A cant strip is a beveled or triangular-shaped roofing component installed where a low-slope roof meets a vertical surface, such as a parapet wall, exterior wall, equipment curb, or roof penetration. Its primary purpose is to create a gradual transition between the horizontal roof surface and the vertical surface, allowing the roofing membrane and flashing to be installed without forcing the material around a sharp 90-degree corner.
Cant strips are particularly important in many built-up roofing (BUR) and modified bitumen roofing systems, although whether one is required depends on the specific roof membrane, manufacturer’s details, and project requirements. The National Roofing Contractors Association (NRCA) identifies cant strips as part of appropriate base-flashing details for certain membrane systems.
Why Are Cant Strips Used?
The junction between a roof deck and a wall or curb is one of the most important areas to detail correctly. Without a cant strip, the membrane may need to make a relatively abrupt change in direction. This can create stress at the bend, particularly as roofing materials experience temperature changes, building movement, and other stresses.
A cant strip changes the geometry of the transition, providing a sloped surface that allows the membrane and flashing to move from horizontal to vertical more gradually. This helps reduce concentrated stress and provides better support for the roofing materials. IIBEC notes that, in traditional built-up roofing, cant strips can change the bend from approximately 90 degrees to a less severe transition and help reduce the potential for membrane damage.
Cant Strips at Wall Transitions
At a roof-to-wall or roof-to-parapet transition, the cant strip is generally positioned at the base of the vertical wall. The roofing membrane extends across the roof and transitions up the wall using the cant as a gradual support.
This detail is important because base flashing must remain properly supported and securely integrated with the roof membrane. A properly designed transition can help reduce the possibility of membrane damage, cracking, splitting, or separation at this vulnerable location.
NRCA guidance specifically describes the use of a cant strip at the base of a vertical wall or penetration to provide a transition for roofing materials and base flashing.
Cant Strips at Roof Curbs
Roof curbs are another common location for cant strips. HVAC equipment, skylights, exhaust fans, and other rooftop equipment often sit on raised curbs that penetrate the roofing system.
The roofing membrane and flashing must transition from the roof surface up the sides of the curb. A cant strip can provide the gradual transition needed for the membrane and flashing assembly, depending on the roofing system being installed.
This is especially important because rooftop equipment areas can experience foot traffic, maintenance activity, vibration, and weather exposure. A properly detailed curb transition helps provide a more durable connection between the roof membrane and vertical flashing.
Are Cant Strips Required on Every Roof?
No. A cant strip should not automatically be specified for every roofing system.
The requirement depends on the membrane type, manufacturer’s installation details, project specifications, and applicable building or roofing requirements. For example, IIBEC guidance notes that some single-ply membrane systems can conform to transitions without a traditional cant strip, while certain modified bitumen systems may or may not require one depending on the manufacturer’s design.
For this reason, roofing contractors should always follow the specific manufacturer’s approved details and project specifications rather than assuming that one cant-strip configuration applies to every roof.
Why Proper Installation Matters
A cant strip is a relatively small component, but it can play an important role in the performance of a low-slope roofing assembly. Improperly installed or missing transition components can contribute to stress concentrations, unsupported membrane areas, flashing problems, and potential water intrusion.
The right approach is to evaluate the entire wall or curb assembly—including the roof membrane, insulation, cant, base flashing, fasteners, termination, and counterflashing—as one integrated waterproofing detail.
For commercial and industrial buildings, professional roof inspections and properly designed flashing details can help identify vulnerable transitions before they become costly leaks.
The Bottom Line
A cant strip creates a smoother transition between a horizontal roof and a vertical wall or curb, helping roofing membranes and flashing handle the change in direction with less concentrated stress. They are especially relevant to many built-up and modified bitumen roofing assemblies, but they are not universally required for every membrane system.
When installing or repairing a commercial roof, the cant-strip requirement should be determined from the roofing manufacturer’s approved details, project specifications, and applicable industry guidance. Properly detailing these small but critical transitions can contribute significantly to the long-term watertight performance of the roof assembly.
Related Questions
- What is a reglet and what types are used in commercial roofing?
- What flashing plies are required at walls and curbs?
- What is a termination bar and how is it sealed?
- What does the code require for roof-to-wall transitions on commercial buildings?
- How do I detail a roof at a masonry parapet with through-wall flashing?
- What is a scupper box and conductor head?
- What is a target patch and where is it used?
Commercial Roof Coatings & Restoration
