The Seals
Four seals keep weather, snow and mice out of a garage, and all four of them are cheap and routinely ignored.
Short answer
The bottom seal on the door, the stop moulding seals down the sides, the header seal at the top, and the seals between the panel sections. In a London winter the bottom seal matters most: it keeps drifting snow out, it stops the door freezing to the slab as readily, and when it hardens it is also the mouse route.
The four seals
The bottom seal, a flexible strip in a retainer along the bottom of the door, which seals to the slab. The one that matters most and the one that fails first.
The side seals, mounted on the stop moulding either side of the opening, which close the gap between the door and the jamb.
The header seal across the top of the opening.
The seals between the panel sections, which on a sectional door prevent air and water passing between panels.
And the threshold seal, which is a separate strip bonded to the slab and which is worth adding where the slab has settled or is uneven.
The bottom seal in winter
A hardened or torn bottom seal lets drifting snow blow in under the door, lets meltwater sit in the opening, and gives mice an entry. It also freezes to the slab more readily because the water sits in the gap. It is an inexpensive part, it slides into a retainer, and replacing it before winter is one of the better small jobs available.
How they fail
The bottom seal hardens and cracks with ultraviolet and cold cycling, and tears where it has frozen to the slab and been forced.
The retainer rusts or deforms, particularly from road salt.
The side seals come loose as the stop moulding they are fixed to rots or splits.
The stop moulding itself rots at the bottom where it sits in the slush.
The panel seals compress permanently and stop sealing.
And the slab settles or heaves, which opens a gap the bottom seal cannot bridge — which is where a threshold seal is the answer.
What good sealing does for a London garage
Keeps drifting snow out, which is the obvious one.
Keeps the garage several degrees warmer, which matters if there is a room above or if anything in there can freeze.
Reduces the water and salt sitting at the threshold, which is what corrodes the bottom panel, the brackets and the cables.
Keeps mice out, which is the reason a surprising number of people get round to it.
Reduces the door freezing to the slab.
And on an insulated garage with a door to the house, it makes a measurable difference to the house as well, because garage air gets drawn in.
Common questions
Which seal matters most?
The bottom seal. It keeps snow and mice out and reduces the door freezing to the slab.
What if the slab is uneven?
A threshold seal bonded to the slab bridges a gap the bottom seal cannot.
Why does the stop moulding rot?
It sits in the slush and salt at the bottom of the opening.