Window frames consist of a wood liner that fits between the actual window, and the rough opening. Also referred to as a jamb frame, it provides a clean, finished surface for the addition of a window.
The rough frame is the stud opening in the wall. It may or may not have siding on the outside, and depending on the type of building, may also have 5/8-inch-thick sheathing under the siding. It might also have 1/2-inch-thick drywall on the interior side.
The jamb frame is the 3/4-inch-thick wood around the inside perimeter of the rough opening. It's a simple liner, and is typically made with your choice of wood. Use hardwood for an upscale jamb frame. Use poplar, fir or pine for an economy frame.
The window frame or sash, is the thicker -- typically 1 1/2-inch -- frame that holds the glass. More exclusive windows might include a grid -- also part of the sash -- that holds individual panes of glass. It may also be just an overlay that gives the appearance of individual panes.
The jamb frame may or may not have window stops. They work like door stops on some windows, to position the window at a particular place in the frame. Depending on the application, windows don't typically need them unless the window is hung with hinges.
The sill is the bottom of the jamb frame. It may or may not be wider than the actual frame, with a lip that extends to the outside, with a slight downward angle to channel moisture away from the window.
Make a Frame
Sheds or outbuilding jamb frames are typically nailed directly to the rough opening. Pocket frames, typically used as a replacement for household windows, are assembled in one piece, and slip into the rough opening as a complete unit. But there's no reason why you can't use pocket windows on sheds.
Measure the sides, top and bottom of the rough opening to determine the overall dimensions of the frame. Write down the shortest measurements for pocket windows, if the opening is out-of-square. If you do plan on making a pocket frame, subtract 3/8-inch from the measurements for overall dimension.
The sides, and top of the frame should be the same thickness, or depth, as the rough opening, including the drywall and siding. For example, if the stud framing is 3 1/2 inches wide, plus 1/2 inch for drywall, and 3/4 inch for siding and sheathing, the three pieces should be 4 3/4 inches. It's recommended to add another 1/4 inch to ensure that the jamb covers adequately, so the total jamb thickness would be 5 inches for the top, and the two sides.
The sill has options for width. It can be wider, with a slight angle to channel water away from the window, or the same width as the sides and top. if you want to install an angled sill, add at least 1 1/2 inches to the width. If water is not an issue, it's fine to build it the same width as the sides and top.
Draw the frame on a piece of paper and add the dimensions. Add 1 1/2-inches to the length of the top and sill, to allow them to overlap the ends of the two side pieces to form butt joints. Order the pieces pre-cut from a home supply store, or cut them yourself with a table saw.
Lay out the pieces on edge, as if the frame were already assembled. Tip the sill at about 10 degrees if you want it sloped. Clamp the frame together with bar clamps.
Mark and drill three pilot holes, spaced evenly through the ends of the top and sill pieces at each joint using a drill/driver. Add glue and screw the frame together with 2-inch screws to build a pocket frame. If you're attaching the four pieces directly to the rough framing, drill pilot holes with a 3/16-inch bit, centered through each piece, and screw the top on first. Place wedges under the front of the sill, and then screw the sill to the sides, with the wedges tilting the sill toward the exterior.
The frame is complete -- if you've chosen to nail the pieces directly to the rough opening, If you've made a pocket unit, place the bottom of it in the rough opening. Tilt the top of the frame up to flush it in the opening.
Tap wedges on both sides, and at the top and bottom, between the pocket frame and rough opening, to secure it. Check for level, and add or remove wedges as needed to level it.
Drill at least three screw holes, centered through each side, and along the top. Place the holes to penetrate through the wedges whenever possible. Screw the frame to the opening with 2-inch screws.