Inserting a door or window in a load-bearing wall can be a tricky situation, but not an impossible one. The reason why this project is so difficult is due to the potential danger—a load-bearing wall supports the structure, so its absence through accidental destruction could lead to the room or entire house collapsing. However, if you know where to cut into the wall, the project is quite straightforward.
Step 1 - Prep
Because of how crucial load-bearing walls are to the stability of your structure, mistakes must be limited and proper preparation is paramount. This begins with understanding where all the wiring, plumbing, and other fixtures are located throughout the wall. Note if, and how, those fixtures can or will be re-routed to accommodate the addition. It is imperative to know inside and out exactly where you will be cutting, so you must know the approximate size of the door or window itself as well (note that “approximate” implies between two to three inches extra all around the desired area). Mark your area, and place sheeting over it. Finally, turn off the power to all circuits within the wall.
Step 2 - Consult an Expert
Before any cutting, you will need an expert. Creating an opening in a load-bearing wall means that the transmitted load down to the floor must be shifted to the sides of the wall. The only way of knowing how much pressure can be allowed is to get an expert to help.
The type of lumber used on the load-bearing wall is an important consideration as well, so check the blueprints, inspect yourself, or call an expert to help out with this. Again, misreading the lumber type can, but not necessarily will, lead to problems, and problems are something to avoid when working with a structure-supporting wall.
Step 3 - Cut the Opening and Shift Support
Use the tip of your saw blade to pierce through the sheeting—this is the starting point. A good tip is to strategically place the rough starting point opening between any wall studs so that a stud is used as part of the framing, cutting time as this eliminates framing for one whole side of the wall. Use the framing square and a pencil to transfer the cut marks to the inside area of the remaining opening, and then continue cutting. If you've determined you have to add temporary support beams or add extra supports to other walls to carry the weight, do that before you make the final cuts.
Next, begin to drill holes at each corner of the opening. These drill marks will help guide your marks on the other side of the wall. Then, with a saw, either hand or miter, continue cutting as needed. If wiring gets in the way, you will need to clamp some wires to reroute them away from the work site.
Step 4 - Frame and Install
After making any and all necessary changes to the opening, including installing framing for the window or door, you're ready to frame and install. Before adding a door jamb, in the case of installing a door, use a level to check the plumb and level of the floor. If the floor is uneven, note that a side of the door jamb will need to be altered so to be completely parallel to the floor. Nail the pieces of your jamb in place with finishing nails and use wood putty to cover the holes. Then, screw the threshold in place, if you need one once it's trimmed to length, and then run a bead of silicone caulk at the edges to seal it properly.
Insert the door in place and use three pairs of wedge shims per side for support and to plumb the jamb. Again note if any changes are needed before securing the hinges to the frame. Then, test the door or window if it is functional, and make final changes as needed.