The Threshold
The gap where the door meets the slab is where a London winter does most of its damage to a garage door.
Short answer
Meltwater carrying road salt runs off the vehicle and sits at the threshold. It freezes the door to the slab, corrodes the bottom panel, the brackets and the cables, and runs into the garage. A bonded threshold seal, a sound bottom seal and a slab that drains outward are the three fixes.
What happens at the threshold
Snow and slush come in on the vehicle and melt on the warmer slab.
That meltwater carries road salt, which is a corrosive solution sitting against the bottom of the door.
It runs to the threshold and sits in the gap under the bottom seal.
Overnight it freezes, which is what bonds the door to the slab.
The salt attacks the bottom panel, the bottom brackets and the lower cable attachment — which is exactly where cables fail.
And on a slab that slopes inward, which many older garages do, it runs into the garage instead of out.
The threshold seal
A threshold seal is a rubber or vinyl strip bonded to the slab across the opening, forming a low ramp the bottom seal closes against. It bridges an uneven or settled slab, keeps water and drifting snow out, and gives the bottom seal something consistent to seal to. It is inexpensive, it is a half-hour job, and on a London garage it is one of the better small investments.
The three fixes
A sound bottom seal on the door, replaced when it hardens, cracks or tears — which is a cheap part in a retainer.
A bonded threshold seal on the slab, which bridges unevenness and keeps water out.
Drainage: the slab or the apron sloping away from the door rather than toward it, which on an existing garage may mean an apron adjustment.
Keeping the threshold clear of snow and ice before operating the door.
Rinsing the threshold and the bottom panel in spring to get the salt off, which costs nothing and extends the life of everything down there.
And lubricating and inspecting the bottom brackets and cable ends annually, because that is where the corrosion is.
What the damage looks like
Rust and paint failure along the bottom edge of the bottom panel.
Corrosion on the bottom brackets and on the cable where it attaches.
A bottom seal that has torn where it froze to the slab and was forced.
A hardened bottom seal that no longer flexes to the slab profile.
Water staining inside the garage along the inside of the threshold.
And in a bad case, the bottom panel perforating, which is a section replacement rather than a repair.
Common questions
What is a threshold seal?
A strip bonded to the slab that the bottom seal closes against. It bridges an uneven slab and keeps water out.
Why does the bottom of my door rust?
Salty meltwater sitting at the threshold all winter.
What is the free fix?
Clearing the threshold before operating the door, and rinsing the salt off in spring.