Why is my roof leaking water during heavy rain?

Your roof leaking during heavy rain indicates that water is penetrating the roofing system at a point where waterproofing is compromised, and heavy rain exposes weaknesses that light rain may not. Common causes include damaged or missing shingles where wind has blown off tabs, exposing underlayment or deck to direct water entry. Flashing failures are the most common cause, as step flashing around chimneys, sidewalls, and skylights, valley flashing, and vent pipe boots can have gaps, rust, cracks, or improper installation that allows wind-driven rain to infiltrate. Clogged gutters can cause water to back up under shingles at eaves. Ice dam damage from previous winter can have lifted shingles. Cracked or deteriorated vent pipe boots allow water to enter around pipes. Improperly sealed nail holes or exposed nails can leak under heavy rain. For older roofs over 20 years, widespread sealant failure and granule loss reduce water shedding ability. Heavy rain also tests roof valleys, as high volume water flow can overwhelm improperly installed valleys. If leaks occur only during heavy wind-driven rain, it suggests flashing or sealant issues where water is being pushed under shingles. You should have a professional inspection during or immediately after heavy rain to trace water entry, as water may travel along rafters and appear far from the actual leak point. Temporary tarping may be needed to prevent interior damage until permanent repair.