When framing exterior walls, some extra work and skill are required. And it is even more complicated if you need to frame a window opening into an existing wall. Before tackling a window framing project, make sure you obtain the rough opening requirements for the window you plan to install. If you don't frame the window opening to the window's rough opening specifications, you either won't be able to insert the window into the opening or won't have the play in the opening to square and plumb up the window in the opening. This latter issue is particularly insidious and painful because of the false sense of security. You might have been able to squeeze the window into the rough window frame opening, but you may not realize until much later that the window will not open and close properly because you weren't able to square it up adequately.
Normally, the rough opening specifications call for the window frame opening to be 2 inches longer and wider than the window itself. This allows for about 1 inch of play all around the window in the opening so that you can square and plumb it up. A carpenter's level and shims are used to ensure the window is level and plumb in the opening.
Also, typically two-by-fours or two-by-sixes are used for residential framing, including the window frame opening.
*How to Frame a Window Opening in New Construction
When you're framing a new wall and you need to frame a window opening, first mark on the top plate and the sole plate the location of the window. Make marks on both plates for a king stud and a jack stud to be positioned on either side of the window opening. Again, the rough opening between the two jack studs should be the required rough window opening specifications, 2 inches wider than the window itself.
Next, nail two king studs between the top plate and sole plate of the wall. Then nail the bottom/inside jack studs to the king studs, again making sure the rough opening between the two jack studs meets the window's rough opening requirements. Also, make sure the height of the jack studs meets the height requirements, minus 1.5 inches for the sill plate, for where you want the window to sit in the wall.
Now attach a sill plate to the top of the jack studs. Note that when the sill plate is nailed into place, the window will now sit on it; at the total height of the sole plate, plus the jack stud, plus the sill plate. Also note that the sill plate sits on its flat side on top of the jack studs. Nail the sill plate to the top of the jack studs.
Next, install trimmer studs on top of the sill plate, and nail them to the king studs. The height of the trimmer studs should be the rough opening window height specification, or 2 inches taller than the window itself. Install any additional jack studs required between the bottom of the sill plate and the sole plate. A jack stud should be installed on 16-inch centers.
Then install a header that rests on top of the trimmer studs. The header is usually composed of two two-by-sixes or two-by-eights that are sistered together and placed on their edge on top of the trimmer studs. Spacers are commonly used between the two sistered pieces of lumber to obtain the same depth/thickness as the framed walls.
Next, nail the header to the king studs and to the trimmer studs.
Lastly, install cripple studs between the window header and top wall plate. A cripple stud should be placed against each king stud, and if necessary, additional ones should go every 16 inches along the top of the window header. Nail the cripple studs to the king studs, header and top wall plate. Typically, the center cripple studs are toenailed into the header and top wall plate.
*How to Frame a Window Opening Into an Existing Wall
The steps for framing a window opening into an existing wall are the same as the steps for framing one into a new wall. However, the existing exterior wall has to be demolished first so there's room for the window frame opening, and the actual framing of the window needs to be stick-built. With stick-building, instead of framing the wall on the ground and then raising the wall into place, you install every window frame stud piece by piece into the existing framed wall. The use of a framing nail gun is highly recommended, as a lot of toenailing is required when stick-building walls.
For the demolition phase of the project, first make sure all electrical power to the wall that you are going to demolish is turned off at the main circuit panel. Also make sure that any plumbing pipes are located and marked on the plasterboard and floor prior to starting demolition. Take care not to cut or damage electrical wires and plumbing pipes.
Once you have addressed the electrical and plumbing concerns, you can start the demolition. Start by removing all the plasterboard on the wall, or at least within 2 feet of each side of where the window is to be framed.
Then remove insulation from the walls.
Next -- and this is where a reciprocating saw really comes in handy -- cut away any vertical stud members between the top wall plate and the bottom sill plate. Note that if you are removing just one or two vertical wall studs, you typically have nothing to fear with structurally impacting the integrity of the home. Any more than two vertical wall studs and you may want to check with your local building inspector or an engineer to ensure that you will not compromise the structural integrity of the home and your personal safety. You may need to temporarily add some bracing between the floor and the ceiling of the room while you frame the window opening.
Once the window opening has been framed, you can remove the plywood and outside house sheathing to allow for the installation of the window. Again, a reciprocating saw is highly useful during this stage.
Mark J. Donovan's website is at http://www.homeadditionplus.com.