Inverted Roofing Systems Explained

An inverted roof is a thermal insulation system specifically designed for flat roofs. Unlike traditional roofing, where insulation is placed beneath the waterproofing membrane, this system reverses the order: the insulation is installed above the membrane and covered with a ballast layer.

This guide explains how inverted roofing works, its advantages and disadvantages, and the best practices for installation and maintenance.

How Does an Inverted Roof Work?

An inverted roof is an innovative insulation system that protects the waterproofing membrane by placing the insulation above it, unlike conventional roofing systems. This layered structure enhances durability and resistance against harsh weather conditions.

Layered Structure for Maximum Protection

An inverted roof consists of multiple superimposed layers, ensuring both waterproofing and insulation. The waterproof membrane is installed directly on the roof structure to prevent water infiltration. Above it, a thermal insulation layer, typically extruded polystyrene (XPS), reduces temperature fluctuations and protects the membrane from external damage.

A filtering fabric can be added to prevent fine particles from penetrating the insulation, extending its lifespan. Finally, a ballast layer (gravel, paving slabs, or green roofing) stabilizes the system and improves resistance to weather conditions. This structure provides greater longevity and protection against UV rays and freezing cycles.

The Role of the Waterproofing Membrane

The waterproofing membrane, installed directly on the roof structure, prevents water infiltration. By being protected by the insulation and ballast, it is less exposed to UV rays, temperature variations, and harsh weather, significantly extending its lifespan.

Depending on budget and project needs, different membrane materials can be used, such as modified bitumen, PVC, or EPDM, each offering specific waterproofing and durability benefits. A well-protected membrane reduces the risk of leaks, minimizes maintenance, and enhances the longevity of an inverted roof.

The Importance of Thermal Insulation (Extruded Polystyrene – XPS)

The thermal insulation layer ensures energy efficiency and must resist moisture, as it is exposed to external conditions.

Extruded polystyrene (XPS) is the most commonly used insulation material because:

• It does not degrade when exposed to water, unlike fiberglass or rock wool.
• It offers excellent thermal insulation, reducing heat loss.
• It withstands compression, allowing the addition of a ballast layer without compromising performance.

The Function of the Ballast Layer

The ballast layer is essential for stabilizing the insulation and preventing it from shifting due to wind or temperature variations. Different materials can be used:

Gravel: An affordable solution that evenly distributes weight and facilitates water drainage.

Concrete slabs: Ideal for creating an accessible terrace, adding durability and aesthetics.

Green roofing: Provides ecological and thermal benefits, improving insulation and retaining rainwater.

Advantages and Disadvantages of an Inverted Roof

An inverted roof is an innovative insulation solution with numerous benefits, but it also comes with challenges to consider before making a decision.

Advantages of an Inverted Roof

Enhanced Protection for the Waterproof Membrane

The insulation and ballast shield the waterproofing membrane from UV rays, weather damage, and temperature fluctuations, extending its lifespan and reducing maintenance costs.

Increased Roof Durability

This layered system is more resistant to extreme temperatures and weather conditions. It withstands freeze-thaw cycles better, making it an excellent choice for harsh climates.

Easier Maintenance and Access

Since insulation is above the membrane, roof access is easier without damaging the waterproofing layer, simplifying inspections and drain cleaning.

Ideal for Flat and Green Roofs

Inverted roofs are compatible with terraces, rooftop parking, and green roofs, offering improved thermal and acoustic insulation while contributing to better stormwater management.

Disadvantages of an Inverted Roof

Potential Thermal Performance Loss if Not Properly Drained

Even though XPS insulation resists moisture, poor drainage can affect its thermal efficiency. Adding a filtering fabric and ensuring proper sloping for drainage helps prevent this issue.

Requires Additional Ballast Weight

Because the insulation is exposed, it needs ballast stabilization. This adds extra weight, requiring a stronger roof structure to support it.

Higher Initial Cost

The installation cost is higher than traditional roofing due to specialized materials and techniques. However, this investment is offset over time through longer durability and lower maintenance costs.

How to Install an Inverted Roof

The installation process follows a specific order of layers to guarantee waterproofing, insulation, and durability.

1. Surface Preparation and Waterproof Membrane Installation

Before installation, the roof surface must be cleaned and prepared to ensure a smooth, dry foundation. The waterproofing membrane—whether EPDM, PVC, or modified bitumen—is then applied using welding, adhesives, or mechanical fasteners for optimal moisture protection.

2. Installation of Thermal Insulation

XPS insulation panels are placed tightly together without gaps to prevent heat loss. This ensures better thermal regulation and protects against temperature fluctuations.

3. Addition of a Filtering Fabric

A filtering layer (geotextile) prevents fine particles from penetrating the insulation while improving water drainage. It also helps prevent water stagnation and extends the system’s durability.

4. Ballast Installation for Stability

The ballast layer secures the insulation and protects the system from wind and weather variations.

Maintenance and Longevity of an Inverted Roof

While highly durable, an inverted roof still requires regular maintenance to ensure long-term performance.

Annual Inspection of Insulation and Ballast

A thorough inspection should be conducted at least once a year, preferably in spring or after severe weather conditions.

• Check ballast stability to ensure it remains evenly distributed.
• Inspect insulation panels for wear, moisture retention, or movement.
• Examine the waterproofing membrane for potential cracks or weaknesses.

Stormwater Drainage Maintenance

Proper water evacuation prevents stagnation and roof deterioration.

• Ensure drains and gutters are free of debris (leaves, dirt, or gravel).
• Identify persistent water pooling areas after rainfall and adjust drainage if needed.
• Verify roof slope efficiency to prevent water accumulation.

Routine Cleaning and Upkeep

• Remove debris regularly to maintain water flow.
• For green roofs, manage plant growth and substrate health.
• Quickly repair minor damages to prevent larger issues.

With proper maintenance, an inverted roof can last up to 40 years.

Why Hire a Professional for Inverted Roof Installation?

Installing an inverted roof requires technical expertise to ensure optimal waterproofing and insulation. Incorrect installation can lead to water infiltration, poor drainage, and premature material degradation.

At Les Couvreurs Duro-Toit, we provide expert roofing solutions, ensuring long-lasting protection and energy efficiency. From material selection to ballast installation, our specialists guarantee top-quality service tailored to Quebec’s climate conditions.

Contact us today for a professional evaluation and a high-performance inverted roof installation!