Short answer: An uneven gap at the bottom of a garage door usually comes from one of three things: a worn bottom seal, an uneven garage floor, or a door that's hanging crooked. Seals and floors are usually simple fixes. A door that sits lower on one side often points to a cable, drum or spring problem, and that needs a professional. Don't adjust the cables yourself.
When your garage door closes, the bottom should meet the floor evenly from side to side. If light shows under one corner, leaves blow in, or the door touches down on one side first, something is off. The fix depends on whether the problem is the seal, the floor or the door itself.
With the door closed, stand inside with the lights off on a sunny day and look along the bottom. Take a photo of each side so you can compare later. Then check these:
A torn or flattened seal leaves a gap in one spot even when the door is level. Our garage door bottom seal guide covers how to check it and replace it. Test the auto-reverse after any seal change.
Concrete settles, and the edge of a driveway can crack and shift. If the door is level but the floor isn't, a few options can help:
After adding a threshold seal or a taller bottom seal, test the auto-reverse, since the door now meets something higher than the floor. Our guide on how to test your safety sensors and auto-reverse shows how. Leave the opener's travel and force settings alone. If the door needs to travel further to seal, or doesn't reverse the way it should, that's a professional adjustment.
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A door that sits lower on one side is a different story. The usual causes are in the parts that carry the weight:
Other signs point the same way: one cable looks slacker than the other, the door shifts sideways as it closes, or a roller sits out of the track. If you see any of these, unplug the opener so no one runs it.
These parts are under heavy tension. Don't loosen, tighten or adjust cables, drums or brackets yourself. A crooked door can also come off its track if it keeps running, so stop using it and have it checked. Our cable warning signs and our guide to a door that's off track explain what to look for.
A gap under the door lets in water, drafts, dust and pests, and it can leave the bottom edge of the door sitting in puddles. A crooked door is a bigger concern. It puts uneven strain on the opener and the hardware, and it wears the parts on the low side faster. Dealing with it early usually means a smaller job. If the door is also hard to lift or drops fast, our guide to a door that's out of balance is worth a read.
A pro will look at the floor, the seal and the door together, then check cable tension, the drums and the balance. If the door is crooked, they'll level it before adjusting anything else. Ask for the price before work begins. If the gap came from a worn seal or a sloped floor, a yearly tune-up is a good time to keep an eye on it.
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Either the floor is higher on that side or the door is hanging crooked. If a level shows the floor is even, the door is likely the cause, often a cable or spring issue, and it should be checked by a professional.
You can replace a worn bottom seal or add a threshold seal for an uneven floor. If the door itself is crooked, leave the cables, drums and springs to a professional.
Lay a level or a long straight board across the floor where the door closes. If the gap under the door lines up with a low spot, the floor is the cause.
It's best not to. A crooked door can come off its track, and the cause is often a cable or spring under heavy tension. Have it looked at first.