Underlayment is one of the most important components of a roofing system. Installed directly over the roof deck and beneath the primary roofing material, underlayment provides a secondary barrier against moisture, wind-driven rain, ice, and condensation. Although it is hidden, its performance has a major influence on roof durability and long-term protection.

This guide explains the role of underlayment, the different types used in roofing, and why proper installation is essential—especially in regions with harsh winter conditions.

1. What Is Roof Underlayment?

Roof underlayment is a protective layer placed between the roof deck and the roofing material. Its primary purpose is to provide:

Underlayment protects the deck long before shingles or metal panels are installed, and it remains a crucial component throughout the life of the roof.

2. Why Underlayment Is Essential in Cold and Wet Climates

Ontario’s climate introduces risks such as:

The underlayment acts as a continuous protective membrane, preventing water from reaching the deck even when the primary roofing material is stressed.

3. Types of Roof Underlayment

There are three primary categories:

A) Asphalt-Saturated Felt (Traditional Felt Paper)

This older underlayment is made of cellulose saturated with bitumen.

Advantages:

Disadvantages:

Felt is rarely recommended in modern cold-climate installations.

B) Synthetic Underlayment

Made of woven polymers, synthetic underlayment is now the standard used in most new installations.

Advantages:

Disadvantages:

Synthetic underlayment performs significantly better than felt, especially in climates with snow and wind.

C) Ice & Water Shield (Peel-and-Stick Membrane)

A self-adhering membrane used in vulnerable areas.

Common applications:

Advantages:

Ice & Water Shield is a key component for preventing winter leakage in northern climates.

4. Where Underlayment Is Installed

Different zones require different protection layers:

Eaves and Lower Roof Sections

Require Ice & Water Shield due to ice dam risk.

Valleys

One of the highest water-volume areas—Ice & Water Shield recommended.

Main Field of the Roof

Typically covered with synthetic underlayment.

Penetrations

Vents, skylights, and chimneys require reinforced membranes.

5. How Underlayment Protects the Roof Deck

Moisture Defense

Underlayment blocks:

UV Protection During Installation

Synthetic underlayment protects the deck if the main roofing is delayed.

Secondary Barrier Against Aging Materials

As shingles or panels age, underlayment becomes increasingly important.

6. Underlayment and Metal Roofing

Metal roofing requires high-quality underlayment because:

Most metal systems use:

Proper underlayment ensures decades of stable performance.

7. Risks of Poor or Missing Underlayment

If underlayment is low-quality or improperly installed, issues may include:

Underlayment quality often determines whether a roof lasts 10 years—or 40.

8. Underlayment Longevity

Synthetic underlayment typically lasts:

Ice & Water Shield can last 40+ years when properly installed.

9. Summary: Why Underlayment Matters

Underlayment:

In cold, wet, and variable climates, high-quality underlayment is a critical foundation of any roofing system.

Learn More at ROOFNOW™ Knowledge Center

For additional roofing science, material breakdowns, and technical explanations, visit:
https://new.roofnow.ca

For Sales & Service (Ontario Homeowners)

For roofing inspections, service details, and replacement options, visit:
https://www.roofnow.ca

Recommended Reading

The Real Cost of a Cheap Roof — Paperback
https://www.amazon.ca/dp/B0G3L5HVVG

Roof Smart. Roof Once. — FREE Kindle Edition
https://www.amazon.ca/dp/B0G3KF4DV1

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