The Cant Strip For Flat Roofs article explains what cant strips are, why they matter for low‑slope and flat roofing systems, and how to choose, install, and maintain them for long‑term roof performance. Readers will learn practical guidance on materials, common mistakes, costs, and code considerations.
| Topic | Key Takeaway |
|---|---|
| What Is A Cant Strip | Provides smooth transition between deck and parapet to prevent membrane stress |
| Materials | Wood, asphalt, or synthetic options chosen by durability and compatibility |
| Installation | Proper slope, adhesion, and fastener pattern are essential |
| Alternatives | Preformed metal fascias, tapered insulation, or chamfered blocking |
What Is A Cant Strip And Why It Matters
A cant strip is an angled filler placed at the junction of a roof deck and parapet wall to create a gradual slope for roofing membranes and flashing. Its purpose is to reduce sharp bends and stress points that cause membrane fatigue and leaks.
On flat and low‑slope roofs, membranes and flashing must accommodate movement from thermal expansion, building settlement, and wind uplift. Using a cant strip extends membrane life by distributing stress over a larger bend radius.
Types Of Cant Strip Materials
Common cant strip materials include softwood (commonly pine), engineered wood block, asphalt‑impregnated fiberboard, rigid foam, and preformed plastic or rubber. Selection depends on compatibility with the roofing system and resistance to moisture and compression.
For built‑up, modified bitumen, and EPDM systems, asphalt‑compatible materials or asphalt‑coated wood are often used. Using incompatible materials can lead to poor adhesion and premature failure.
Standard Sizes And Profiles
Cant strips are available in various bevel angles and sizes, commonly 1″ x 3″ and 2″ x 4″ cross sections with a 45° or custom bevel. Larger profiles provide a gentler transition and are preferred where equipment or heavy foot traffic is expected.
Custom tapered shapes can be fabricated on site to match parapet heights and insulation thicknesses. Choosing the correct profile ensures the membrane is not excessively folded or stretched.
When To Use A Cant Strip On A Flat Roof
Cant strips are recommended at parapet walls, curbs, penetrations, and edge terminations where a roof membrane changes plane. They are particularly important where flashing must be sealed to vertical surfaces.
They may be unnecessary on roofs with substantial tapered insulation achieving smooth transitions or on some metal roof details where different flashing systems apply. Evaluate the roof assembly to confirm whether a cant strip will improve durability.
Step‑By‑Step Installation Guide
Preparation begins with a clean, dry substrate and trimming the cant strip to fit the perimeter. Proper adhesion and mechanical fastening are critical to prevent displacement during membrane application.
Typical steps: position the cant strip against the parapet, fasten per manufacturer recommendations, seal joints with compatible adhesive or bitumen, and lap the membrane over the strip allowing adequate cover and adhesion. Installers should avoid voids beneath strips and ensure continuous support.
Fastening And Adhesion Best Practices
Fasteners should penetrate backing materials and be spaced to resist wind uplift and dynamic loads. Use washers or fastener plates where materials are compressible. Manufacturer guidelines for spacing, penetration depth, and adhesive type must be followed.
For hot‑applied systems, tacking with bitumen secures strips before membrane rolling. For cold adhesives or single‑ply systems, use compatible primers and adhesives. Incompatible adhesives can cause delamination or chemical degradation.
Common Installation Mistakes To Avoid
Frequent errors include undersized cant strips, gaps at joints, improper fastener spacing, and using materials incompatible with the roof membrane. These mistakes concentrate stress and lead to premature flashing failures.
Other issues: embedding wood strips in continuous wet conditions without protection and not providing a termination bar or cover flashing. Design details that ignore moisture and movement cause rot, swelling, or membrane separation.
Alternatives To Traditional Cant Strips
Tapered insulation can create gradual transitions without additional wood or block, and preformed metal or PVC transition pieces can serve in specific assemblies. Alternatives are useful where moisture concerns or long‑term compression resistance are priorities.
Another option is factory‑made flexible cants specifically matched to single‑ply membranes that resist rot and provide consistent shapes. Synthetic cants reduce variability and installation labor.
Cost Considerations And Lifespan
Costs vary by material, labor, and roof perimeter complexity. Wood cant strips are inexpensive in materials but labor‑intensive. Preformed synthetic options cost more but may lower long‑term maintenance. Budgeting should include both initial installation and expected longevity.
Lifespan depends on material durability and exposure; protected strips beneath membranes can last as long as the roofing system, while exposed or moisture‑saturated wood can deteriorate rapidly. Choosing rot‑resistant or coated materials improves service life.
Maintenance And Inspection Tips
Regular roof perimeter inspections should check for membrane cracks at transitions, displacement of strips, and moisture intrusion. Early detection of separation or blistering at cant transitions prevents larger repairs.
If repairs are needed, replace water‑damaged strips and restore proper adhesion and flashing continuity. Documenting repairs and materials used helps future maintenance planning. Maintenance records support warranty claims and long‑term roof management.
Building Code And Permit Considerations
Local codes and manufacturer specifications may dictate acceptable materials, fastening, and fire performance for cant strips, especially on commercial buildings. Consult local building officials and roof system manufacturer requirements before installation.
Historic buildings or special occupancies may require specific noncombustible details; in those cases, noncombustible cants or engineered solutions might be mandated. Permits and inspections can prevent costly rework later.
Compatibility With Common Flat Roof Systems
Built‑up roofing and modified bitumen systems traditionally use asphalt‑coated cant strips or wood guarded with bitumen, while EPDM and TPO benefit from synthetic cants designed for single‑ply adhesion. Compatibility ensures long‑term bond and reduces chemical interactions that degrade membranes.
Always follow the membrane manufacturer’s accessory guidance for cant material and installation details to preserve warranties and performance. Using unapproved materials can void warranties.
Selecting A Professional Contractor
Choose contractors with flat roofing experience and references showing successful perimeter and detail work. Request written detail drawings and material specifications for cant strips and flashing. Qualified contractors will provide documentation tying materials to manufacturer details.
Verify worker training, roofing system certifications, and adequacy of insurance and warranties. Obtain multiple bids and compare scope, not just price. Skillful detail work at transitions often distinguishes long‑lasting roofs from short‑lived ones.
Frequently Asked Questions
Are Cant Strips Required On Every Flat Roof?
Not always; their necessity depends on roof design and whether tapered insulation or other transitions already create a smooth slope. A design review determines whether a cant strip will prevent membrane stress.
Can Cant Strips Be Installed Over Existing Membranes?
Retrofitting often requires partial removal of membrane at the parapet to properly seat and fasten the cant strip. Surface preparation and compatible adhesives are essential. Improper retrofits risk trapping moisture and causing failures.
How Long Do Cant Strips Last?
Durability depends on material and exposure; synthetic or properly coated cant strips under well‑installed membranes can last the roof’s life, while untreated wood may deteriorate in moist conditions. Material choice and installation quality determine longevity.
Resources And Further Reading
Refer to NRCA details, ASTM standards for roofing materials, and single‑ply manufacturer installation manuals for specific cant strip recommendations. Manufacturer technical data sheets provide exact fastening patterns, adhesives, and compatibility notes.
Consult local building codes and recognized roofing trade organizations for regional practices and inspection checklists. Combining manufacturer guidance with local code requirements yields compliant, durable installations.