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:
- Secondary moisture protection
- Wind resistance
- Stabilization of the roofing system
- Protection during installation
- Insurance against unexpected leaks
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:
- Heavy snowfall
- Ice dams
- Wind-driven rain
- Condensation
- Freeze–thaw cycles
- Moisture infiltration
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:
- Low cost
- Simple installation
Disadvantages:
- Poor moisture resistance
- Tears easily
- Short lifespan
- Sensitive to temperature changes
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:
- High tear resistance
- Lightweight
- Excellent traction
- Handles moisture well
- UV resistance during installation
- Longer lifespan
Disadvantages:
- Quality varies between manufacturers
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:
- Eaves
- Valleys
- Around chimneys
- Roof penetrations
- Low-slope sections
Advantages:
- Forms a watertight seal
- Self-heals around nails
- Essential for ice dam protection
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:
- Wind-driven rain
- Backflow from ice dams
- Condensation
- Minor leaks
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:
- Metal panels shed water faster
- Condensation can occur beneath metal
- Temperature fluctuations are more intense
- Noise reduction and airflow matter
Most metal systems use:
- Synthetic underlayment for the field
- Ice & Water Shield for edges and valleys
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:
- Leaks during storms
- Deck rot
- Mold formation
- Heat blistering on shingles
- Condensation damage
- Premature roofing failure
- Water infiltration around penetrations
Underlayment quality often determines whether a roof lasts 10 years—or 40.
8. Underlayment Longevity
Synthetic underlayment typically lasts:
- 25–35 years on average
- Longer when combined with metal roofing
- Shorter when installed under aging asphalt systems
Ice & Water Shield can last 40+ years when properly installed.
9. Summary: Why Underlayment Matters
Underlayment:
- Protects against water infiltration
- Enhances durability
- Supports winter performance
- Reduces long-term maintenance
- Plays a major role in overall roof lifespan
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