Changing an old, worn, out-of-style front door with a modern one can increase curb appeal and replacing a draughty, ill-fitting with a properly fitting door and frame can lower heating and cooling costs. Pre-hung exterior doors and frames are sold ready for installation.
Remove the trim on both the inside and outside of the old door. Place a putty knife against the plasterboard under the edge of the trim. Slide the pry bar under the trim but above the putty knife. Pull the trim off. Remove any nails with a hammer. This exposes the rough door opening, which is the space in the framing that the door package fits into.
Measure the height and width of the door, exterior casing, jam width, trim width and the rough opening with a tape measure. Round the size of the door up to the next full centimetre.
Buy a door package that fits your measurements. Pay attention to the rough-opening measurements. The new door frame must be slightly smaller than the rough opening for it to fit.
Remove the old door's hinge pins with a hammer and punch. Set the old door aside.
Remove the old door frame. Cut through the nails that hold it in place with a reciprocating saw. Slide the door frame out after the nails have been cut.
Clean up debris with a broom and dust pan. Clean out the gaps around the rough opening.
Inspect the rough opening and sill for damage. Replace all damaged areas.
Wrap the rough opening with strips of felt paper. Secure it with roofing nails.
Apply caulking to the sill and the new door casing. The caulking beads must be unbroken and extend completely across the sill and all the way around the casing. Fill the bottom corners.
Set the door frame in place. Push the casing tight to the wall.
Centre the door frame in the rough opening. Use a bubble level to check the frame for plumb. Push shims between the frame and the rough opening at the top, bottom and middle. Tack the frame corners in place with 9 cm (16d) nails.
Double-check the door frame for plumb and level. Place a bubble level on each side of the jam and the top plate. Adjust the shims as needed.
Hammer 9 cm (16d) nails every 40 cm (16 inches) along the hinge side of the exterior casing. Open and close the door to verify that the door works properly.
Finish securing the door frame casing. Drive 9 cm (16d) nails every 40 cm (16 inches) along the top and latch side. Screw the threshold to the sill. Remove one screw from each hinge. Install a 7.5 cm (3 inch) screw in its place with a drill.
Fill the area between the door frame casing and the rough opening with a non-expanding foam sealant. Allow the foam to dry. Remove any excess with a putty knife.
Install the trim with finishing nails. Caulk the edges and seams. Large gaps require a foam-caulking backer.
One person can complete this project but it is much easier with a helper.