Walk past a row of Toronto shops in late March and the damage is all at the same height. The bottom six inches of the door, the threshold, the lower corner of the frame. Everything above waist level looks fine. That is not coincidence, and it is not simply wear. It is road salt, carried in on boots, dissolved in meltwater, and left to sit exactly where a door is least able to shed it.

Salt damage is slow, which is why it gets ignored until something stops working. By the time a door drags or a closer gives up, the corrosion has usually been underway for two or three winters.

How salt actually gets at a commercial door

Calcium and sodium chloride do not attack metal directly in the way people imagine. What they do is keep moisture liquid at temperatures where it would otherwise freeze, and make that moisture far more conductive. A damp door in a dry freeze is relatively safe. A door standing in salty slush at just above zero is in the worst condition metal can be in.

An entrance door collects that slush in a very specific way. Customers knock snow off their boots in the doorway. Meltwater runs toward the threshold because the sidewalk slopes that way. The door sweep traps it against the bottom rail. Then the shop heats up, the water evaporates, and the salt stays behind as a crust that pulls fresh moisture out of the air on the next damp day. The cycle repeats every thaw.

The bottom rail and the glass pocket

On an aluminium storefront door, the bottom rail is a hollow extrusion with the glass seated in it on a setting block. Water that gets past the glazing gasket sits in that pocket. Salt gets in with it. Aluminium resists corrosion well in clean conditions because it forms its own oxide layer, but chlorides break that layer down locally and you get pitting instead of an even dulling.

The visible signs are a white powdery bloom on the inside face of the rail, bubbling under paint or anodising, and in advanced cases a corner joint that has opened enough to see daylight. Once the corner stakes or screws are sitting in corroded metal, the door starts to rack, and a racked door is a door that no longer meets the frame squarely. Commercial door repair at that stage is a structural question about the leaf, not a hardware adjustment.

Thresholds and the floor under them

The threshold is the single worst-hit component. It is horizontal, it is walked on, and it is where the salt concentration is highest. Aluminium thresholds pit and lose their surface. The fasteners holding them down corrode faster than the threshold itself, because dissimilar metals in a salty electrolyte make a small battery, and the less noble metal gives way.

Under the threshold is worse. Water tracks through the fixing holes into the substrate. On a concrete slab that means spalling and a soft bed for the anchors. On a wood sub-floor it means rot that is invisible until the threshold flexes underfoot. A threshold that rocks slightly when stepped on has usually lost its substrate, and bolting it down harder only crushes what is left.

Hinges, pivots and closers

Bottom pivots on a storefront door sit directly in the splash zone. A pivot is a small bearing carrying the entire weight of the leaf, and once salt has worked into it the bearing stiffens, then wears oval. A door with a worn bottom pivot drops at the lock edge and starts catching the strike, which is the complaint that eventually gets reported.

Butt hinges on a hollow metal door corrode at the knuckle and at the screw heads. Overhead closers are usually high enough to escape, but the arm and the bracket are not immune, and a closer that is working against a door which has dropped will be blamed for a problem it did not cause.

Why the symptoms show up late and all at once

Corrosion does not degrade function gradually. A door that is slightly out of alignment still closes, because there is tolerance built into every part of the assembly. The tolerance absorbs the first winter and most of the second. When it runs out, the door goes from fine to not latching inside a few weeks, usually during the coldest part of the year when everything is also contracting.

That is why so many Toronto businesses discover the problem in January, at the point where it is most disruptive and most expensive to deal with. The underlying damage happened the previous two springs.

What actually slows it down

None of this is preventable outright on a door that opens onto a salted sidewalk. It is manageable.

• Rinse, do not just sweep. Dry brushing moves the visible salt and leaves the crust. A bucket of clean water over the threshold and the bottom rail during a thaw does more than anything else on this list.
• Get the water away from the door. Proper matting inside and out, long enough that boots are dry before they reach the threshold, keeps the worst of it off the metal.
• Check the sweep and the glazing gasket in autumn. A hardened sweep channels water into the rail instead of away from it. A perished gasket lets it into the glass pocket.
• Look underneath once a year. Pull the mat, look at the threshold fasteners, press on the threshold and see whether it moves.
• Rinse and lightly lubricate the bottom pivot rather than only adding grease, which traps salt against the bearing.

On glass entrance doors the rails and the glazing are the parts to watch, since the glass itself is unaffected but everything holding it is not.

When it becomes urgent

A storefront door that will not latch is a security problem that night, not a maintenance item for next month. The same goes for a door that has dropped far enough to bind, because the force used to pull it shut starts breaking other things, and for a threshold that has lifted enough to catch a foot. Those are the cases where emergency door repair is the right call rather than waiting for a scheduled visit.

The useful mental model is that salt damage is a floor-up problem on a system designed from the hinge out. Everything the door depends on structurally is sitting in the one place the salt collects. Winter is not kind to a Toronto storefront door, but most of the harm is done in the thaw, which is also the easiest moment to do something about it.

Frequently asked questions

Does salt damage aluminium or only steel?

Both, differently. Steel rusts visibly and generally. Aluminium pits locally under its oxide layer, which is harder to spot early and concentrates the damage at joints and fasteners.

Can a corroded bottom rail be repaired?

Sometimes. Surface corrosion can be cleaned and treated. Once the corner joint has opened or the metal is perforated, the rail or the leaf usually needs replacing, since the rail is what holds the door square.

Is a rubber mat enough protection?

It helps a great deal, provided it is long enough and gets lifted and cleaned. A short mat that stays down all winter holds salty water against the floor and the threshold.

Why does the door only stick in winter?

Cold contraction plus a stiffened weatherstrip can make an already misaligned door bind. If it sticks every winter and a little more each year, the alignment is drifting and something mechanical is wearing.

How often should a commercial entrance be inspected?

Twice a year suits most Toronto sites, once before winter to check seals and sweeps and once after the thaw to check what the salt did. High-traffic entrances benefit from more.