A door hanging with one side higher than the other has one of three causes: a lift cable that has stretched, snapped or jumped its drum, a drum set screw that has slipped on the shaft, or an opening that is no longer square. All three put uneven load on the door, so the opener comes out of service until it is sorted.
Read the closed door first
Close the door as far as it will go and look along the bottom edge. Uniform contact with the slab and a level top edge means the door is fine and something else brought you here. A wedge of daylight that grows steadily from one side to the other means the door is hanging out of level. A door that is level at the bottom but leaves a triangle of gap at the top of one side is a different story, and that one points at the opening rather than the door.
Cables: the most common answer
Each lift cable carries roughly half the weight of the door on wire about an eighth of an inch thick. They stretch unevenly with age and they fail at the bottom end, in the last six inches above the bottom bracket, where the wire flexes over a pin and sits in the wet zone. With about 37 inches of rain a year and a hose regularly run across the slab, that section rusts from the inside out.
When one cable lets go, that side drops and the door jams at an angle. When one has only stretched, the door lifts crooked without any drama. Either way both cables get replaced as a pair, because the survivor has taken the same cycles, the same moisture and the same freezes. A pair fitted is an estimated $145 to $295 per door.
A drum set screw that has slipped
The cable drums at each end of the torsion shaft are clamped by set screws onto flats on the shaft. If one loses grip, that drum turns a little behind the shaft, that cable pays out further, and the door hangs low on that side. It is a quick diagnosis for a technician, and a slipped drum often takes a scored groove with it, in which case the drum gets replaced at an estimated $155 to $295 rather than just re-torqued.
When the opening has racked
This is the one that gets misdiagnosed the most in this part of the Metroplex. The soil under Richardson is Blackland Prairie clay, the Houston Black series, and it shrinks and swells with moisture. TDLR's own technical guidance on foundations notes that expansive soils which shrink and swell with moisture are a common cause of foundation problems, and you can read what the state regulates and what it does not on the Texas Department of Licensing and Regulation site.
When a slab edge lifts or a jamb leans, the two vertical tracks stop being parallel and the header stops being level. The door is still square. The hole it lives in is not. Fitting new cables to that door will make it lift evenly for a while and then go crooked again, because nothing about the geometry changed. The tell is a door that seals on one bottom corner while gapping at the other and a gap at the top that mirrors it.
Why the opener has to stay off
A crooked door drags one roller hard against the track flange and puts the whole weight on one cable and one spring end. Cycling it wears the flange, elongates hinge holes, and can pull a section out of the track. Unplug it, use the side door, and if the door is already partway up, clamp a sound section of vertical track under a seated roller so it has something to rest on.
What a proper visit does
- Door clamped, spring tension controlled, then both cables inspected end to end.
- Cables replaced as a matched pair and seated in the drum grooves with the door flat on the slab.
- Both drums set with even tension, set screws torqued onto the shaft flats.
- Vertical tracks plumbed and measured for parallel, horizontal tracks checked for level.
- The opening measured for square, and the finding said out loud if it is not.
- Door cycled by hand and watched at both ends, then the opener limits and force reset.
If the answer turns out to be the opening rather than the door, the honest version of that conversation includes the fact that a garage door contractor is not who fixes a foundation. What a technician can do is shim and realign so the door works with the movement, and tell you plainly whether the movement is still going.