French doors are a popular architectural element used to separate patios and balconies, as well as for some interior locations. They are made from wood and glass panels, which allow plenty of light and exterior views into the home. These doors are typically sold as prehung units, which means that the door(s) are already hung on the frame. This makes installation much easier for the DIY homeowner, though these doors must still be properly fit to your walls in order to work correctly.
Measure the size of the rough opening using a tape measure. This is the dimension from stud to stud, width-wise, and from the floor to the top of the opening, height-wise. Measure the thickness of the wall to determine the width of the jambs of the frame.
Purchase a prehung door set that is 1 inch smaller in width and height than the rough opening. This extra space will allow for clearance to install the door. If your existing opening is a standard size, you'll have no trouble finding a unit that fits. The jambs should be the same thickness as the existing wall so that they can butt flush against it to create a smooth line.
Adjust your rough opening as necessary to fit the doors you'd like to install. Use 2 inch by 4 inch or 2 inch by 6 inch studs to frame out the rough opening based on the door you've chosen. Add these studs and nail them in place alongside existing framing members. Remember to leave a 1 inch clearance in height and width.
Slide your French door unit into the opening bottom first. Plan this carefully so that the doors swing the way you want them to. It is standard for these doors to swing outside of the home. Add a couple of nails in each jamb to hold the unit in place as you adjust it.
Place wooden shims under the legs of the jamb as needed to level the head of the frame. Slide more shims between the jambs and the wall studs to further even out the frame. Check the unit with your level to ensure it is level before proceeding.
Fill any remaining gaps along the head and jambs with batt insulation. Pack the insulation so that it fills the space completely but do not force it in. Finish nailing or screwing the unit in place according to the manufacturer's instructions. Add trim or casing to cover the joints between the frame and the wall.