CSLB C-39 Roofing ContractorLow-Slope RoofingEasy
A roofer installs a 45-degree wood cant strip at the base of a parapet wall before applying modified bitumen base flashing up the wall. What is the primary purpose of the cant strip?
- ATo provide additional surface area for gravel embedment
- BTo eliminate a sharp 90-degree bend in the membrane where it could crack under stress
- CTo create positive slope on the field of the roof toward the drain
- DTo increase the R-value of the wall insulation
Show answer & explanationAnswer & explanation
Correct answer: B. To eliminate a sharp 90-degree bend in the membrane where it could crack under stress
Membranes are stiff and can crack, fatigue, or delaminate if forced around a sharp 90-degree corner. A cant strip transitions the membrane gradually from horizontal to vertical, reducing stress concentration at the base flashing.
Why the other options are wrong
- A. Gravel embedment relates to the field surfacing, not the cant detail.
- C. Field slope is achieved with tapered insulation, not a cant strip at the wall base.
- D. Cant strips are not insulation and add negligible thermal value.
Cant Strip
A 45-degree tapered strip (wood, fiber, or foam) installed at the base of vertical walls, curbs, or parapets to ease the transition of roofing membrane from horizontal to vertical.
- Prevents sharp 90° bends that crack membranes
- Commonly required under base flashing in BUR and mod-bit systems
- Typically 4x4 inch triangular cross-section
Memory trick: Cant strip = 'can't bend it sharp' so ease the angle.