Cant Strip on Flat Roofs: Installation and Maintenance
Cant strips are angled boards installed at roof edges to redirect water away from vertical details and to protect roofing membranes. On flat roofs, cant strips help manage transitions between parapets, parapet corners, and roof decking, reducing membrane wear and helping with gutter performance. This article explains what cant strips are, why they matter on flat roofs, materials and installation steps, common issues, and maintenance best practices to extend roof life.
What Is A Cant Strip?
A cant strip is a beveled piece of wood or composite material installed along the edge or corner of a roof. On flat roofs, cant strips create a smooth, angled transition between horizontal surfaces and vertical planes. This angle allows water to shed more effectively and minimizes stress on the membrane at sharp corners or abrupt changes in plane. Cant strips can be used at parapet walls, edge details, and around penetrations to prevent cracking and punctures.
Why Cant Strips Matter On Flat Roofs
On flat roofs, water drainage is critical. Cant strips help direct water toward drains, scuppers, or gutters and reduce pooling near the edge. They also protect the edge membrane from abrading against sharp corners, which can cause premature failures. By softening transitions, cant strips reduce wind uplift stress on the roofing system and help maintain a watertight seal around flashings and terminations. Correct installation contributes to longer membrane life and improved roof performance.
Materials And Tools
Select cant strips that match the roofing system and climate. Common options include:
- Plywood or OSB with weatherproof coating for exterior use.
- Composite or PVC foam cant strips for moisture resistance.
- Pressure-treated lumber suitable for exterior exposure, treated against rot.
- Metal cant strips for highly durable edge details on some membranes.
Tools commonly needed:
- Measuring tape, square, and pencil
- Circular saw or handsaw for cutting angles
- Construction adhesive compatible with roofing membranes
- Roofing nails or screws with corrosion resistance
- Sealant compatible with the membrane system
- Protective gear and ladder safety equipment
Installation Steps
Follow these steps to install cant strips on a flat roof edge effectively:
- Assess and prepare: Inspect the edge for damage, clean the surface, and ensure proper drainage paths. Repair any membrane damage before adding cant strips.
- Cut to fit: Measure the length of each edge and cut cant strips at the correct angle to match the roof slope and parapet profile. Typically a 45-degree bevel is used, but consult the membrane manufacturer’s guidelines.
- Prime and seal: Apply a compatible primer or adhesive to the contact surface if required by the product, ensuring a clean, dry substrate.
- Attach strips: Position cant strips along the edge, ensuring a snug fit with no gaps. Secure using corrosion-resistant screws or fasteners as recommended, driving them at regular intervals.
- Seal joints: Apply sealant or flashing tape at joints and transitions to create a continuous waterproof seam. Pay attention to corners and penetrations.
- Overcoat and protect: If the roof system specifies, install a protective layer or additional flashing over the cant strip to shield against weather exposure.
Common Issues And Troubleshooting
Several issues can arise with cant strips on flat roofs. Address them promptly to preserve waterproofing:
- Membrane punctures at the contact edges due to poor fit or movement. Remedy by resealing and replacing damaged sections, ensuring proper edge support.
- Water ponding near cant strips indicates drainage problems. Reevaluate slope, drain location, or gutter connection and adjust as needed.
- Delamination or warping from moisture exposure. Use moisture-resistant materials and verify that installation won’t trap water behind the strip.
- Inadequate adhesion causing loose strips. Clean surfaces thoroughly and follow manufacturer’s curing times and temperature ranges.
Maintenance And Longevity
Regular upkeep extends cant strip performance on flat roofs. Consider these practices:
- Annual inspection of edge details, flashing, and sealant joints for signs of wear, cracking, or moisture intrusion.
- Clean drainage paths to ensure water flows away from edges and does not back up along cant strips.
- Re-seal as needed to maintain watertight joints, especially after severe weather or freeze-thaw cycles.
- Material checks for rot, swelling, or corrosion and replace compromised cant strips promptly.
- Compatibility verification with membrane products when performing repairs or roof resurfacing to avoid chemical incompatibilities.
Safety Considerations
Installation and maintenance involve working at height. Follow OSHA guidelines, use fall protection, secure ladders, and wear appropriate PPE. If the roof is actively leaking or structurally compromised, consult a licensed roofing professional before proceeding.
Choosing A Professional Or DIY Approach
For complex edge details, high-slope implications, or membranes with strict specifications, hire a roofer with experience in cant strip installation. A professional can assess substrate conditions, select compatible materials, and ensure proper flashing and drainage integration. Some flat roof systems require specific cant strip materials or installation sequences to maintain warranty terms. Always verify product compatibility with the existing roofing system.
Conclusion
Cant strips on flat roofs improve edge detailing, water management, and membrane longevity. Proper material selection, precise installation, and proactive maintenance are key to preventing leaks and membrane damage. When in doubt, consult a roofing professional to ensure the job meets current standards and warranty requirements.