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All sectional garage doors are ordered by the internal frame dimensions and all the doors have a steel fixing frame surrounding the door panels to fix the gear and tracking to the garage opening.
For example a sectional garage door 7'6" x 7' (2286 x 2125) refers to a door which ideally if it were to be fitted BEHIND or INSIDE the garage opening would mean ideally your opening size was exactly the same dimensions 7'6" x 7' (2286 x 2125).
As a sectional garage door does not go through the opening at any time during operation you can install a sectional door behind an opening that is smaller than the ordering sizes as long as there is sufficient room inside to take the frame sections. The headroom required inside is the one big variable with sectional doors as different manufacturers use different spring lifting systems and these will vary between single width and double width door sizes.
The sectional doors can be installed INBETWEEN the structural opening and then you have to be sure of the OVERALL dimensions of the door with the frame legs and frame head included and leave some tolerance for installing too.
Generally most of the sectional door require about 100 -125mm of headroom for single width doors (up to 10' wide) and 210 - 240mm for double width doors. If there is insufficient headroom then low headroom options are available to help install the door and maintain a sensible height.
On widths most sectional doors have steel frame legs that are around 90mm wide with a bit of tolerance and we suggest you allow 100mm either side to be sure unless the opening is absolutely square. So overall size required for installation with some tolerance is the ordering size width plus the 100mm x 2 = 200mm or as little as 180mm if you feel the opening is 100%.
It can be quite complicated so again we encourage you to email or call us directly for help on deciding the best gear and deciding on the measurements if required.
For Hormann sectional doors we have a page to show you more information on sizing - DIY installation help >
For Carteck sectional doors we also have a DIY help page - DIY installation help >