What Is Roof Underlayment? (ROOFNOW™ Ontario Guide)
Roof underlayment is one of the most important layers in a roofing system, yet most homeowners never see it and rarely hear about it. This hidden component sits directly on top of the roof deck and acts as a secondary weather barrier underneath shingles or metal roofing. In Ontario’s climate—where heavy snow, wind-driven rain, and freeze-thaw cycles are constant—underlayment is essential for long-term roof protection.
What Is Roof Underlayment?
Underlayment is a protective sheet installed between the roof deck and the roofing material. It shields the deck from moisture, prevents leaks during ice damming, and provides temporary protection if shingles are damaged. Even though the roofing material sheds water, underlayment is what keeps the deck dry when weather conditions become extreme.
Types of Roof Underlayment
- Synthetic Underlayment – The modern standard for roofing systems
- Ice & Water Shield – A self-sealing membrane used in vulnerable areas
- Felt Paper (Tar Paper) – Older technology, used less today
For Ontario homes, synthetic underlayment and ice & water shield offer the best long-term performance because they resist moisture, temperature swings, and wind uplift.
Where Underlayment Is Installed
Underlayment is typically installed across the entire roof deck, with ice & water shield added in high-risk areas:
- Along eaves to prevent ice dam leaks
- In valleys where water flow is concentrated
- Around chimneys, vents, and skylights
- At roof transitions and slopes
These areas experience the most stress during storms, winter melt, and freeze-thaw cycles.
Why Underlayment Matters in Ontario
Ontario winters produce heavy snow loads, ice dams, and rapid temperature fluctuations. These conditions allow water to back up under shingles, making underlayment the final line of defense for the roof deck.
- Prevents leaks caused by melting snow under shingles
- Protects the deck during storms or missing shingles
- Improves roof longevity by reducing moisture exposure
- Seals around fasteners to block wind-driven rain
Without proper underlayment, even a brand-new roof can fail prematurely.
Common Underlayment Issues
- Insufficient coverage near eaves
- Improper installation around penetrations
- Using outdated felt paper on modern roofs
- Not sealing valleys with ice & water shield
These mistakes lead to leaks, deck rot, and other long-term structural problems.
Choosing the Right Underlayment
For Ontario homes, the most effective combination is:
- Synthetic underlayment for full-roof protection
- Ice & water shield on all eaves, valleys, and penetrations
This setup creates a moisture-resistant barrier capable of withstanding harsh weather conditions year-round.
Learn more at www.roofnow.ca
Expert Notes (ROOFNOW™)
Roof underlayment is the hidden backbone of a roofing system. In Ontario’s climate, proper underlayment installation is essential for preventing leaks, protecting the roof deck, and maximizing long-term performance. Whether a homeowner chooses asphalt or metal roofing, underlayment plays a critical role in roof engineering.
ROOFNOW™ provides Ontario homeowners with technical, engineering-based roofing knowledge covering attic airflow, soffit performance, winter moisture behaviour, and long-term roof durability. Explore more at the ROOFNOW™ Knowledge Center, www.roofnowontario.com, or visit the ROOFNOW™ main website at www.roofnow.ca.
For deeper insights into long-term roofing performance, read the ROOFNOW™ educational book:
Roof Smart. Roof Once.
🏠 STOP RE-ROOFING. ROOF SMART. ROOF ONCE. ROOFNOW™.
#roofnowontario