How to Prepare a Window Opening

Before the new replacement window can be installed, the existing window framing must be prepared by removing all debris and correcting any defects. Ensure that the window opening is the correct height, width, and depth to meet the specifications of the window being installed. Check the opening for square by performing corner-to-corner diagonal measurements.
If the opening is out of square, correct it until it is square. Measure again to ensure that the window will fit into the opening.
Wood windows must have all of the chips and rough edges (caused by the removal of the blind stops and the parting stop) removed and/or smoothed to avoid impeding the installation.
Aluminum windows must have wood blocking fastened into the sash tracks of the sill to help support the weight of the new window. The wood blocking must also be fastened into the jamb and header sash tracks to give an additional fastening reinforcement to the screws that will be used to attach the replacement window to the frame. Cut the wood blocking flush with the top edge of the sash track leg.
After the window opening has been inspected and corrections, if any, have been made, the window is ready for installation.

Before the new replacement window can be installed, the rough framing must be prepared by removing all debris and correcting any defects.
Clean the rough frame opening. Remove all dirt, dust, nails, old putty, and sealants.
Ensure that the window opening is the correct height, width, and depth. Check the opening for square by performing corner-to-corner diagonal measurements.
If the measurements indicate that the opening is not correct, build up the rough framing with wood bucks or small pieces of wood-framing materials until the opening is within specifications. If the opening is small, remove the rough frame material at the area where the measurement is incorrect until the opening is within specifications. This recommendation is not intended to correct for mis-measured or mis-ordered windows.
After the rough frame opening has been inspected and corrections, if any, have been made, the window is ready for installation.