Why is there a gap under my garage door when it's closed?

Let the shape of the gap tell you the cause

Close the door, go inside, kill the lights, and look along the bottom. The pattern of daylight is diagnostic. A uniform strip of light the full width points at the seal or the stopping point. A wedge of light that grows toward one jamb says the door itself is not landing level. A gap only in the middle, with both ends tight, usually means a bowed bottom section or a slab that crowns in the center.

Each of those has a different repair, and the uniform kind is by far the cheapest to fix.

Even gaps: seal or limit

• The bottom seal is shot. Astragal rubber flattens, cracks, and loses its memory — fast in Oklahoma, where it bakes in summer and goes brittle in winter. Replacing it is routine work.
• The down limit stops short. If the opener quits an inch early, the seal never compresses. See adjusting opener travel limits.
• The seal is the wrong profile. A universal seal that does not match the retainer will not close the gap no matter how new it is.
• The slab has settled. Then the door is fine and the floor moved — the situation covered in sealing a door against an uneven floor.

Tapered gaps are a tension problem

When one side lands and the other does not, one lift cable is holding more door than the other. That happens when a cable has slipped a wrap on its drum, when a drum set screw has walked loose, or when one spring has lost tension faster than its partner. The door is arriving out of square and will keep doing so until the tension is reset.

This is not just a draft problem. A door landing crooked loads its hinges and the bottom brackets unevenly, and the bottom brackets are attached to the cables under full spring tension — which is why that hardware carries warning labels. The background is in why one side sits higher and what loose cables mean.

Why closing the gap is worth doing here

In this climate a bottom gap is not only a comfort issue. It is the entry point for the red dust that coats everything in an OKC garage, for driving rain off a south wind, and for the mice and snakes that come in out of a cold snap. It also undoes much of what an insulated door is for, which is the point made in keeping dust out of the garage.

A seal replacement is quick work and we do it as part of a maintenance visit. If the gap is the tapered kind, say so when you call — that one gets a balance and cable inspection rather than just a new strip of rubber.