CSLB C-39 Roofing ContractorLow-Slope RoofingMedium
A roofing crew is installing a metal expansion joint cover across a building's structural expansion joint on a low-slope roof. Why is the joint cover fabricated with a raised, bellows-shaped profile rather than a flat sheet?
- ATo provide a walking surface for maintenance personnel
- BTo reduce the amount of metal needed for fabrication
- CTo increase the surface area for solar reflectance
- DTo allow the cover to accommodate building movement without tearing the waterproofing
Show answer & explanationAnswer & explanation
Correct answer: D. To allow the cover to accommodate building movement without tearing the waterproofing
Structural expansion joints allow buildings to move due to thermal expansion, seismic activity, or settlement; the bellows or loop shape in the expansion joint cover provides slack material that can flex and accommodate this movement without splitting or pulling apart the roof's waterproofing seal.
Why the other options are wrong
- A. Expansion joint covers are not designed as walking surfaces and can be damaged by traffic.
- B. The shape actually requires more material, not less, than a flat sheet.
- C. Solar reflectance is unrelated to the joint's functional shape.
Expansion Joint Cover
A flexible metal or membrane assembly with a raised bellows profile installed over structural expansion joints to accommodate building movement while maintaining a watertight seal.
- Bellows shape provides slack for lateral and vertical movement
- Prevents membrane tearing at building movement joints
- Must be detailed to remain watertight through full range of motion
Memory trick: Give it slack so the building can move without a crack.