First check whether the door itself is level
A gap under a closed garage door does not automatically mean the door needs mechanical adjustment.
Start by looking at the closed door from a normal, safe position.
Ask whether the bottom edge appears reasonably level, whether the gap is fairly even or concentrated in one area, whether one side sits noticeably higher, whether the bottom weather seal is visibly damaged, and whether the floor beneath the door appears uneven.
Do not repeatedly operate the door just to reproduce a gap that is already visible.
If the door is level but light or air comes through
If the door appears straight and reaches its normal closed position, the bottom weather seal is an appropriate place to inspect.
Look for cracks, tears, flattened or hardened material, missing sections, separation from its normal position, areas where the seal no longer contacts the floor, or daylight and drafts concentrated beneath part of the seal.
A bottom weather seal is intended to help reduce drafts and keep water, dirt, debris, and pests from entering beneath the door.
Visible deterioration or loss of contact can therefore explain a gap even when the door itself appears mechanically normal.
Check whether the floor contributes to the gap
Look at the shape of the gap without crawling beneath the door or manipulating any hardware.
If the door remains level but the gap is concentrated over a low or irregular area of the floor, the floor-to-seal contact may be part of the problem.
Bottom seals can accommodate some surface irregularity, but they cannot necessarily compensate for every floor shape or difference in height.
Do not change the door’s mechanical alignment simply to make it conform to an uneven floor.
If one side of the door is higher
This is a different diagnostic path.
If one bottom corner is visibly higher than the other, or the entire door appears crooked or out of level, do not assume that a thicker seal will solve the problem.
Uneven door position can involve cables, springs, rollers, tracks, or other mechanical components.
Stop repeated operation and use Garage Door Opens Unevenly or Looks Crooked.
Do not push or pull one side into position, adjust springs or lifting cables, loosen bottom brackets, straighten or realign tracks, reseat an off-track roller, increase opener force, or change opener settings merely to force the door farther down.
Look for mechanical warning signs
Treat the problem as more than ordinary weather sealing if you can see a loose, displaced, frayed, or broken lifting cable; an obviously damaged spring; a roller outside its track; bent, displaced, or severely damaged track; a visibly crooked or unstable door; or severe binding or abnormal movement.
These conditions take priority over the bottom gap.
Do not touch a suspicious cable or spring to confirm whether it is damaged.
What about replacing the bottom seal?
A worn bottom seal may need replacement, but the correct seal and installation method depend on the exact door and retainer system.
If the door is straight and mechanically normal and you want to determine whether visible seal wear or reduced sealing function is enough to justify replacement, use How to Tell if a Garage Door Bottom Seal Needs Replacement.
Use the garage door manufacturer’s instructions for the specific model when identifying replacement weatherstripping.
Do not turn seal replacement into adjustment of the door’s counterbalance or track system. In particular, do not loosen or remove bottom brackets or cable attachments. These components can be associated with the door’s tensioned lifting system.
If the replacement procedure for your particular door requires work you cannot clearly identify as homeowner-serviceable, arrange professional service instead.
When the gap may be a weather-seal problem
A weather-seal path is more likely when the door appears straight and level, reaches its normal closed position, and the primary symptom is light, air, water, debris, or pests entering beneath the door.
It is also consistent with a bottom seal that is visibly worn, damaged, hardened, or incomplete, or with a localized floor irregularity while the door itself remains normally positioned.
These observations do not require changing springs, cables, tracks, or opener force.
When to stop DIY
Stop treating the gap as an ordinary sealing problem if the door is crooked or uneven, one side is clearly higher, the door is off track, a lifting cable or spring appears abnormal, the door binds or moves abnormally, or correcting the gap would require adjusting tensioned or structural door hardware.
A visible mechanical abnormality takes precedence over improving the weather seal.
When to call a garage door professional
Professional inspection is appropriate when the door does not sit normally, the cause of the gap is unclear, mechanical damage is visible, or correcting the condition would require work on springs, cables, bottom brackets, tracks, door alignment, or other hazardous hardware.
If the door itself remains straight and mechanically normal but the bottom seal is simply worn, follow the manufacturer’s guidance for the exact door and seal system or have the seal replaced professionally.
The important distinction is simple: a worn seal can leave a gap under a normal door; a crooked or mechanically abnormal door is not merely a weather-seal problem.