A rough opening too small for the window or door to fit presents a problem with a limited number of options to fix. Explore them and choose the best way. The options are: replace the window or door with one that will fit, remove the jack studs and make them smaller, or rebuild the opening.
What to do if the Rough Opening is too small
If the window can be replaced and still meet egress requirements, or the door downsized without any consequences, then this is the easiest way to fix the problem. Most of the time, this is not an option.
The easiest way to make the rough opening bigger is to remove the jack studs. According to The International Residential Code on page 151, it is possible to replace the jack studs with approved framing anchors for instances requiring only one jack stud. There is a maximum of 3″ to be gained by this.
There is only one way, from my perspective, to make a rough opening bigger, it’s sad, but it’s true; rebuild the opening. Start by removing the jack studs. A flat bar, hammer, sledge hammer, reciprocating saw, and some wedges will prove handy. The following instructions assume that the center must remain the same. If this is not critical, then do nothing to one of the jacks.
Use the flat bar or wedges to establish a small crack between the sheathing on the outside of the wall and jacks. Saw the fasteners with a reciprocating saw. Saw the nails at bottom of the jacks and the side of the header. Knock out the jacks hammering away and in from the center with a sledge hammer. You will encounter resistance from the nails in the sheathing above the header. Overcome this by hammering out out the sheathing while force is applied inwards on the jacks.
Saw the nails holding the header to the sheathing and to the upper cripples. Be careful not to let it fall. It really takes two people for this.
Clean up all nails with the reciprocating saw on the inside of the sheathing, the top wall plate, and anything else that might be in the way.
The jacks are reusable unless they are just damaged too bad. The header is too short and there is no hope for it here.
Re-install the jacks and then the new header. Nail back the sheathing and the rough opening is ready!