Key points to remember
- The underlayment acts as a secondary water barrier between the roof decking and the surface covering.
- There are three main types: felt paper, synthetic underlayment, and self-adhesive frost-proof membrane.
- The Quebec Building Code requires specific protection for eaves to prevent ice dams.
- The choice of underlayment depends on the type of roofing material, the roof pitch, and local weather conditions.
- The best underlayment is one that meets both the code requirements and the recommendations of your flooring manufacturer.
What is the real purpose of a roof underlayment?
No roofing material is completely watertight on its own. A strong wind can lift a shingle. Water can seep in around a chimney or skylight without you even noticing. And in winter, ice dams force water to seep back up under the shingles, sometimes far enough to reach the wood.
The underlayment is designed to intercept this water before it causes damage. It slows down or stops water infiltration before it reaches the deck. It also plays a role during the construction process itself: between the time the old roofing is removed and the new one is installed, it protects the roof if the weather doesn’t cooperate.
What types of underlayment are there?
Felt paper is the oldest product. Made from paper saturated with asphalt, it comes in two thicknesses: No. 15, which is lighter, and No. 30, which is more tear-resistant. Both products meet the requirements of the National Building Code for the underlayment layer beneath shingles. Roofing felt is still used, but it is more fragile to handle on a busy construction site than newer options.
Synthetic underlayment is now the most common choice for roof repairs. It is lighter, less likely to tear during installation, and holds up better if exposed to the elements for a few days while the work is in progress. For most residential shingle roofs, it is the standard choice.
The self-adhesive ice and water barrier, often called an “ice and water shield,” is in a class of its own. It adheres directly to the wooden sheathing—without nails—and automatically seals around fasteners to prevent any water infiltration. It is used in the most vulnerable areas: eaves, the valleys between two roof slopes, and around chimneys or skylights.
What the regulations require in Quebec
The underlayment is not a matter of individual discretion for each roofer. The Quebec Building Code specifies where and how it must be installed.
For eaves, protection is mandatory in the vast majority of heated residential homes. The Code requires a covering that extends up the roof slope at least 900 mm from the eave, up to 300 mm inside the inner face of the exterior wall. This distance covers the area where ice dams typically form during Quebec winters, where standing water can seep up under the shingles.
A qualified roofer is familiar with these requirements and follows them without being asked. This is one of the reasons why any contractor performing construction work in Quebec must hold a license from the Régie du bâtiment du Québec. Before signing anything, make sure your contractor is listed in the Registry of License Holders.
Which type should you choose based on your situation?
| Location | Recommended Type | What You Need to Know |
| Sloped roof, asphalt shingles | Synthetic or No. 30 felt | Standard, well-established solution |
| Eaves and Areas Prone to Ice Jam Formation | Self-adhesive ice-barrier membrane | Required by the Building Code |
| Metal Roofing | Synthetic or self-adhesive | Check the manufacturer’s requirements |
| Valley sections, chimneys, skylights | Self-adhesive ice-proof membrane | Areas where water accumulates |
What to Check Before You Begin
Ask your roofer what type of underlayment he plans to use and exactly where. A qualified roofer will install the self-adhesive membrane along the eaves, in the valleys, and around penetrations right from the start, without waiting to be asked.
Best Practices After Installation
Once the roof has been redone, the underlayment requires no maintenance. What is worth doing, however, is an annual visual inspection of the roofing, ideally in the fall before the first snowfall. Look for loose, cracked, or missing shingles, and check around chimneys and skylights. A small problem spotted early costs much less to fix than a leak discovered on the ceiling in March.
A Solid Foundation for Your Roof
The underlayment isn’t visible, but it plays a crucial role. A roof installed with the right materials in the right places, in compliance with the Building Code, performs better over time and saves you from unexpected repairs. If you’re planning a roof renovation, now is the right time to make sure all components are up to standard—not just the visible roofing material.
Do you have any questions about your roof?
Our team can assess the condition of your roof and explain which materials are best suited for your building.
Contact Toitures LV for a no-obligation consultation.
