Framing for a French door is similar to framing for a single door except there are two doors and each door will be hung one on each side of the opening. However, French doors are usually worth the extra effort, for once they are up and everything is in order they make a handsome addition to your living space, whether they are interior or exterior.
How to Frame a French Door
The most complex part of the task is measuring the doors and calculating the width of the opening for the set of double doors. When calculating the width remember there are three gaps between the doors instead of two, plus a jamb on each side of the opening. So if the doors are 24-inches wide, leave a 1/4-inch gap between doors and between each door and the jamb. And don't forget that the jamb is usually 1-inch thick with another 1/4-inch space between the frame and the jamb. This adds up to a total of 51 1/4 inches for the width of the opening. Now add another 1 1/2 inches to each side of the opening so you can side nail a vertical wall plate to the bottom plate. That brings the total to 54 1/4 inches for the opening width.
Calculate the height of the opening. This task is easier to complete, but you still must allow for the height of the actual doors, a gap for the flooring underneath the doors and a space for the piece of jamb that goes above the door. So allow an extra inch at the bottom and two inches at the top for the jamb. For example, if the doors are 84-inches high (don't forget French doors can be very tall), then the opening will be 88 inches. (Please note that due to the way the top jamb is installed, the vertical measurements can be loose.)
Provided the wall in which the door is to be installed is not a support wall for the roof, cut the door opening in the 2x4 frame after the wall is built. Mark the exact location of the opening on the bottom plate. Use a carpenter's square and pencil to place two marks that are exactly 88 inches apart. Continue the marks across the top face of the bottom plate with a carpenter's square to form a line that is perpendicular with the length of the bottom board.
Remove all 2x4s that are between the two marks by knocking the bottom of each board sideways until it comes loose. Then take the free end of the 2x4 and twist it until the top comes loose. Remove all nails and save the 2x4s for later use. Any nails that are sticking straight up or straight down through one of the plates can be pounded with a hammer and bent over until the nail lies flat.
Cut the portion of the plate out that will form the opening for the French doors. Use a circular saw combined with a carpenter's square or a reciprocating saw if space is tight. The length of this opening has to be 54 1/4 inches, the distance that you calculated in step 1.
Cut a 2x4 to length (that's from the subfloor to the bottom of the top plate) and side nail the bottom end of the freshly cut 2x4 to the end of the bottom plate. Add a couple of angled nails above the first nails that you installed to make sure the bottom end is tight.
Set the top end with a 4-foot level. Mark the spot on the top plate where the 2x4 is exactly level and then toenail the top of the board so it is even with this mark. After the stud is in place, check it again with the 4-foot level to be sure it is level.
Repeat steps 6 and 7 for the other side of the opening.
Make a perpendicular mark with your carpenter's square and pencil at the 88-inch mark (the height of the opening calculated in step 3) and measure the distance between the two upright 2x4s. This should be 51 1/4 inches. (Refer to step 1 to see why.)
Make a header to fit between the two uprights. This header will consist of two 2x6s and a piece of 1/4-inch plywood cut to fit between the 2x6s. Don't forget the plywood needs to be 5 1/2-inches wide just like the 2x6s. Nail the header together with the #8 rosin coated nails. Install the header by driving #16 framing nails through the 2x4 upright into the side of the header. Be sure to keep the bottom of the header flush with the line across the upright stud.
Add another upright 2x4 to the outside of each vertical 2x4 to form a double post on the outside of the header. Be sure to drive some nails into the bottom and top plates at an angle as well as nailing directly into the other upright.
Measure the distance between the top of the header and the bottom of the top plate and cut four 2x4 pieces to fit. Keeping with the existing spacing of the wall studs, nail these four pieces with #16 framing nails. Now the opening should be complete.