When an older window is replaced with a new one, it is easier to use the existing frame to mount the new window. There are times, however, when the frame needs to be rebuilt to accommodate a new window. Building a wooden window frame is essentially building a box in a wall in which a window is going to be installed.
Remove the existing window from the wall. Examine the window to see how it was originally installed and what tools will be needed to remove it. An existing window may have been installed using screws or nails that need to be removed and sometimes cut with a Sawzall.
Pull the old window out of the opening, and clean up and debris, such as nails and flakes of paint, from the window opening.
Take out the old window frame. The frame can be attached by screws or nails. A drill can be used to remove the screws. Another option would be to use a Sawzall to cut the nails that were used to secure the window frame to the studs.
Insert a new window sill. Cut a piece of 5 cm x 10 cm (2 x 4 inch) board the width of the window opening. Secure it to the side and bottom wall studs at the bottom of the window opening.
Replace the studs that can be used to hold the header beam if the wall is load bearing. Measure the height of the window opening, and subtract the height of the header beam. Cut two pieces of 5 cm x 10 cm (2 x 4 inch), and attach them to the existing studs, making sure they are positioned on top of the window sill. Insert the header beam so that it sits on top of the new stud beams. Secure the header beam to the existing wall studs.
Measure the width of the opening to determine how wide and high it needs to be for the replacement window. Place wood surrounding the opening until the desired measurements are reached. Make the opening slightly larger than the window so that it will fit without problems.
A replacement window should be used if the frame is being built in an existing wall. A replacement window will be much easier to install. A wall that is not load bearing does not require a header beam. It is easier to shim the window than trim the opening to make room for the window.
Building a wooden frame in an existing wall may require tearing out the drywall to properly frame the opening.