Will a roof leak if shingles blew off?

Yes, a roof will very likely leak if shingles blew off, especially if underlayment is also damaged or missing and it rains. Asphalt shingles are installed in an overlapping system where each row covers the nails and seams of the row below. When shingles blow off, this water-shedding system is broken, exposing the underlayment or bare wood deck directly to weather. If only the shingle is missing but the felt or synthetic underlayment underneath remains intact and undamaged, you may have a short grace period of a few days in dry weather before leaks occur, as underlayment provides a secondary barrier. However, underlayment is not designed for long-term exposure to sun and rain and will deteriorate quickly, and wind-driven rain can easily get under surrounding shingles and cause leaks. If the missing area is in a critical area like a valley, ridge, or near flashing, or if it exposes bare wood, leaks can occur with the next rain, even light rain. Even a single missing shingle can allow water to travel sideways and cause interior damage far from the missing spot. Therefore, any missing shingles should be covered with a tarp within 24 hours and permanently repaired within a week to prevent deck rot, attic mold, and interior ceiling damage. Document damage for insurance before tarping.