In Vancouver, roof deck rot is primarily caused by the region’s unique climate and specific structural or maintenance issues. The high annual rainfall (over 1,300 mm) and constant humidity prevent trapped moisture from evaporating, creating an ideal environment for wood rot and fungi to thrive.

According to Paragon Roofing BC, the most common causes include:

• Direct Leaks: Moisture entering through cracked or missing shingles, gaps in flashing, or moss growth.
• Poor Ventilation: Inadequate attic ventilation traps warm, damp air against the underside of the roof decking.
• Condensation: Compressed or inadequate insulation causes warm indoor air to condense on the cold roof boards.
• Ice Dams: During cold snaps, ice dams can force water to back up underneath the shingles.
• Aging Materials: Old asphalt shingles that have lost their protective granules allow water to penetrate the wooden sheathing.
• Lack of Maintenance: Preventable issues like popped nails or missed inspections allow minor leaks to escalate into widespread decay.

• Rotted Roof Decking: Complete Guide to Signs & Repair