Uneven Gap Under Garage Door: Causes and How to Fix It

A crooked garage door is one of the clearest warning signs your system needs attention. In my 15+ years servicing doors across Toronto and the GTA, an uneven bottom gap almost always traces back to a cable, roller, or track problem — and occasionally to a concrete slab that our freeze-thaw winters have pushed out of level. This guide walks through every cause, how to tell them apart, and what is safe to fix yourself.

Why Is the Gap Under My Garage Door Uneven?

There are two broad categories: the door is crooked, or the floor is crooked. Telling them apart takes 30 seconds. Close the door, then look at the bottom edge of the door against the floor line:

• The door's bottom edge is tilted (not parallel to the floor): The problem is mechanical — a cable, roller, spring, or track issue is letting one side ride lower or higher than the other.
• The door's bottom edge is straight and level, but the floor slopes away: The problem is structural — the concrete slab has settled or heaved, leaving a tapered gap even though the door is hanging perfectly square.

That single check tells you whether you are looking at a repair (door hardware) or a seal upgrade (floor). Below are the specific causes within each category, ranked by how often we see them on GTA service calls.

1. Frayed or Broken Lift Cable (Most Common)

Each garage door has a steel lift cable on the left and right that wraps around a drum at the top and connects to the bottom bracket. If one cable frays, snaps, or jumps off its drum, that side of the door loses support and drops. The result is an instantly crooked door with a wedge-shaped gap on the failed side. You may also notice the cable hanging loose, kinked, or pooled at the bottom of the track. This is the number-one cause of a sudden uneven gap and it should never be ignored — a door under spring tension with one failed cable can come down hard. Learn the warning signs in our guide to a garage door cable that came off.

2. Worn Bottom Roller or Bent Track

The bottom rollers carry the weight of the door at the floor line. A flattened, seized, or broken roller lets one corner sag, and a vertical track that has been bumped by a vehicle or knocked out of plumb does the same thing. If the door scrapes, jerks, or makes a metallic pop as it nears the floor on one side, suspect a roller or track issue. Replacing worn rollers is a straightforward maintenance task — see our garage door roller replacement service for details.

3. A Spring That Is Out of Balance

On a two-spring (double torsion) system, if one spring weakens or breaks while the other stays strong, the door can lift unevenly and settle crooked. A door that feels heavy, slams down, or will not stay halfway open usually has a spring problem. Never adjust or replace torsion springs yourself — they store enormous energy. Our broken garage door spring page explains the safe repair path.

4. Damaged or Loose Bottom Bracket

The bottom bracket anchors the cable to the door at each lower corner. If it pulls loose, bends, or its fasteners back out, that corner sags. Bottom brackets are under the same tension as the cable, so they are a pro-only repair.

5. Settled or Heaved Concrete Slab

In older Toronto, Mississauga, and Brampton homes especially, repeated freeze-thaw cycles and soil movement can lift or sink one corner of the garage slab. The door is fine — it is the floor that is no longer level. According to the Natural Resources Canada guidance on home air sealing, gaps like this are a meaningful source of heat loss in winter, so even a "cosmetic" tapered gap is worth sealing.

How to Diagnose the Cause Yourself (Safely)

Work through these steps in order. Stop immediately if you find a broken cable or spring — those are not DIY fixes.

1. Close the door and measure the gap at each end. Note which side is larger and by how much. A 1/4-inch difference is minor; an inch or more is significant.
2. Check whether the door edge or the floor is tilted using the level-line test described above. A 4-foot spirit level on the floor confirms a sloped slab.
3. Inspect both lift cables from a safe distance. Look for fraying, slack, kinks, or a cable off its drum. Do not touch a loose cable.
4. Look at the rollers and tracks on the low side for damage, debris, or a visibly bent rail.
5. Do a balance test only if cables and springs look intact: pull the red release cord, lift the door halfway by hand, and let go. A balanced door holds position. If it drops or rises, the springs are the issue.

Causes at a Glance: Symptoms, DIY Risk & Cost

This table helps you match what you are seeing to the likely cause and whether it is safe to handle yourself. GTA repair prices below come from our live pricing page.

Cause	Telltale Symptom	DIY?	Typical GTA Cost
Broken / frayed cable	One side drops suddenly; cable loose or off drum	No — call a pro	$180–$220 (+$260 with bottom brackets)
Worn bottom roller	Scraping or popping near the floor; corner sags	Light DIY / pro	Often part of a tune-up $100–$120
Bent vertical track	Door binds or rubs on one side	Pro recommended	Varies by damage
Unbalanced / broken spring	Door heavy, slams, won't stay open	No — pro only	Single spring $280; double $320–$460
Loose / bent bottom bracket	One lower corner sags; bracket visibly moved	No — under tension	$260 (cables + bottom brackets)
Settled concrete floor	Door edge level, floor slopes; tapered gap	Yes — seal upgrade	Seal $80–$260

DIY Fixes for an Uneven Gap

Two of the causes are genuinely homeowner-friendly: a worn roller and a sloped-floor gap. The rest involve cables, springs, or brackets under tension and should go to a technician.

Fixing a Tapered Gap from a Sloped Floor

If your door hangs level but the floor slopes, the fix is a flexible bottom seal that conforms to the slab:

• Adjustable bottom seal (astragal): A U-shaped or T-style rubber seal that slides into the retainer on the door bottom and compresses to fill an uneven gap up to about 1.5 inches.
• Concrete threshold seal: A rubber strip glued to the floor itself. Because it follows the slab, it seals a tapered gap from the floor side and also blocks water and pests — ideal for GTA driveways that slope toward the door.
• Brush or vinyl side seals: If the wedge runs up one jamb, refreshing the side weatherstripping completes the seal.

For a full walkthrough of options, see our side and top seal replacement guide.

Replacing a Worn Bottom Roller

If a single bottom roller is the culprit and the door is fully closed and resting on the floor (so the bottom roller is out of the curved track), an intermediate roller swap is manageable for a confident DIYer. However, the very bottom roller sits in the bottom bracket, which is under cable tension — that one should be left to a professional. When in doubt, a tune-up covers roller inspection and replacement.

When to Call a Professional

Call a technician right away if any of the following are true. Trying to "muscle" a crooked door level can turn a $200 cable repair into a bent door or a fall hazard.

• A cable is frayed, slack, snapped, or off its drum.
• The door dropped on one side suddenly or will not close evenly.
• A torsion spring is broken, gapped, or the door fails the balance test.
• A vertical track is bent or the door is binding in the tracks.
• The bottom bracket is loose, bent, or pulling away from the panel.

Royal Garage Doors covers Toronto and the entire GTA, including Mississauga and Toronto, with same-day service. If the door is beyond economical repair — rusted panels, repeated cable failures, or a frame that has racked — ask about garage door replacement options instead. You can also reach our team any time to talk through symptoms.

Note that Ontario follows national door safety standards: every modern opener must meet the UL 325 entrapment-protection requirements, which include auto-reverse and photo-eye sensors. A door that hangs crooked can interfere with the bottom seal contact that those safety systems rely on, which is another reason not to keep operating it. You can read the official standard summary from UL Solutions.

Frequently Asked Questions

Why is the gap under my garage door uneven on one side?

A gap on one side usually means a frayed or broken lift cable on the low side, a bent vertical track, or a worn bottom roller. When one cable slips off its drum or snaps, that side of the door drops and the door hangs crooked. Stop using the door and inspect the cables and tracks before opening it again.

Is it safe to use a garage door that sits crooked?

No. A crooked door often signals a cable or spring problem that puts uneven load on the system. Continuing to operate it can pull the door out of the tracks, damage the opener, or cause the door to fall. Disconnect the opener with the red release cord and call a technician.

Why is there a gap at the bottom of my garage door even when it is closed?

An even gap along the whole bottom is usually a worn or missing bottom weather seal, or the down travel limit on the opener set too high. An uneven gap that is larger on one end points to a structural cause such as a settled concrete floor, bent track, or cable problem.

Can a cracked or uneven concrete floor cause a gap under the garage door?

Yes. In older GTA homes, freeze-thaw cycles and soil settling can heave or sink the garage slab so it is no longer level. The door bottom is straight but the floor is not, leaving a tapered gap. A threshold seal or adjustable bottom seal usually solves this without major concrete work.

How much does it cost to fix an uneven garage door in the GTA?

Cable and bracket repair runs $180 to $220 plus tax, with bottom brackets at $260. A new weather seal is $80 to $260 depending on door size. A single torsion spring is $280 plus tax. Royal Garage Doors includes a FREE service call with any repair across Toronto and the GTA.

How do I stop a draft from the uneven gap under my garage door?

Once the door itself hangs level, install an adjustable bottom seal (a U-shaped or T-style astragal) or a concrete threshold seal that conforms to a sloped floor. These compress to fill a tapered gap and block drafts, water, and pests, which matters during GTA winters.