Why Cant Strips Matter in Roofing

In India’s diverse climate—from the heavy monsoon rains of Kerala and the Northeast to the intense summer heat of Rajasthan and Gujarat—roof waterproofing is one of the most critical aspects of building performance. While membranes and waterproofing systems often receive the most attention, a small but vital component called the cant strip plays a major role in preventing roof failures.

Whether you’re designing a commercial building, industrial warehouse, hospital, educational institution, or residential complex, understanding when and where cant strips are required can significantly improve the life of your roof.

What Is a Cant Strip?

A cant strip is a triangular-shaped section installed where a horizontal roof slab meets a vertical surface such as a parapet wall, lift machine room, staircase headroom, equipment curb, or service duct.

Instead of forcing the waterproofing membrane to bend sharply at a 90-degree angle, the cant strip creates a gradual transition of approximately 135 degrees. This reduces stress on the waterproofing layer and provides continuous support at one of the most vulnerable parts of the roof.

Why Are Cant Strips Important?

Most roof leakages in India occur not through the middle of the roof slab but at junctions, including:

• Roof-to-parapet intersections
• Expansion joints
• Pipe penetrations
• Equipment bases
• Skylights
• Lift overrun walls

These locations experience constant movement due to:

• Thermal expansion and contraction
• Heavy monsoon rainfall
• Ponding water
• Building settlement
• Wind loads

Without proper support, waterproofing membranes may crack, wrinkle, separate, or lose adhesion, eventually allowing water to penetrate the building.

Cant strips help distribute stresses more evenly and improve the durability of flashing details.

Cant Strips in Built-Up Bituminous Roofing

Traditional built-up bituminous roofing systems have been widely used on industrial buildings, factories, warehouses, and institutional projects across India.

The reinforcing felts used in these systems are relatively stiff. When bent sharply around a 90-degree corner, they may:

• Develop cracks over time
• Pull away from the substrate
• Create unsupported voids
• Lose adhesion due to thermal movement

Installing a cant strip provides a smoother transition, reducing stress concentrations and improving the long-term performance of the waterproofing system.

For conventional bituminous roofing, cant strips remain a recommended best practice.

Modified Bitumen Membranes

APP and SBS modified bitumen membranes are among the most widely used waterproofing systems in India because of their excellent resistance to UV radiation, heat, and heavy rainfall.

Modern modified membranes are reinforced with polyester, making them much more flexible than traditional roofing felts.

Some membrane manufacturers specify that cant strips are optional in torch-applied systems, while others continue to recommend them as part of their standard installation details.

Before omitting a cant strip, contractors should always follow the manufacturer’s installation manual and project specifications.

Selecting the Right Cant Strip Material

Different roofing systems require different cant strip materials.

Cement Mortar Fillets

In India, cement mortar fillets are among the most common solutions.

A 75 mm × 75 mm (or project-specified) triangular mortar fillet is constructed at the junction of the roof slab and parapet before waterproofing is applied.

Advantages include:

• Easy availability
• Low cost
• Compatibility with cementitious waterproofing
• Suitable for most RCC roofs

However, workmanship is critical. Poorly compacted or cracked mortar fillets can compromise waterproofing performance.

Insulation-Based Cant Strips

For insulated roofing assemblies, cant strips manufactured from rigid insulation boards such as perlite or mineral fibre are commonly used.

These provide:

• Smooth membrane support
• Improved thermal continuity
• Reduced thermal bridging

They are particularly suitable for modern commercial roofing systems.

Metal and Concrete Cant Strips

Where heavy foot traffic or mechanical impact is expected, concrete or metal cant strips may provide additional durability and support.

Metal Compatibility Matters

Roofing details often combine waterproofing membranes with metal flashings, gutters, and edge trims.

Incorrect combinations of metals can lead to galvanic corrosion, especially in India’s humid coastal regions.

Galvanized Steel

Galvanized steel is commonly used for flashings and gutters because its zinc coating protects the underlying steel.

However, galvanized steel should not be installed in direct contact with copper where moisture is present. Rainwater can create an electrolytic reaction that accelerates corrosion of the zinc coating.

This consideration is particularly important in coastal cities such as Mumbai, Chennai, Kochi, Visakhapatnam, and Goa, where high humidity and salt-laden air increase corrosion risks.

Lead

Lead develops a natural protective oxide layer that minimises corrosion. Although it is less commonly used in modern Indian roofing projects due to environmental considerations, it generally exhibits minimal galvanic interaction with other metals.

Best Practices for Indian Projects

To maximise roof performance:

• Always provide properly designed flashing details at parapet walls and penetrations.
• Follow the waterproofing manufacturer’s installation recommendations.
• Use compatible cant strip materials for the selected roofing system.
• Ensure proper substrate preparation before waterproofing.
• Avoid incompatible metal combinations in flashings and roof drainage systems.
• Pay special attention to detailing before the onset of the monsoon season.

Final Thoughts

A roof’s waterproofing system is only as good as its detailing. In India, where buildings are exposed to intense UV radiation, prolonged monsoon rains, temperature fluctuations, and increasing weather extremes, roof-to-wall junctions require particular attention.

Cant strips may appear to be a minor component, but they play a significant role in reducing membrane stress, improving flashing performance, and extending the service life of roofing systems. Whether using conventional bituminous roofing, modified bitumen membranes, or advanced single-ply systems, selecting the appropriate cant strip—and installing it correctly—can help prevent costly water ingress and reduce long-term maintenance.

For architects, structural consultants, waterproofing specialists, and roofing contractors, investing time in proper transition detailing is one of the simplest ways to achieve a durable and leak-free roof.