The short answer

Trace the gap with the door closed, then observe the opening and closing behavior without reaching near moving parts. Side seals can deteriorate, but a persistent gap may reflect jambs, tracks, rollers, door sections, floor slope, or spring balance. A thicker seal can hide the symptom and make the door bind.

Check the boundary

Area Observation What it separates
Side seal Compression, cracks, and continuity Material failure versus alignment.
Jamb Straightness and attachment Framing movement or damaged trim.
Track Parallel, secure, and clear Track movement versus seal condition.
Door sections Even contact at multiple heights Door out of square or damaged hardware.
Floor Slope, gap, and threshold Building geometry versus door hardware.

Photograph the gap from a safe position, note whether it appears on one side or both, and record weather or seasonal changes. Do not remove cables, loosen spring hardware, force a stuck door, or disconnect safety features to test the seal.

What a service estimate should separate

Ask whether it includes seal replacement, jamb repair, track alignment, roller or hinge work, door balance, floor or threshold work, and opener safety testing. A weatherstrip estimate should not quietly include high-energy spring work without identifying it.

Practical decision notes

The key question for this topic is whether light or air enters at one side only when the door closes. Before making a change, focus on checking track alignment, stop molding, seal compression, floor level, and seasonal movement. Record the starting condition with a photo, measurement, weather note, and the symptom that needs explaining. Compare more than one location when possible, follow the manufacturer or owner manual, and separate observations from assumptions. Choose a next step that produces evidence or improves safety rather than hiding the symptom. Stop at concealed water, combustion, energized wiring, structural movement, or unsafe access.

Limits

This is a planning guide, not a garage-door repair procedure. Damaged cables, springs, tracks, or an uncontrolled door need a qualified garage-door professional.