Materials and what they suit
Steel is the workhorse: strong, well priced, available in a wide range of finishes, and the standard on most new installations. Its weak point is corrosion at the bottom edge where water sits, which is why the bottom seal and the floor level matter more than people expect.
Aluminium is lighter and does not rust, which makes it a sensible choice on exposed openings and on very wide doubles where weight is a concern. Timber gives the appearance people usually want on a period property, at the cost of real weight, seasonal movement and a maintenance commitment. Glass fibre sits in between — light, stable, and the most forgiving in a coastal or high-moisture position.
Measuring: headroom, side room and backroom
Three clearances decide whether a door can be fitted at all. Headroom is the distance from the top of the opening to the ceiling or the nearest obstruction. Side room is the space either side of the opening for the vertical tracks — usually around 90mm each side. Backroom is how far the horizontal tracks can run into the garage, which must exceed the door height plus the motor rail.
Obstructions are the usual surprise: a lintel, a lighting rail, a roller-shutter box from a previous door, a geyser, or a beam that crosses exactly where the tracks want to go. All of them have solutions; none of them has a solution that can be chosen from a photograph.
Single, double and two singles
A double opening can take one wide door or two single doors, and the choice is not purely aesthetic. One double door is a single mechanism to maintain and a cleaner opening to drive through. Two singles mean one door can stay working while the other is repaired, and each door is lighter, which is gentler on springs and motors.
Where there is a central pillar you have no choice. Where there is not, the width of the opening and the weight of the finished door usually decide it — beyond about 5 metres, a single door starts asking a lot of its springs and its top section.