Why Is There a Gap Under My Garage Door? Causes, Fixes & What It Costs to Put Right

Introduction

You close the garage door for the night, glance back, and see it: a thin line of daylight running along the bottom, or worse, a wedge of light at just one corner. In winter that gap is where the cold pours in and the heat leaks out. In autumn it is where the leaves blow through. In spring it is where the meltwater and the mice find their way in. A gap under a closed garage door is one of the most common things London homeowners call us about, and it is also one of the most misunderstood, because the same symptom can come from four completely different causes, ranging from a five-minute fix to a real repair.

This guide walks through every reason a garage door does not sit flush to the floor, how to tell the causes apart from your driveway without any tools, which ones you can safely fix yourself, which ones need a technician, and what the professional fixes actually cost in southern Ontario in 2026. A gap is not just an annoyance. It drives up your heating bill, invites water and pests, and in a few cases is an early warning that something in the door's balance or tracking is going wrong. Knowing which kind of gap you are looking at tells you whether to reach for a new seal or reach for the phone.

First, Decide What Kind of Gap You Have

Before you can fix a gap you have to describe it accurately, because the shape of the gap points straight at the cause. Step outside with the door fully closed and look at the bottom edge from a few feet back, then crouch down for a closer look. Ask yourself three questions.

Is the gap even across the whole width, or worse at one end? An even gap along the entire bottom almost always means the rubber bottom seal is worn, torn, or simply too small for the opening. A gap that is tight at one end and wide at the other means the door itself is sitting crooked, which is a different and more serious problem.

Is the gap under the door, or is it up the sides and across the top? Light leaking around the sides and the top of the door is not a bottom-seal issue at all. That is the perimeter weatherstripping on the door stops, and it is a separate fix covered in our jamb and header seal guide. This article is about the gap at the floor.

Is the floor itself flat, or does it dip and rise? Lay a straightedge or a level across the concrete under the door opening. Many older London driveways and garage slabs have settled or heaved over the decades, leaving a floor that a straight-bottomed door can never seal against. That is a floor problem, not a door problem, and it has its own fix.

With those three answers in hand, you can match your gap to one of the causes below.

Cause 1: A Worn, Torn, or Hardened Bottom Seal

This is by far the most common reason for a gap, and the good news is that it is also the cheapest to fix. The bottom seal, sometimes called the bottom weatherstrip or astragal, is the flexible rubber or vinyl strip that runs along the very bottom of the door and squashes against the floor when the door closes. It slides into an aluminium retainer channel on the bottom of the door, or on older wooden doors it is nailed on. Its whole job is to fill the small, uneven space between a rigid door and a rigid floor.

Rubber does not last forever. Ontario's climate is especially hard on it: the seal bakes in summer sun, freezes and cracks in winter, and gets ground against grit and road salt every time the door closes. Over five to ten years it hardens, shrinks, splits, or tears away entirely. Once it stiffens, it can no longer flatten out to fill the gap, and daylight appears. Sometimes a corner tears off and you get a gap at just one end that looks, at first glance, like a crooked door.

How to confirm it is the seal

Raise the door a couple of feet and look at the bottom edge. A healthy seal is soft, pliable, and shaped like a wide U or a P that mushrooms out when pressed. A failed seal is hard, cracked, flattened, split down its length, or missing chunks. Press it with your thumb: if it feels like a hockey puck instead of a soft eraser, it is done.

The fix

Replacing a bottom seal is one of the few garage door repairs that a reasonably handy homeowner can do safely, because it involves no springs, no cables, and no parts under tension. You slide the old strip out of the retainer, buy a matching replacement by the foot, and feed the new one in with a little dish soap as lubricant. We walk through the whole job, including how to measure the retainer channel so you buy the right profile, in our bottom seal replacement guide. If the retainer channel itself is bent or corroded, that is a slightly bigger job that is worth handing to a technician.

Cause 2: An Uneven or Settled Garage Floor

Sometimes the seal is fine and the door is hanging perfectly straight, but there is still a gap, and it is always in the same spot. The culprit is the floor. Concrete slabs move. Over the decades a garage floor can settle toward a low corner, heave upward from frost, or crack and tilt. The door's bottom edge is a straight, rigid line, so where the concrete dips away beneath it, a triangular gap opens up that no standard flat seal can close.

You confirm this the way we described above: with the door open, lay a long level or a straight board across the floor under the opening and look for daylight beneath the straightedge. If the floor rises and falls by more than a small amount, that unevenness is your gap.

The fix

You do not need to repour your floor. There are two practical solutions. The first is a taller, more flexible bottom seal: a deep U-shaped or bulb-style astragal has enough travel to squash flat on the high spots and still reach the floor on the low spots. The second, and often better, solution is to add a garage door threshold seal, which is a low rubber ramp glued to the floor itself. Because the threshold is bonded to the concrete, it follows the contour of the slab and meets the door seal partway, closing gaps that no amount of seal replacement on the door alone can fix. A threshold is also the right answer if your driveway slopes water toward the garage, because it forms a small dam at the opening.

Cause 3: A Door That Is Sitting Crooked

If the gap is tight at one side and wide at the other, the door is not level when it reaches the floor. This is more serious than a worn seal because it usually points at the mechanism that lifts and balances the door, not just the rubber at the bottom. A door that closes crooked is telling you that one side is travelling further or faster than the other.

The common causes are a lift cable that has slipped or come off its drum on one side, cables that have stretched unevenly, a bottom bracket that has shifted, or a door that has come slightly out of its tracks. Any of these lets one bottom corner drop lower than the other, so the door lands unevenly and leaves a wedge-shaped gap. We cover the full diagnosis in our guide to a garage door that closes crooked, and the related tracking faults in our garage door track problems guide.

Why this one is a professional job

Cables and the bottom brackets they anchor to are under the full tension of the counterbalance system whenever the door is closed. Adjusting them is not a homeowner task, and forcing a door that is off its tracks can turn a small alignment issue into a bent panel or a derailed door. If your gap is clearly worse at one end, stop cycling the door more than necessary and book a service call. A technician can re-seat a cable, rebalance the door, and re-square it in a single visit, and while they are there they can run the balance check that catches the underlying cause.

Cause 4: A Misadjusted Down Limit or a Reversing Door

A quieter cause of a gap is the opener itself. Every automatic opener has a travel setting that tells the motor exactly how far down to drive the door before it stops. If that "down" limit is set too high, the opener stops the door a fraction early, before the seal fully compresses, and you are left with a gap even though every mechanical part is fine.

There is a related trap here. If the down limit is set too far, the opener drives the door into the floor, the seal folds, the opener senses resistance, and it reverses the door back up a few inches for safety. Homeowners often read that as "the door will not stay closed" or "there is a gap," when the opener is actually doing its job. If your gap appeared right after a power outage, a new opener, or a battery change, suspect the travel limits first. Our monthly safety test includes the balance and reverse checks that help you tell a limit problem from a hardware problem.

The fix

Travel limits are adjustable, usually with two dials or buttons on the back of the opener head, and the procedure is in your opener's manual. It is a safe adjustment to make yourself if you are comfortable on a step stool, though it takes a little patient trial and error to get right. If the door also reverses, or if adjusting the limit does not close the gap, the problem is mechanical rather than electronic and it is time to call.

Why a Gap Is Worth Fixing Promptly

It is tempting to live with a small gap, especially in summer when the consequences are invisible. Here is why closing it earns its keep.

Energy loss

A one-inch gap across a sixteen-foot door is the same as leaving a small window wide open all winter. If your garage is attached, or if there is a room above it, that lost heat shows up on your bill and as cold floors in the room over the garage. Sealing the bottom is one of the cheapest efficiency upgrades on the whole house, and it works hand in hand with the insulation covered in our garage door R-value guide. A well-insulated door with a bad bottom seal is still leaking energy at the floor.

Water and ice

Meltwater and driving rain run under a gapped door and pool on the garage floor, where in winter it freezes. That is how doors end up frozen to the slab, a problem we cover in our frozen garage door guide. A good bottom seal and, where the floor is uneven, a threshold, keep that water outside where it belongs.

Pests

A gap of even a centimetre is an open invitation. Mice can squeeze through a space the width of a pencil, and insects need far less. The garage is often the first room they colonize before moving into the house. Our guide to keeping pests out of the garage starts, unsurprisingly, with sealing the bottom of the door.

Drafts, dust, and noise

Beyond the big three, a sealed bottom keeps out blown dust and grit, cuts the draft across the floor that makes a garage workshop miserable in January, and modestly reduces the road and wind noise that comes in under the door.

The Safe Homeowner Inspection, Step by Step

You can diagnose almost any gap in ten minutes from the floor, with no tools beyond a level or a straight board and your phone. Here is the routine we would run.

1. Close the door and look from outside. Note whether the gap is even, worse at one end, or only along the sides and top. Take a photo for reference.
2. Raise the door and inspect the seal. Press the bottom rubber with your thumb. Soft and springy is good. Hard, cracked, split, or missing means the seal is the problem.
3. Check the floor. Lay a level or straight board across the concrete under the opening. Look for daylight under the straightedge that reveals dips and high spots.
4. Watch the door close. Stand to one side and watch the bottom edge land. Does it meet the floor level, or does one corner touch down first? An uneven landing points at the cables or tracks.
5. Note when the gap started. A gap that appeared gradually is usually a wearing seal. A gap that appeared overnight, after a power outage, a repair, or a bump, points at the opener limits or a mechanical fault.

What you should not do: do not adjust or loosen the lift cables, the bottom brackets, or the springs. Those parts are under high tension whenever the door is down, and they are the one area of a garage door where a homeowner mistake causes real injuries. Seals, thresholds, and opener travel dials are fair game. Cables, brackets, and springs are for the technician.

Realistic 2026 Costs in Southern Ontario

Prices on this page are before HST.

What you pay depends entirely on which of the four causes you have. Here are realistic 2026 numbers for the London area.

• Bottom seal, do it yourself: $15 to $45 in materials for the rubber strip, depending on door width and profile.
• Bottom seal replaced by a technician: typically $120 to $220 all-in for a single or double door, including the part and the minimum service call.
• Threshold seal (floor-mounted ramp): $50 to $120 in materials to do it yourself, or roughly $200 to $350 installed for a double door, since it must be measured, cut, and bonded to a clean floor.
• Re-seating a slipped cable and re-squaring a crooked door: typically $150 to $280, depending on what caused the door to go crooked in the first place.
• Opener travel-limit adjustment during a service call: usually rolled into a tune-up at $120 to $220, or included if the technician is already on site for something else.

For the bigger picture on what garage door work costs across the board in this market, our Ontario garage door cost guide puts these repair numbers in context against full replacement. In almost every case, closing a gap is one of the highest-return, lowest-cost jobs you can do on a door, because a $20 seal is the cheapest thing on the door and stops the draught the rest of your heating is fighting and prevents the water and pest damage that costs far more to undo.

How to Keep the Gap From Coming Back

Bottom seals are wear parts, so a gap will eventually return no matter what. A few habits stretch the interval between replacements.

• Keep the seal clean. Sweep grit, salt, and gravel out of the door opening so the rubber is not ground down every time the door closes. This is part of the routine in our garage door maintenance guide.
• Manage garage moisture. Chronic condensation and standing water degrade the seal and the retainer from below. Our condensation guide covers how to keep the floor dry.
• Do not let ice bond the seal to the floor. Chipping a frozen door free tears the rubber. A threshold and a dry floor prevent the freeze in the first place.
• Inspect the seal every fall. A thirty-second thumb press each autumn catches a hardening seal before it splits open in the first cold snap, when replacements are least convenient.

Beyond the seal itself, keeping the whole door balanced and tracking straight prevents the crooked-close version of a gap. That is what an annual professional tune-up buys you, and it is the same visit that keeps the springs, cables, and rollers healthy.

When to Call a Professional

Reach for the phone rather than the seal kit if any of the following is true: the gap is clearly worse at one end, the door lands unevenly or looks tilted when closed, you can see a cable that is loose or off its drum, the door has come out of its tracks, or adjusting the opener's down limit does not close the gap. All of those involve the tensioned parts of the door and are genuinely unsafe to work on without training and the right tools.

Industry bodies say the same. The Door & Access Systems Manufacturers Association's garage door safety tips are explicit that springs, cables, and the bottom brackets they connect to should be adjusted only by a trained technician. Their homeowner maintenance guide is a good printable checklist for the seasonal upkeep that keeps seals and hardware in shape, and the International Door Association maintains the broader professional reference for the trade. When the gap is more than a tired piece of rubber, a technician can diagnose and fix the underlying cause in a single visit.

Frequently Asked Questions

Why is there suddenly a gap under my garage door when there was not one before?

A gap that appears gradually over months is almost always a bottom seal that has hardened, shrunk, or torn with age. A gap that appears suddenly, especially right after a power outage, a new opener, a repair, or a bump against the door, points instead at the opener's travel limits being off or a cable having slipped. The speed of the change is your best first clue.

Can I fix a gap under my garage door myself?

Often yes. Replacing the rubber bottom seal, adding a floor threshold, and adjusting the opener's down-travel limit are all safe homeowner jobs that need no special tools. What you must not do yourself is adjust the lift cables, the bottom brackets, or the springs, which are under high tension whenever the door is closed and are the leading cause of do-it-yourself garage door injuries.

What is the difference between a bottom seal and a threshold seal?

A bottom seal is the flexible rubber strip attached to the bottom edge of the door itself; it moves up and down with the door. A threshold seal is a low rubber ramp bonded to the garage floor at the opening; it stays put. Seals fix worn rubber, thresholds fix uneven floors and water that runs in from the driveway. On a badly uneven floor, using both together gives the best seal.

My garage door has a bigger gap on one side than the other. Is that dangerous?

It is worth taking seriously. A gap that is worse at one end usually means the door is landing unevenly, which points at a cable that has slipped or stretched, a shifted bottom bracket, or a tracking problem. Those are tensioned parts, so it is best to stop cycling the door more than needed and book a technician. A crooked-closing door left alone can worsen and, in the worst case, come off its tracks.

Will a new bottom seal fix a gap caused by an uneven floor?

Only partly. A taller, more flexible seal can absorb small dips, but if the concrete rises and falls significantly, a flat-bottomed door will always leave a gap somewhere. The reliable fix for an uneven floor is a threshold seal bonded to the concrete, which follows the shape of the slab and meets the door's seal partway.

How long does a garage door bottom seal last in Ontario?

Typically five to ten years, though sun exposure, road salt, grit, and standing water all shorten that. A south-facing door in full sun tends to be at the shorter end. An annual thumb-press check each fall will tell you when the rubber has hardened enough to need replacing before it splits open in winter.

Does a gap under the door really affect my heating bill?

Yes, especially with an attached garage or a room above it. A continuous gap along a wide door is the equivalent of an open window, and it lets cold air pour in and warm air escape all winter. Sealing the bottom is one of the least expensive efficiency improvements you can make, and it multiplies the benefit of an insulated door.

Conclusion

A gap under a closed garage door looks like a single problem but is really four, and telling them apart is most of the job. An even gap is a worn seal, a cheap and often do-it-yourself fix. A gap that follows the dips in the floor is a threshold job. A gap that is worse at one end is a crooked door and a signal to call a technician before a small cable or tracking issue becomes a big one. And a gap that showed up overnight is often just an opener limit that drifted out of adjustment. In every case the gap is worth closing, because the rubber at the bottom of your door is the last line of defence against the cold, the water, and the pests that make a garage uncomfortable and expensive.

If you have looked at the bottom of your door and you are not sure which kind of gap you have, or if it is clearly worse at one corner, our London team is happy to take a look, seal it properly, and run the balance check that catches the underlying cause. Call 519-619-7901, reach us through our contact page, or price out a new door and opener on our price estimator. AA Advantage Doors has kept London-area doors sealed, balanced, and running since 2009; our BBB profile has the service history.