Low-slope roofs: what you need to know

Low-slope roofing is becoming increasingly common in Quebec, especially on modern buildings, garages, and home extensions. While this type of roof may seem simple at first glance, it requires specific technical choices, particularly in terms of covering materials, upstands, and minimum slope.

Here are the key points to understand before starting your project.

What is a low-slope roof?

A roof is considered “low-slope” when the angle of its surface is less than 4/12 (or 18.4 degrees). In other words, for every 12 inches horizontally, the roof rises less than 4 inches vertically. This is sometimes also expressed as a slope percentage. For reference:

• A 3/12 slope equals about 14%.
• A 2/12 slope equals about 9%.
• Below 2/12, the roof is almost flat and requires a specific system.

This technical detail may seem minor, but it directly impacts the type of roofing material you’ll need.

Why does a low slope require special treatment?

Unlike steeper roofs, a low-slope roof drains water more slowly. This increases the risk of water infiltration, especially if the membrane isn’t completely watertight. Choosing the wrong material or poor installation can lead to moisture issues, mold, or premature deterioration of the roof.

That’s why it’s essential to reinforce low-slope roofs with suitable flashings and membranes—especially around chimneys, skylights, and side walls. These upstands prevent water from seeping in at the junctions between surfaces.

What is the best covering for a low-slope roof?

Several roofing materials are available, but not all are suitable for reduced slopes. Based on our expertise, here are the most effective options:

1. Two-ply elastomeric membrane

This is the solution we most often recommend. The elastomeric membrane combines two layers of modified bitumen: a self-adhesive or torched base, and a finishing membrane offering excellent waterproofing. It typically lasts between 25 and 30 years and performs very well in Quebec’s climate.

2. Self-adhesive systems like IKO Roof-Fast

Very popular for slopes from 2/12 to 4/12, these cold-applied systems are ideal for small residential roofs. Installation is quicker, which can reduce labor costs.

3. TPO or EPDM for very low slopes

Primarily used for flat or near-flat roofs, these single-ply membranes are durable, flexible, and recyclable. They’re usually installed mechanically or with adhesive.

4. Asphalt shingles: to be used with caution

While asphalt shingles are common in Quebec, they’re not suited for all slopes. For low-slope roofs, you must have at least a 2/12 pitch (around 9%) and add a full-surface self-adhesive underlayment.

Even with these precautions, the risk of leaks remains higher than with continuous membranes. We recommend shingles only if the slope is near the minimum limit and if ice and water accumulation are properly managed.

How to ensure the longevity of your roof

Beyond choosing the right covering, several best practices can extend the life of your roof:

• Regularly inspect joints, upstands, and drains.
• Prevent snow and ice buildup, especially near edges.
• Make sure downspouts are clear.
• Avoid temporary patch-type repairs.

We recommend a full inspection every 3 to 5 years to identify potential issues before they cause significant damage.

Why trust us with your low-slope roof?

At Duro-Toit, we have years of experience installing, repairing, and maintaining low-slope roofs. We work with precision, no matter the chosen roofing system. We also support you in selecting materials, designing upstands, proper installation, and long-term maintenance.

We understand that a roof is not just a covering: it’s protection from the elements, thermal insulation, and an essential part of your home’s architecture.