Underlayment for Installing a Metal Roof Over Asphalt Shingles

The decision To Install A Metal Roof Over Existing Asphalt Shingles Raises Practical Questions About Underlayment, Moisture Control, And Long-Term Performance. This Article Explains Best Practices, Materials, And Installation Steps To Help Homeowners And Contractors Make Informed Choices.

Key Topic	Quick Take
Is Underlayment Required	Often Recommended For Vapor Barrier, Secondary Water Protection, And To Smooth The Surface
Best Underlayment Types	Synthetic High-Temp Underlayment Or Self-Adhered Modified Bitumen
Ventilation Need	Critical To Prevent Trapping Moisture Between Layers
Fastening Method	Roofing Screws With Neoprene Washers For Metal Panels; Avoid Penetrating Shingle Deck Where Possible

Why Underlayment Matters When Installing Metal Over Shingles

Installing A Metal Roof Directly Over Asphalt Shingles Creates A Multi-Layer System That Can Trap Heat And Moisture, Affect Flashing Performance, And Change Load Characteristics. Underlayment Acts As A Secondary Water Barrier, A Slip Layer To Protect The Metal, And As A Thermal Separator. It Also Helps Smooth Minor Irregulaties In The Shingle Surface And Can Extend The Life Of The Metal Roof.

Building Code And Manufacturer Considerations

Local Building Codes And Manufacturer Warranties Can Dictate Whether A Metal Roof May Be Installed Over Existing Shingles And What Underlayment Is Required. Many Codes Allow Re-Roofing Over One Existing Layer, But Some Areas Require Tear-Offs Or Specific Underlayment Types For Wind Or Fire Ratings. Always Verify With The Local Building Department And The Metal Roof Manufacturer Before Proceeding.

Common Underlayment Options And Their Pros And Cons

Synthetic High-Temperature Underlayment

Synthetic Underlayments Are Lightweight, Tear-Resistant, And Often Rated For Higher Temperatures Than Traditional Felt. They Provide Good Slip Resistance For Installers And Drainage For Any Water That Penetrates The Metal Seams. These Are A Top Choice When Installing Metal Over Shingles Due To Durability And Breathability.

Self-Adhered Modified Bitumen (Peel-And-Stick)

Self-Adhered Underlayments Provide Reliable Waterproofing Especially Around Eaves, Valleys, And Penetrations. Peel-And-Stick Membranes Form A Strong Bond To The Shingle Surface, Reducing The Risk Of Wind-Driven Rain Intrusion. Caution: Adhesion Can Be Affected By Extreme Heat And Oil Migration From Old Shingles.

Asphalt-Saturated Felt (15# Or 30#)

Traditional Felt Is Less Expensive But Offers Lower Tear Resistance And Can Absorb Oils From Old Shingles, Causing Slippage Under Metal Panels. It Is Generally Not Preferred Under Metal Over Shingles Unless Budget Constraints Demand It And Proper Fastening And Venting Are Provided.

Ice And Water Shield

Ice And Water Shield Is Commonly Used At Eaves, Valleys, And Around Penetrations. When Installing Metal Over Shingles, Extending Ice And Water Shield In Vulnerable Areas Reduces The Risk Of Leakage Below The Metal Panels. It Should Be Used In Addition To A Field Underlayment, Not As The Sole Layer Unless Manufacturer Allows.

Preparing The Roof Deck And Shingle Surface

Surface Preparation Is Critical For Underlayment Performance. Loose Or Curling Shingles Should Be Nailed Down Or Removed, Flashings Should Be Inspected, And The Deck Must Be Structurally Sound For The Added Weight Of Metal Panels And Underlayment. Remove Excess Granules And Debris To Improve Adhesion Of Peel-And-Stick Products.

Ventilation And Condensation Control

Trapping Airspace Between Metal Panels And Old Shingles Can Create Condensation Risks, Especially In Humid Climates Or Poorly Ventilated Attics. A Continuous Ventilation Strategy, Including Ridge And Soffit Vents Or Mechanical Ventilation, Is Essential To Prevent Moisture Problems. Consider Installing A Breathable Air Gap Layer Or Raised Furring Strips To Promote Airflow Under The Metal.

Installation Methods For Underlayment Over Shingles

Direct Lay With Synthetic Underlayment

Roll Out Synthetic Underlayment Over The Shingle Surface Starting At The Eave And Working Upward, Overlapping As Recommended By The Manufacturer. Secure With Non-Corrosive Cap Nails Or Staples At Specified Intervals To Prevent Wind Uplift. Keep Fasteners Clear Of Seams And Ensure Proper Overlap.

Peel-And-Stick Application

Clean The Shingle Surface Then Apply The Peel-And-Stick Membrane Over Critical Areas First—Eaves, Rakes, And Valleys. Ensure Firm Adhesion By Pressing Or Rolling The Membrane To Remove Air Pockets And Follow The Manufacturer’s Temperature Guidelines For Best Bonding.

Using Furring Or Purlins For An Air Gap

Install Pressure-Treated Furring Strips Or Metal Purlins Over The Shingles To Create A Ventilated Air Gap And A Solid Attachment Point For The Metal Panels. This Method Reduces Heat Transfer, Helps Prevent Condensation, And Often Extends The Life Of Both The Shingles And The Metal Roof. It Also Changes Load paths so Verify Structural Limits.

Fastening And Flashing Practices

When Installing Metal Over Shingles, Fastening Strategy Is Critical. Use Screws With Neoprene Washers And Follow Panel Manufacturer Patterns To Ensure Weatherproof Seals. Flashings May Need To Be Replaced Or Modified To Fit Over The Added Thickness. Pay Special Attention To Rake, Valley, And Chimney Details To Maintain Water Shedding Performance.

Thermal Expansion And Attachment Considerations

Metal Expands And Contracts With Temperature Changes; allowing For Movement During Installation Prevents Panel Buckling And Seal Damage. Follow Manufacturer Guidelines For Clip Types, Fastener Spacing, And End-Lap Detailing To Accommodate Thermal Movement. Avoid Overdriving Fasteners Into The Underlayment Or The Shingles.

Common Problems And How To Avoid Them

• Moisture Trapping: Provide Adequate Ventilation Or Use A Breathable Underlayment.
• Oil Migration From Shingles: Select Synthetic Or Self-Adhered Products Rated For Compatibility.
• Fastener Failures: Use Corrosion-Resistant Screws And Proper Washer Types.
• Poor Adhesion: Prepare Shingle Surface And Follow Temperature Limits For Peel-And-Stick Membranes.

Cost Considerations And Lifespan Impacts

Installing Underlayment Adds Material And Labor Costs But Can Significantly Improve The Performance And Lifespan Of A Metal Roof System Installed Over Shingles. Using Higher-Quality Underlayment Like Synthetic Or Self-Adhered Membranes Increases Upfront Cost But Reduces Long-Term Repair Risk. Factor In Potential Savings From Not Removing The Old Shingles When Comparing Full Tear-Off Costs.

When A Tear-Off Is The Better Option

In Some Cases, Removing The Existing Shingles Is The Safer Long-Term Choice. If The Shingles Are Heavily Damaged, The Deck Is Deteriorated, Or The Additional Layer Exceeds Local Code Limits, A Tear-Off Ensures A Proper Substrate For Underlayment And Metal Panels. A Tear-Off Also Makes It Easier To Install Optimal Insulation, Ventilation, And Flashing Details.

Checklist For A Successful Underlayment Installation Over Shingles

1. Confirm Local Code And Manufacturer Requirements Regarding Overlays And Underlayment Types.
2. Conduct A Thorough Roof Inspection For Structural Integrity, Flashings, And Shingle Condition.
3. Select An Appropriate Underlayment (Synthetic Or Self-Adhered) And Ice/Water Shield For Vulnerable Areas.
4. Plan For Ventilation Or Add An Air Gap Using Furring Strips Or Purlins.
5. Follow Proper Fastening And Flashing Practices With Corrosion-Resistant Hardware.
6. Document Materials And Installation Steps To Support Warranties And Future Maintenance.

Maintenance Tips After Installation

Regular Inspections Twice Annually And After Major Storms Help Catch Issues Early. Check Fasteners For Tightness, Inspect Flashings And Sealants, And Watch For Signs Of Condensation Or Staining Inside The Attic.Clearing Debris From Roof Surfaces And Maintaining Proper Ventilation Extends The System’s Service Life.

Resources And References For Further Reading

Consult Building Code Resources, Metal Roofing Manufacturer Installation Guides, And Local Roofing Contractors For Specific Product Recommendations. Manufacturer Technical Bulletins Often Contain Critical Compatibility Information For Underlayment And Metal Panels. Local Building Departments Can Clarify Permit Requirements And Overlay Limits.

Practical Example: Typical Installation Sequence

A Common Process For Installing A Metal Roof Over Shingles Includes: Remove Loose Shingles And Clean Surface; Install Ice And Water Shield At Eaves And Valleys; Roll Out Synthetic Underlayment Over The Field; Attach Furring Strips Or Purlins If Adding An Air Gap; Fasten Metal Panels Per Manufacturer Pattern; Install New Flashings And Trim; Verify Ventilation And Seal Penetrations. Each Step Should Follow Product Specs For Overlap, Fastener Spacing, And Temperature Limits.

If Additional Visual Aids Would Help, Contractors Often Provide Detail Sheets Or CAD Drawings For Flashings And Attachment Methods That Match Specific Metal Panel Profiles.