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How to Properly Apply Roof Underlayment Step-by-Step

A watertight layer beneath shingles is your home’s defense against leaks and damage.

By Medha deb

Created on Sep 26, 2025

A watertight layer beneath shingles is your home’s defense against leaks and damage.

How to Apply Roof Underlayment

Roof underlayment is an essential part of a well-built roofing system. This protective barrier is installed between the roof deck and the outer roofing material, acting as the last line of defense against water infiltration, wind, and damage caused by harsh weather. Mastering the process of applying roof underlayment is crucial for homeowners and contractors seeking to extend the lifespan of their roofs and secure their investment.

What Is Roof Underlayment?

Roof underlayment refers to a thin, water-resistant or waterproof layer laid directly on the roof deck underneath shingles, metal panels, or tiles. It protects against moisture, snow, wind-driven rain, and provides temporary covering if exposed sheathing is caught in a storm before roofing is complete.

Types of Roof Underlayment

Asphalt-Saturated Felt: Traditional, also called “tar paper.” Reasonably water-resistant but heavier and less durable than newer synthetics.
Synthetic Underlayment: Lightweight, tear-resistant, and typically offers superior water and UV resistance.
Rubberized Asphalt: Self-adhering, highly waterproof, and commonly used in valleys and areas prone to ice dams or water pooling.

When and Why Apply Roof Underlayment?

Modern roofing codes require underlayment as a critical backup barrier. It’s especially vital:

For new roof installations, replacement, or over an existing deck.
As protection before the final roofing material goes on.
In climates with significant rainfall, snow, or wind-driven moisture.

Tools and Materials Needed

Roofing underlayment rolls (asphalt felt, synthetic, or rubberized asphalt)
Roofing nails, cap nails, staples, or self-adhering underlayment
Tape measure and chalk line
Utility knife or heavy scissors
Hammer, nail gun, or cap nailer
Roofing gloves and safety harness
Broom or leaf blower (for cleaning deck)
Drip edge flashing
Roofing sealant or patch materials (for repairs)
Ladder and roof brackets

Step-by-Step Guide: How to Apply Roof Underlayment

1. Preparation: Clean and Inspect the Roof Deck

Before any underlayment is installed, it’s vital to ensure the roof deck is clean, dry, and structurally sound:

Remove all debris, dust, nails, and leftover shingle pieces using a broom or blower.
Check for damaged, rotted, or warped sheathing. Replace compromised sections.
Ensure all surfaces are flat with no protruding fasteners.
If removing an old roof, strip off any previous underlayment for best results.

2. Install Drip Edge Flashing

Drip edges are metal strips installed along roof eaves and rakes to channel water away from the roof edge and into the gutters, preventing rot and damage to the fascia. Steps:

Place the drip edge along the eaves first. Align flush with the edge and secure every 12 inches with roofing nails.
Overlap sections by at least 2 inches to ensure a tight seam.
Cut angles at corners and create tabs for bends at valleys following the roof’s profile.
For synthetic underlayment, install over the eave drip edge and under the rake drip edge^*.

^*Always refer to the underlayment manufacturer’s instructions, as installation order can vary.

3. Measure and Cut the Underlayment

Measure the length of the roof horizontally and cut the underlayment to manageable lengths (typically 10-15 feet per section). This makes handling and placement easier, especially on steep pitches or in windy conditions.

Rolls are typically marked with lines to guide overlaps and alignment.
Use a utility knife and a firm edge for straight, clean cuts.

4. Lay the First Course of Underlayment

Start at the bottom (eaves) and work upwards. This prevents water from running under the seams, directing it over the underlayment and off the roof.

Position the first row so it overhangs the eaves by 1 inch to help direct water away from the roof and into the gutters.
Use a chalk line to snap a straight horizontal line across the deck as a guide.
Smooth out the underlayment as you go to avoid wrinkles or trapped air.

5. Secure the Underlayment

Use roofing nails or staples with plastic caps for secure installation; this prevents tearing and water penetration.
Fasten along the upper edge first, spacing fasteners about 12 inches apart, unless specific product guidance differs.
Do not overdrive fasteners, as this can damage or tear the material.
For “peel-and-stick” underlayment, peel a section of the backing, align, press in place, and work gradually across, smoothing as you go.

Some synthetic underlayments have pre-printed fastener marks to indicate placement.

6. Overlap and Stagger the Rows

Start each new course above the previous one, overlapping by at least 6 inches at horizontal seams and 3 inches at vertical seams (verify with product specs).
End joints should be offset at least 6 feet from the joints in the course below to avoid a continuous seam.
In low-slope roofs (less than 4:12), even greater overlap (up to 22 inches) may be required per code or product instructions.

7. Finish Edges, Valleys, and Penetrations

Apply extra layers or specialty underlayment (like ice and water shield) in valleys or at eaves where leaks are more likely to occur.
Cut underlayment to fit tightly around chimneys, vent pipes, and skylights. Create tabs and carefully layer pieces for full coverage.
Seal seams and nail holes with recommended sealant or tape as directed by manufacturer.
At rakes (gable ends), the underlayment should extend slightly past the roof edge; then, install additional drip edge if directed.

Pro Tips for a Successful Roof Underlayment Job

Work in calm weather: Windy or wet conditions can make underlayment harder to align and secure.
Use roof jacks and harnesses: Safety is paramount on steep or high roofs.
Choose the right fasteners: Products like cap nails greatly reduce tear-out, especially on synthetic products.
Don’t stretch underlayment: Allow a natural fit to avoid tears when temperatures change.
Follow all manufacturer and local building code requirements.

Common Mistakes to Avoid

Installing on dirty or wet surfaces – Moisture and debris undermine underlayment adhesion or cause rot.
Insufficient overlap or improper alignment – Gaps can lead to major leaks.
Underdriving or overdriving fasteners – Can cause tearing, uplift, or leaks around nail holes.
Forgetting about flashing – Skipping proper flashing around valleys and penetrations is a recipe for leaks.
Sacrificing safety – Always use proper fall protection, even for simple jobs.

Roof Underlayment Comparison Table

Type	Pros	Cons	Best Use Scenarios
Asphalt Felt	Affordable, basic moisture barrier, traditional choice	Heavier, less durable, can wrinkle or tear, absorbs water	Budget-friendly, moderate climates
Synthetic	Lightweight, tear-resistant, longer UV resistance, easier to install	Higher material cost	Most modern roofs, extreme weather, DIY and professional jobs
Rubberized Asphalt	Highly waterproof, self-sealing, ideal for critical areas	Expensive, not usually necessary for entire roof	Valleys, eaves, hips, ice dam prone regions

After Applying the Underlayment: Next Steps

Once all underlayment is secure and all flashing, penetrations, and edges are watertight, proceed to install your roof covering (shingles, tiles, or panels) as soon as possible to prevent UV degradation.
Regularly inspect underlayment for tears or unfastened edges until the roof is fully completed.
If more than a few days pass before roofing, reinspect and repair as weather or sun may cause damage or lifting.

Frequently Asked Questions (FAQs)

Q: Can I install roof underlayment myself, or should I hire a professional?

A: Many DIYers can install underlayment on simple roofs if comfortable with heights and follow safety protocols. For steep, complex, or high roofs, hiring a professional is safer and yields better results.

Q: How long does roof underlayment last?

A: Synthetic underlayments may last 20-30 years, asphalt felt typically 10-15 years beneath intact roofing. Exposure to prolonged sunlight or weather can drastically shorten its useful life before the finished roof is applied.

Q: What happens if underlayment gets wet before installation is complete?

A: Most products tolerate a brief period of moisture if promptly covered, but persistent or standing water can degrade performance. Always check manufacturer’s maximum exposure period and replace any damaged sections.

Q: How much should roof underlayment overlap?

A: Minimum of 6 inches horizontally and 3 inches vertically; always check product-specific requirements and local codes.

Q: Do I need ice and water shield if I use synthetic underlayment?

A: Ice and water shield is extra insurance in leak-prone areas like valleys, eaves, and around roof penetrations, even with synthetic underlayment. Use both where manufacturer’s instructions or local building codes require.

Safety Tips for Roofing Projects

Use a stable ladder and roof brackets.
Always wear a harness and non-slip shoes on the roof.
Keep children and pets away from the work zone.
Never work in wet, icy, or excessively windy conditions.
If in doubt, consult a licensed roofing contractor.

Final Thoughts on Installing Roof Underlayment

Properly applying roof underlayment is a straightforward yet critical task for protecting the integrity, value, and safety of your home. Whether you’re refreshing your roof as a homeowner or guiding clients as a professional, following these step-by-step instructions, using quality materials, and respecting safety at all times will ensure a durable, weather-tight barrier beneath your finished roof.

References

Author

medha deb

Medha Deb is an editor with a master's degree in Applied Linguistics from the University of Hyderabad. She believes that her qualification has helped her develop a deep understanding of language and its application in various contexts.

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