Why Is My Roof Deck Rotting Under New Shingles?

New shingles create a tight seal over your roof. If the underlying deck was already damp or poorly ventilated, that moisture gets locked in. This creates a perfect environment for wood rot fungi to thrive unseen. The problem often starts before the new roof is installed, with the new layer hiding the damage until it becomes severe.

The rotting process requires consistent moisture and limited airflow. Even a small, persistent leak from an old flashing or a single missing vent can introduce enough water. Once the new shingles go on, evaporation slows dramatically. The deck stays wet, and the structural plywood or boards begin to soften and decay from the inside out.

You might notice signs inside your home before seeing exterior damage. Look for water stains on ceilings or walls, a musty odor in the attic, or sagging rooflines. From the outside, check for cracked or curling shingles, which can indicate a soft, moving deck underneath. Moss growth in Portland's climate can also trap moisture against shingles and decking.

The only reliable way to assess the damage is a professional inspection. Local water damage pros use moisture meters and thermal imaging to map wet areas without removing shingles. They can pinpoint active leaks and measure the moisture content deep within the wood. This tells you if the rot is localized or widespread, guiding the repair plan.

Prevention starts with choosing a qualified contractor for any roof work. Ensure they plan to inspect the existing deck thoroughly and replace any questionable sections. Ask about their ventilation strategy to meet local building codes. Proper ventilation is crucial in an oceanic climate with mild wet winters and dry summers to manage condensation and drying cycles.

Schedule regular roof inspections, especially after severe storms. Keep your gutters clean to prevent water backup under the shingles. Ensure attic insulation does not block soffit vents. If you see any interior water stains, address them immediately to prevent the problem from reaching the deck. Proactive maintenance is far less costly than deck replacement.

• Demand a full deck inspection before a reroofing job.
• Verify the contractor's plan for balanced intake and exhaust ventilation.
• Clean gutters and downspouts twice a year to prevent ice dams and backups.
• Trim tree branches that overhang and scrape the roof surface.
• Inspect the attic seasonally for signs of leaks, condensation, or blocked vents.
• Act quickly on any interior ceiling stains to trace the source.