Consider hiring a garage door contractor to remove the doors if you have this style spring.
How to install a garage door in a brick wall.
Garage door torsion springs mounted above the door are under high tension and can unwrap with great force.
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Fill all nail holes with caulk and insulate with weather stripping where applicable.
Apply paintable caulk and insulate around the entire door frame.
If you are going to install a door into the frame take into account the exact dimensions of the door when you make your measurements on the wall.
The first step in converting a garage is to remove the door and build a wall.
It would be a lot more convenient access if there was also a personnel door from the garden.
This old house general contractor tom silva shows how to assemble and hang a new garage door.
Adjust the threshold for a tight seal as long as it is adjustable as directed by the manufacturer.
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Determine how tall and how wide you want the doorway to be.
Use chalk to mark the areas where you want to cut through the brick.
Finish the door as directed by the manufacturer.
Here is a step by step tutorial on how to install a prehung exterior door frame in a concrete wall.