Settled Slab: Gap at One Corner and a Door That Rubs

The straightforward version of a settled slab, and the one people are usually right about.

Soil under one corner of the garage has lost volume — dried out and shrunk, or consolidated — and the concrete above it has followed. The result is a floor that is no longer flat under the door's landing line, with the low point at or near one end of the opening.

The signature is the shape of the gap. Concrete doesn't tilt as a rigid plate the way a door does; it dishes and curves. So the daylight under the door is a lens or a crescent that gets deeper toward the corner rather than a clean triangular wedge. Lay a straightedge on the slab and the drop is measurable in the concrete itself, independent of the door.

Whether this is a problem or a nuisance depends entirely on how much movement there is and whether it's finished. A quarter inch at a corner on a slab that stopped moving a decade ago is a sealing question. Continuing movement is a different conversation, and the way to know is to measure it and look again later.

How you can tell: The door's bottom edge sights straight and level. A straightedge on the concrete shows a definite drop toward one corner, and the gap's shape follows that drop rather than tapering evenly.

The mirror image, and the one that gets misfiled as a foundation problem more often than the other way round.

A door hangs level because both lift cables carry it equally. If one cable has slipped, spooled unevenly on its drum, or come off, that corner of the door rides low. The gap under the door then tapers in a clean straight line from one end to the other — because the door is a rigid assembly and it tilts as one piece.

That straight taper is the tell. Concrete curves; a tilted door doesn't. Put a level on the slab and the floor is flat; put your eye along the door's bottom edge and it isn't parallel to the concrete.

This version is considerably more urgent than a settled slab, and that's the reason the distinction is worth the fifteen minutes. An unevenly hung door runs its rollers at an angle the track was never designed to take, and it's the state that precedes a door coming off its track. Nobody should be shopping for foundation work while the door is doing this.

How you can tell: A straight, even taper in the gap. A flat floor under the straightedge. Often a visibly slack lift cable, or cable spooled unevenly on one drum at the top of the track.

The combination version, and the one that actually brings most people to this page.

When a slab moves under one side of a garage opening, the wall framing on that side goes with it. The jamb tilts, and the vertical track bolted to that jamb tilts with it. The door — which is still perfectly square and hasn't changed in any way — now has to run in an opening that is a parallelogram rather than a rectangle. It touches the frame somewhere in its travel, and it leaves a gap somewhere else.

The rubbing is the part to take seriously. Every cycle now pushes a roller sideways in a track built for vertical load. Roller stems bend, track flanges wear, and the resistance shows up at the opener as extra force and at the cables as uneven loading. This is the mechanism by which a slow, undramatic building movement becomes a mechanical failure years later.

The good news is that garage door assemblies have adjustment in them. Track brackets sit in slotted holes, stop molding can be reset, and a door can often be brought back into a workable relationship with an opening that has shifted — within limits.

How you can tell: A corner gap and a scuff or polished stripe where the door contacts the jamb, molding or track. Usually on the same side as the gap, and usually at a consistent height.

Expansive soil doesn't only settle. It swells too, and around here the cycle is annual — soil takes on moisture through a wet winter and the monsoon, then dries through the long stretch in between.

What that produces is a gap that isn't stable. It opens through the dry months, and closes again, partly or fully, after sustained wet weather. Homeowners notice it in July, get a quote, get busy, and by February the problem appears to have solved itself. Then it comes back.

Recognizing seasonal movement changes the plan in a useful way. A door reset to suit the floor at its driest point may bind when the soil swells back. So the sensible approach is to measure at both extremes before committing to anything permanent, and to prefer solutions with tolerance in them — a deeper seal, a threshold — over solutions that assume the floor has finished moving.

How you can tell: The gap is noticeably different at different times of year, and you can remember it being better or worse than it is now.

Expansive soil moves because of moisture, so anything changing the moisture next to the garage changes the ground under it. This is the part of the problem homeowners can genuinely influence, and it's usually cheap.

The usual sources: an irrigation line or drip emitter running close to the foundation; a landscaped bed against the garage wall that gets watered on a timer; a roof scupper or downspout discharging next to the slab edge; a hose bib that drips; grading that has been built up over the years so water runs toward the wall instead of away from it. A pool or a plumbing leak on that side of the house does the same thing more dramatically.

Movement driven by one of those is often the version that stabilizes once the cause is removed, and it's why we always ask what changed outside before assuming the ground is doing it unprompted.

How you can tell: The change followed something — a new irrigation zone, a landscaping project, a leak, a driveway resurfacing, or a season of unusual watering. Often the low corner is the one nearest the water.

Sometimes it is, and we'd rather say so than sell a threshold to somebody whose house is moving.

The signs that this has outgrown a garage door: cracks in the slab wide enough to have a lip you can feel with a foot; separation opening between the slab and the wall; drywall cracking at the corners of doors and windows inside the house; interior doors that have started sticking or swinging on their own; a visible step or dip across the garage floor; or movement that is measurably continuing rather than cycling.

None of that is a garage door's business, and the honest thing for us to do is to tell you what we measured, hand over the numbers, and stay out of the way. What we can usefully contribute is confirmation that the door and its frame are square and level as an assembly — which rules the door out as a cause and gives whoever comes next a cleaner picture.

How you can tell: More than one symptom, in more than one place, and the garage door is the one you happened to notice first.

Worth setting out plainly, because this is the information that's missing from every article that sends people straight to a foundation contractor.

A deeper seal. A hollow bulb astragal collapses considerably further than a flat strip, so it follows a floor with a modest drop that a standard seal simply spans. It's the first thing to try and it resolves more of these than people expect.

A bonded threshold. Where the drop is beyond what rubber can bridge, a threshold bonded to the slab raises the closing surface back to a consistent height. On a post-tension slab this is specifically the right approach, because it's adhered rather than fastened — no drilling into concrete whose tendons you can't see.

Re-squaring the door in the opening. Vertical track brackets sit in slotted holes precisely so the door's relationship to the frame can be set. Within that range, a door running in an opening that has shifted can often be brought back to running clean. That range isn't unlimited, and it's the thing we assess on the visit.

Re-leveling how the door hangs. If cable spooling is part of the picture, that gets corrected properly under controlled tension — never as an adjustment on the fly.

What none of that does is fix the floor. If the concrete keeps moving, the door will need looking at again. But there's a large space between “a small gap at one corner” and “lift the house,” and most of the doors we see land in it.

Two reference points worth having in the conversation. First, the target isn't negotiable: DASMA's consumer checklist, TDS #167, asks whether “the door fully close[s] and rest[s] on the floor with no gap,” and treats a No as a reason to get a technician out. So a corner gap isn't something to live with because the cause happens to be concrete. Second, if you ever end up replacing that slab or the apron, DASMA's TDS #197 specifies the detail to build in — a sloped sill or a notch below slab grade for the door to close into. Our page on water under the door covers the actual dimensions.

How you can tell: You've established the floor has moved by a modest amount, the movement is stable or seasonal rather than accelerating, and the rest of the building is behaving itself.