How Is Underlayment Installed During Roof Replacement? Step-by-Step Guide

When replacing a roof, most homeowners focus on shingles, roofing materials, and installation costs. However, an equally important protective layer sits underneath the visible roofing material: roof underlayment.

If you’re wondering how is underlayment installed during roof replacement, understanding the process can help you evaluate contractor estimates, recognize installation mistakes, and protect your home against future leaks.

Roofing underlayment acts as a secondary moisture barrier between the roof decking and the outer roofing material. Proper installation helps protect your home from wind-driven rain, moisture intrusion, and certain types of storm damage.

This guide explains the installation process, common underlayment materials, fastening methods, costs, and warning signs homeowners should understand before replacing a roof.

Quick Answer

During roof replacement, contractors remove old roofing materials, inspect and repair the roof decking, and install underlayment starting at the bottom edge of the roof. Each new row overlaps the previous row to direct water downward. Workers secure the material using approved fasteners or adhesive before installing flashing and the new roofing system.

What Is Roof Underlayment and Why Is It Important?

Roof underlayment is a protective material installed directly over the roof decking and beneath the primary roofing material.

The decking is the wooden surface, usually plywood or oriented strand board (OSB), that supports the roof covering.

Although asphalt shingles, metal panels, or tiles provide the main protection against weather, moisture can occasionally reach the layers underneath.

Underlayment provides additional protection against that moisture.

Its primary functions include:

• Secondary water protection: Helps prevent moisture from reaching the wooden roof decking.
• Wind-driven rain resistance: Provides another protective layer when rain moves beneath shingles or other roofing components.
• Temporary construction protection: Helps protect properly prepared decking during roof installation, within the product’s exposure limits.
• Additional protection around vulnerable areas: Works with flashing and specialized membranes near roof edges, valleys, and penetrations.
• Support for roofing system requirements: Helps the completed roof meet applicable installation specifications and warranty conditions.

However, underlayment is not a substitute for correctly installed shingles, flashing, or roof drainage.

Even high-quality underlayment cannot compensate for major installation defects.

Types of Underlayment Used During Roof Replacement

Roofing contractors generally choose underlayment according to the roofing material, local climate, roof slope, building codes, and manufacturer requirements.

Three common options are asphalt-saturated felt, synthetic underlayment, and self-adhering waterproof membranes.

Asphalt-Saturated Felt Underlayment

Traditional roofing felt is made from a reinforcing material saturated with asphalt.

It has been widely used under residential asphalt shingles for many years.

Common classifications include No. 15 and No. 30 felt, although modern products are specified by applicable performance standards rather than weight alone.

Advantages:

• Generally inexpensive
• Familiar to most roofing contractors
• Suitable for many conventional shingle installations
• Readily available

Limitations:

• Can wrinkle when exposed to moisture
• May tear during installation
• Typically heavier than comparable synthetic products
• Often has more restrictive weather-exposure limitations

Felt can still be a reasonable option when installed according to the roofing manufacturer’s requirements.

Synthetic Roof Underlayment

Synthetic underlayment typically consists of engineered polymer materials designed to provide moisture resistance and durability.

It has become a common choice for residential roof replacement.

Advantages:

• Lightweight and relatively easy to handle
• Often more tear-resistant than traditional felt
• Available in large rolls that can speed installation
• Many products offer improved resistance to wrinkling

Limitations:

• Usually costs more than basic felt
• Fastening requirements vary by manufacturer
• Some products can become slippery in certain conditions
• Not every synthetic product is suitable beneath every roof covering

Synthetic underlayment is often a practical choice for asphalt-shingle replacements, but contractors should confirm compatibility with the complete roofing system.

Self-Adhering Ice and Water Protection

Self-adhering membranes have an adhesive backing that bonds to the roof deck.

They are commonly installed in locations that need additional water protection, including roof valleys, eaves, and certain penetrations.

In colder regions, these membranes can help protect against water backing up behind ice dams.

They may also provide additional protection against wind-driven rain in vulnerable areas.

However, ice and water protection is not automatically required across the entire roof. Coverage depends on climate, code requirements, roof design, and the specified roofing system.

Roof Underlayment Comparison

Underlayment type	Main advantage	Main limitation	Common application
Asphalt felt	Lower material cost	More vulnerable to tearing and wrinkling	Traditional shingle roofs
Synthetic	Lightweight and tear-resistant	Product-specific fastening and compatibility	Many residential replacements
Self-adhering membrane	Enhanced water protection	Higher material cost and deck preparation requirements	Eaves, valleys, critical details
High-temperature membrane	Designed for elevated temperatures	Often more expensive	Certain metal and other specialized roofs

The best selection is the product approved for your roof design, not necessarily the most expensive material.

How Is Underlayment Installed During Roof Replacement? 8 Main Steps

Professional underlayment installation follows a sequence designed to direct water toward the roof edges without allowing it to enter the decking.

The following process describes a typical residential asphalt-shingle replacement.

Metal, tile, and low-slope roofing systems may require different installation details.

Step 1: Remove the Existing Roofing Materials

Contractors begin by removing the old shingles or other roof covering.

They also remove worn underlayment, unnecessary fasteners, and damaged components scheduled for replacement.

Removing these layers exposes the roof decking.

This allows the crew to examine areas that were previously hidden.

Common findings include:

• Water-stained plywood
• Rotten or weakened roof decking
• Loose or raised nails
• Soft areas around roof penetrations
• Damage near chimneys and roof valleys
• Previous repairs that concealed moisture problems

The roof deck must provide a suitable surface before new underlayment is installed.

Important: Installing fresh underlayment over rotten decking does not repair structural damage.

Step 2: Inspect and Repair the Roof Decking

After removing the old roofing materials, contractors inspect the exposed roof surface.

The decking should be structurally sound, properly attached, sufficiently dry, and free of sharp debris.

Damaged sections may need replacement.

For example, if rainwater has repeatedly leaked around a chimney, nearby plywood may have deteriorated.

The affected decking should be repaired before installing the new roof system.

Contractors may also inspect roof ventilation openings and identify signs of moisture trapped in the attic.

Poor ventilation can contribute to condensation problems and premature deterioration of roof components.

A qualified professional should evaluate extensive rot, sagging, or suspected structural damage before roofing work proceeds.

Step 3: Install Ice and Water Protection Where Required

Before covering the main roof surface, contractors install self-adhering waterproof membranes in designated vulnerable areas.

These locations may include:

• Eaves in regions with ice-dam risks
• Roof valleys
• Around skylights and chimneys
• Certain roof-to-wall intersections
• Other details specified by the roofing manufacturer

Installation usually involves positioning the membrane, removing its protective backing, and pressing it firmly against a compatible, prepared surface.

The membrane must be installed with the correct overlaps, adhesion conditions, and transitions to adjacent roofing components.

For ice-dam protection, coverage may need to extend a specified distance beyond the exterior wall line, depending on the applicable code and roof assembly.

Homeowner tip: Ask your contractor where self-adhering membrane will be installed. The estimate should identify these areas rather than simply stating “roof underlayment.”

Step 4: Position the First Row Along the Eaves

Once the decking and required protective membranes are ready, workers begin installing the main underlayment.

The first course usually runs horizontally along the lower edge of the roof.

This edge is called the eave.

Starting at the eave allows subsequent courses to overlap the material below them.

The arrangement resembles the overlapping pattern of roof shingles.

Water that reaches the underlayment can travel downward over the laps instead of entering between them.

Contractors align the first course carefully and keep the material smooth.

Wrinkles, folds, and unnecessary tension should be avoided.

The exact relationship between underlayment and metal drip edge depends on roof location and the approved system details.

According to the Asphalt Roofing Manufacturers Association’s drip-edge installation guidance, the general arrangement for asphalt-shingle roofs places the eave drip edge beneath the underlayment and the rake drip edge above it, with specified exceptions for certain self-adhering systems.

image

Asphalt Roofing Manufacturers Association (ARMA)

Correct sequencing helps direct water off the roof rather than behind its perimeter components.

Step 5: Overlap Each Additional Underlayment Course

After positioning the first course, contractors continue installing additional rows toward the roof ridge.

Each higher course overlaps the course below.

This overlapping arrangement is sometimes called shingle fashion.

The overlap distance depends on the underlayment product, roof slope, local code, and manufacturer’s instructions.

For example, some products require a minimum 2-inch horizontal overlap, while others specify 4 inches or more.

End joints, where two sections meet within the same course, often need wider overlaps.

Workers also stagger end joints when required to avoid creating a continuous weak line across the roof.

The Asphalt Roofing Manufacturers Association’s residential roofing guidance discusses underlayment laps and emphasizes following product-specific requirements, particularly for synthetic materials.

image

Asphalt Roofing

A properly installed system should have:

• Consistent overlap widths
• Smooth, flat material
• Correctly positioned end joints
• No unnecessary openings
• No reversed laps that direct water beneath the material

Small installation errors can become important during severe weather.

Step 6: Secure the Underlayment With Approved Fasteners

Most mechanically attached underlayments are secured using roofing nails with caps or other manufacturer-approved fasteners.

Cap nails have a wider head or attached cap that helps distribute pressure across the material.

This reduces the likelihood of the underlayment tearing around the fastening point.

The contractor follows the manufacturer’s specified fastening pattern.

Fastener spacing may differ at roof edges, overlapping seams, and the center of each sheet.

Areas exposed to strong winds may require enhanced fastening.

Contractors should not assume ordinary staples are acceptable. Some underlayment manufacturers prohibit staples or restrict their use.

Fasteners should be installed straight and seated correctly without damaging the material.

For self-adhering membranes, the installation relies primarily on the approved adhesive application process rather than the same general fastening pattern used for mechanically attached sheets.

Step 7: Complete Valleys, Roof Edges, and Penetrations

Roof valleys, wall intersections, chimneys, plumbing vents, and skylights need careful detailing.

These areas are vulnerable because water can concentrate or change direction around them.

Contractors coordinate the underlayment with flashing and specialized waterproofing membranes.

For example, a roof valley may receive self-adhering membrane before the surrounding field underlayment.

Flashing materials must then be integrated so water drains properly.

A plumbing vent may require correctly layered underlayment and a compatible flashing boot.

Similarly, chimney flashing must be integrated with the roof covering and surrounding water-control layers.

Simply applying roofing sealant over a poorly constructed intersection is not an adequate substitute for correct flashing.

At roof ridges, contractors must also follow the specified details for ridge ventilation.

Underlayment should not block an intended ventilation opening.

Step 8: Inspect the Underlayment Before Installing Roofing Materials

Before installing shingles or another outer roof covering, the crew should inspect the underlayment.

The inspection helps identify:

• Loose or missing fasteners
• Torn sections
• Improper overlaps
• Wrinkled material
• Incomplete waterproofing around roof details
• Incorrect drip-edge transitions
• Uncovered decking
• Damage caused by foot traffic or equipment

Any significant defects should be corrected before they become concealed.

Workers then install the remaining roofing components according to the manufacturer’s system instructions.

Underlayment provides secondary protection, but it should not be treated as an indefinite substitute for the finished roof covering.

How Underlayment Installation Changes With Roof Slope

Roof pitch describes how steeply a roof rises.

For example, a 4:12 roof rises 4 inches vertically for every 12 inches horizontally.

Roof slope affects how quickly water drains and how much moisture protection a roof assembly may require.

Standard-Slope Roofs

Many conventional asphalt-shingle roofs with slopes of 4:12 or greater use a single approved field underlayment layer, plus specialized protection where required.

Proper overlaps and approved fasteners are still essential.

Low-Slope Asphalt-Shingle Roofs

Asphalt-shingle roofs between 2:12 and less than 4:12 generally need additional underlayment protection under commonly adopted roofing codes and manufacturer instructions.

This may involve a specially overlapped double-layer application or an approved self-adhering alternative.

A standard single-layer installation may not provide adequate protection.

Asphalt shingles generally are not intended for slopes below 2:12.

Very low-slope roofs often require a different roofing system designed for slower drainage.

Steep Roofs

Steep roofs may shed water quickly but create greater installation and fall hazards.

Specialized access methods, anchoring, and fastening arrangements may be required.

Homeowners should never assume a steep roof is easier to waterproof simply because rain runs off faster.

Does the Installation Process Differ by Roofing Material?

Yes. Although the basic goal of protecting the roof deck remains the same, different roofing systems can require different underlayment products.

Asphalt Shingles

Asphalt-shingle installations commonly use felt or synthetic underlayment over suitable decking, with additional membranes in vulnerable locations.

The shingle manufacturer’s instructions determine many of the final details.

Metal Roofing

Metal roofing may require synthetic or specialized high-temperature underlayment.

Compatibility is particularly important because roofing materials and roof decks can experience significant temperature changes.

Certain metal roofing assemblies also require carefully designed ventilation, separation layers, or condensation-control details.

Tile Roofing

Concrete and clay tile roofing often uses underlayment systems designed for tile installations and regional weather exposure.

In some climates, the underlayment plays a particularly important role because wind-driven rain can travel beneath individual tiles.

Flat and Low-Slope Roofing

EPDM, TPO, and other low-slope roof systems may use insulation, cover boards, vapor-control layers, and waterproof membranes rather than a conventional shingle-style underlayment arrangement.

These assemblies require system-specific installation methods.

A contractor should not automatically apply asphalt-shingle underlayment practices to a commercial membrane roof.

How Much Does Roof Underlayment Cost During Replacement?

The cost of installing roof underlayment depends on the material, roof size, complexity, and labor involved.

Traditional felt generally costs less than synthetic underlayment.

Self-adhering membranes and specialized high-temperature products typically cost more.

However, material prices represent only part of the total expense.

Important factors include:

• Roof area: Larger roofs require more material and fasteners.
• Roof pitch: Steeper roofs can increase installation difficulty.
• Roof complexity: Valleys, dormers, and penetrations require more detailed work.
• Underlayment selection: Premium or specialized membranes can increase costs.
• Regional requirements: Wind and ice-dam protection may add materials and labor.
• Deck repairs: Damaged plywood or OSB replacement increases overall project costs.

For illustration, a roof with 2,000 square feet of actual roof area requires more than 2,000 square feet of underlayment material once laps, trimming, and waste are considered.

Roofing contractors commonly estimate materials using roofing squares, with one square representing 100 square feet of roof area.

Because labor and product pricing vary significantly by region, ask for an itemized quote rather than relying on a generic national price.

A useful estimate should identify the underlayment brand or product type, ice and water protection coverage, fastening requirements, and whether decking repairs are included.

How Long Does Roof Underlayment Last?

Roof underlayment is typically designed to function as part of the complete roofing system.

Its service life depends on the product, installation quality, moisture exposure, temperature, ventilation, and condition of the outer roof covering.

Synthetic underlayments may offer better tear resistance and longer permitted construction exposure than traditional felt.

However, that does not mean every synthetic product lasts longer in every completed roof.

Even durable underlayment can deteriorate if repeated leaks allow moisture to enter the roof assembly.

Homeowners should also distinguish between two different manufacturer claims:

• Exposure rating: How long a particular product may remain uncovered under specified conditions.
• In-service performance: How the product functions beneath the finished roofing system.

An advertised 90-day or 180-day exposure rating should not be interpreted as a roof lifespan or a guarantee of leak-free protection during that period.

In most full roof replacements, replacing the old underlayment is the practical approach, especially when existing roofing materials have been removed.

Can Poorly Installed Underlayment Cause Roof Leaks?

Yes. Improper installation can increase the risk of moisture reaching the roof decking.

Several problems are particularly concerning.

Incorrect Overlaps

If the upper course is placed underneath the lower course, water may flow into the seam rather than over it.

Tears and Punctures

Damaged underlayment creates possible pathways for water intrusion.

Poorly Secured Materials

Loose fastening may allow underlayment to shift, wrinkle, or tear during construction and windy conditions.

Incomplete Waterproofing

Missing protective membranes in required locations can leave vulnerable roof areas exposed.

Incorrect Flashing Integration

Even good underlayment cannot reliably prevent leaks if chimney, skylight, and roof-to-wall flashing are installed incorrectly.

After roof replacement, homeowners should watch for ceiling stains, attic moisture, damp insulation, and unexplained water marks.

However, visible leaks do not automatically prove an underlayment defect. Flashing problems, damaged shingles, ventilation-related condensation, and other conditions may produce similar symptoms.

A professional inspection is the safest way to determine the cause.

Expert Tip

Expert Tip: Request Photos Before the Shingles Go On

Ask your roofing contractor to photograph the completed underlayment, roof valleys, eaves, and flashing preparation before installing the final covering. These photos can help document concealed work, confirm the agreed materials were used, and provide useful records if a future roofing issue arises.

When to Consult a Licensed Roofing Contractor

Professional roofing assistance is especially important when an installation involves structural damage, significant leaks, complex roof details, or potentially disputed insurance damage.

Consult a properly licensed roofing contractor where licensing is required if:

• Your roof has widespread damaged or missing shingles.
• Water stains continue appearing after previous repairs.
• The decking feels soft or appears to sag.
• Storm damage affects multiple roof sections.
• The roof requires major flashing replacement.
• Your home has a steep or complex roof.
• You are uncertain about code or warranty requirements.
• A contractor proposes installing new roofing over visibly damaged materials.

Structural concerns may also require evaluation by a qualified engineer or other appropriate building professional.

Roofing work carries serious fall hazards. The Occupational Safety and Health Administration’s residential fall-protection guidance explains the risks associated with roofing and the protective methods used during construction.

image

Occupational Safety and Health Administration

Homeowners should not climb onto a roof to inspect underlayment, measure overlaps, or photograph installation details.

Instead, request documentation from the contractor and observe from a safe ground-level location.

For insurance-related damage, document the condition of the property and coordinate with your insurer before authorizing non-emergency work that could affect the claims process.

Common Mistakes Homeowners Should Avoid

Understanding common installation problems can help you ask better questions when evaluating roofing estimates.

• Choosing underlayment based only on price: The least expensive option may not suit your climate or roof covering.
• Ignoring manufacturer requirements: Incorrect products or installation methods may compromise performance or warranty eligibility.
• Skipping decking repairs: New underlayment cannot compensate for rotten or structurally unsound wood.
• Overlooking ice and water protection: Vulnerable locations may need additional protection beyond standard field underlayment.
• Accepting an unclear estimate: Ask contractors to identify specific materials and installation details.
• Assuming all roofing systems are identical: Metal, tile, asphalt, and low-slope membranes have different requirements.
• Ignoring ventilation problems: Existing condensation or attic moisture issues should be investigated during replacement.
• Attempting unsafe inspections: Roof surfaces can be slippery, unstable, and hazardous even when the weather appears clear.
• Failing to document concealed work: Once the final roof covering is installed, many underlayment details are difficult to inspect.

A clearly written scope of work helps prevent misunderstandings about material selection and installation quality.

What Should Homeowners Check Before Approving the Installation?

Although homeowners should leave rooftop inspections to professionals, they can review the proposed installation details before work begins.

What to check	Why it matters	What to ask
Underlayment material	Affects compatibility and performance	What product will you install?
Roof deck condition	Supports the entire roof assembly	How will damaged decking be handled?
Ice and water protection	Adds protection in vulnerable areas	Where will protective membrane be installed?
Fastening method	Helps keep material secure	What fasteners does the product require?
Overlap specifications	Helps direct water away from seams	Will you follow the manufacturer’s lap requirements?
Flashing details	Protects vulnerable intersections	Which flashing components will be replaced?
Warranty coverage	Clarifies product and workmanship protections	What exclusions and installation conditions apply?
Installation documentation	Provides records of concealed work	Can you provide progress photographs?

It is also worth confirming who supervises the installation and how the contractor handles unexpected decking damage.

A detailed written agreement is generally more useful than verbal assurances alone.

FAQs

1. Is roof underlayment always replaced when installing new shingles?

During a complete tear-off roof replacement, contractors generally install new underlayment after removing the old roofing materials. Reusing old underlayment is usually not appropriate because it may have holes, deterioration, or concealed damage.

The exact scope should follow local requirements and the roofing manufacturer’s installation instructions.

2. Does underlayment go over or under the drip edge?

For typical asphalt-shingle roofs, the eave drip edge generally goes beneath the underlayment, while the rake drip edge goes above it.

However, certain self-adhering membrane systems use different approved details. The contractor should follow the applicable manufacturer instructions and local building code.

3. How much should roof underlayment overlap?

Overlap requirements vary by product and roof slope. Some materials specify a 2-inch horizontal lap, while others require 4 inches or more.

Low-slope roofs may need substantially greater overlap or a different approved system.

Always follow the specific product instructions rather than assuming one measurement applies to every roof.

4. Can synthetic underlayment be installed directly over plywood?

Yes. Many synthetic roofing underlayments are designed for installation directly over properly prepared plywood or OSB decking.

The decking must meet the product’s requirements for condition, attachment, dryness, and surface preparation.

Special membranes may require additional preparation or primers.

5. Can roof underlayment be left exposed before shingles are installed?

Some underlayment products have manufacturer-rated exposure periods, but the limits vary significantly.

An exposure rating does not guarantee that an unfinished roof will remain watertight.

Contractors should follow manufacturer instructions, protect vulnerable details, and complete the roofing system as promptly as practical.

6. Does roof underlayment prevent all leaks?

No. Underlayment provides secondary moisture protection, not complete leak prevention.

Incorrectly installed flashing, damaged shingles, open seams, and roof penetrations can still allow water to enter.

A reliable roof depends on the entire roofing assembly working together.

7. Will incorrect underlayment installation void my roof warranty?

It can affect warranty eligibility or coverage, particularly if the installation violates manufacturer requirements or causes damage.

However, warranty consequences vary by manufacturer, product, defect, and warranty language.

Ask your contractor for the actual product and workmanship warranty documents, including installation conditions and exclusions.

Conclusion

Understanding how underlayment is installed during roof replacement helps homeowners make better decisions about roofing materials, contractor quality, and long-term moisture protection.

The process begins with removing old roofing materials and inspecting the decking. Contractors then install required protective membranes, position underlayment from the lower roof edge upward, overlap each course, secure it properly, and coordinate it with flashing and the finished roofing system.

The most important factors are proper decking preparation, approved materials, correct overlaps, secure fastening, and careful treatment of vulnerable roof areas.

Before approving your replacement project, ask your roofing contractor for a written installation plan that identifies the underlayment product, protective membrane locations, and warranty requirements. These details help you evaluate the work with confidence and protect your investment.