A garage door that does not quite meet the floor is one of those faults people live with for years. It lets in a draft, then some leaves, then a mouse, and eventually a puddle that freezes into a ridge of ice the door has to climb over every morning. By then the seal is usually not the only thing that needs looking at.

There are three separate causes behind a gap under a garage door, and they get fixed in very different ways. Knowing which one you have saves you from buying a seal that will not solve anything.

Cause one: the bottom seal has simply worn out

The rubber or vinyl strip along the bottom of the door is a wear part. It gets compressed every time the door closes, dragged across grit, baked in summer and frozen in winter. Eventually it hardens, cracks and stops conforming to the floor.

How to tell

Close the door and look along the bottom from inside with the lights off outside. Daylight showing in a thin, even line across the whole width points to a flattened or hardened seal. Run your hand along the rubber: if it feels stiff and does not spring back when you squeeze it, it is done.

Also check the retainer — the aluminium track the seal slides into. If the retainer is bent or the seal has pulled out of one end, no amount of new rubber will seat properly until the retainer is straightened or replaced.

What it takes to fix

On most sectional doors the seal slides out of the retainer and a new one slides in. The awkward parts are matching the bead profile to the retainer and getting the door low enough to work on safely. A seal that is the wrong profile will either fall out or refuse to go in at all.

Cause two: the floor is not flat

This one catches people out, because the seal looks fine and the gap is still there. Concrete slabs settle unevenly. Driveways heave over winters. A garage floor poured with a drainage slope was never flat to begin with.

How to tell

Look at where the daylight is. A gap that is wide in the middle and tight at both ends, or tight on one side and open on the other, is a floor problem rather than a seal problem. The door is straight; the ground is not.

What actually works

A standard seal can only bridge a small amount of unevenness. For bigger variations the options are a deeper bulb-style seal with more compression travel, or a threshold seal bonded to the floor itself so the concrete is effectively levelled at the door line. Threshold seals have a trade-off worth knowing about: they also stop water draining out of the garage, so if your floor slopes inward they can trap water instead of keeping it out.

In the worst cases the door itself has to be reassessed, because a bottom section that was cut to match an old floor line will never sit well on a floor that has moved. That is the point where the door section itself becomes part of the discussion rather than the weatherstrip.

Cause three: the door is not closing all the way

If the door stops short of the floor, the seal never gets a chance to compress. People assume a worn seal and buy a new one, and the gap stays exactly the same.

Where to look

• Opener travel limits. Nearly every opener has a down-travel adjustment. If it has drifted, or was set when the floor was different, the door parks early.
• Safety reversal. A door that touches down and then lifts back up a few centimetres is hitting its force setting or the photo eyes, not finishing its travel.
• Cable tension. If one lift cable has stretched or slipped on its drum, one side of the door comes down lower than the other. That produces a wedge-shaped gap and is a genuine safety issue, not a cosmetic one.
• Obstruction in the track. Debris or a bent track section can stop the rollers travelling the last few centimetres.

Uneven cable tension is the one to take seriously. Cables are under high tension and a frayed or mis-wound cable can fail suddenly. A visibly crooked door bottom is a reason to stop using the opener and get the cables inspected rather than adjust the limits to compensate.

Why the gap matters more in a Canadian winter

A summer gap is an annoyance. A winter gap turns into a cycle: meltwater runs under the door, refreezes overnight, and the door now closes onto ice. The seal gets crushed against a hard ridge instead of compressing against concrete, which shortens its life. If the door is forced down onto ice repeatedly, the opener’s force setting gets pushed up to compensate, and that masks the next genuine obstruction.

Blown snow through a bottom gap also collects on the inside of the bottom section, where it melts and sits against the steel. Over several winters that is how the bottom of a door starts to rust from the inside out.

What to check before calling anyone

1. Close the door and note where the daylight is: even line, middle only, or one side.
2. Squeeze the seal. Stiff and cracked means replacement regardless of anything else.
3. Check whether the door is sitting level at the bottom edge.
4. Look at the retainer for bends and pulled-out ends.
5. Watch a full close cycle and see whether the door reverses or stops short.

Those five observations will tell a technician more than a photo will, and they determine whether this is a twenty-minute seal swap or something that needs the door and cables looked at. On older doors where the bottom section has corroded through, replacing the door sometimes works out better than patching a section that will keep failing at the same spot.

Frequently asked questions

Can I just add a second seal on top of the old one?

It rarely works. Two seals stacked do not compress evenly and the door can end up resting on the stack instead of closing fully, which trips the opener’s force setting.

Will a threshold seal stop water coming in?

It can, as long as the floor drains away from the door. If the slope runs into the garage, a threshold can hold water inside instead.

How often should the bottom seal be replaced?

There is no fixed interval — it depends on use, sun exposure and how clean the floor is. The honest answer is to replace it when it stops springing back when squeezed.

My door closes level but there is still a gap at one end. Why?

Usually the floor at that end has settled or the seal has pulled out of the retainer there. Check the retainer end caps first.

Is a gap under the door a security problem?

A large gap gives something to pry against at the bottom edge, which is the weakest point of most doors. Closing the gap properly removes that purchase.

Summary

Three causes, three different fixes: a worn seal gets replaced, an uneven floor needs a deeper or bonded seal, and a door that stops short needs its travel, cables and tracks sorted before any seal will help. Work out which one you have by looking at where the daylight shows, and you will not waste money on the wrong part.