but seams and fastener penetrations make it a water-shedding barrier rather than a hermetic submersion seal.
Q6: How far up the roof does ice and water shield need to go?
Building codes require ice and water shield to extend from the roof edge up to a point at least 24 inches inside the home's exterior heated wall line.
Q7: Can felt paper be used instead of synthetic underlayment?
Yes, #30 asphalt felt meets building code minimums, but synthetic underlayment offers far greater tear resistance, longevity, and weatherproofing.
Final Thoughts & Key Takeaways
In conclusion, understanding what goes under shingles on a roof? provides essential clarity, practical strategies, and actionable advice. By incorporating these foundational insights, adhering to verified safety guidelines, and following structured best practices, you ensure reliable, long-term outcomes while preventing common mistakes. Stay informed, consult certified professionals when needed, and maintain consistent quality care.