Wood is the traditional choice for window frames, because of its availability, ease of use and appearance. It exudes warmth and adds texture to any home. A frame is exactly what the name suggests, as it surrounds the window much like a picture frame. However, wood is prone to rot and warp over time and has a higher maintenance requirement than other types of framing. If you want the look of timber frames, be prepared to paint every three to five years.
Remove the old window, if necessary, and inspect the opening for signs of moisture.
Determine the size of the rough opening and the depth needed for the frame. The depth should be the same as the thickness of the wall, plus the interior drywall and the outside siding materials. Measure the horizontal and vertical openings at each end and in the middle, and use the smallest dimension in each instance.
Add sill flashing or flexible tape to the outer edges of the rough opening.
Cut a rebate into the pieces of timber lumber. A rebate is a ledge, or step, cut into a portion of the wood. It should be 2cm (3/4-inch) deep and 5cm (2 inches) wide. This will be the ledge into which you will sit the window and screw it in.
Cut four pieces of rebate-cut timber into two pieces as wide as needed and two pieces as long as needed with the saw. Angle cut the corners at 45 degrees.
Glue and nail these pieces together to form a box. Make certain the rebates are all facing the same way. Check each corner for squareness with a carpenter's square.
Install the frame into the opening, using wood screws. Check for level, straight across the opening.
Paint or stain the frame before installing the window. Let the paint dry completely.
Lay down a bead of silicon and insert the glass window into the ledge created by the rebate.
Screw the window into place with the screwdriver. Finish by adding a bead of caulk around the window.