Since most exterior doors are made of wood and they are exposed to the outdoors, eventually many of them will incur some type of rotting or moisture damage, especially near the bottom, that will require replacement. Leaving rotted wood on the frame will only encourage more damage, so knowing how to remove and repair that rotted door frame is a good ability to have.
Inspect the rotted doorframe to assess how much of the frame will need to be removed. Press the wood with your thumb or a screwdriver to test for softness or "give." Also check for insect damage and/or mould. Make a line with a pencil at least 2 inches higher than the rotted areas.
Determine the style of door frame you have and locate suitable replacement material. Some frames may be able to be "pieced together" with stock frames or wood from a local home improvement store, but for custom frames it may be necessary to contact the builder or manufacturer to obtain the right replacement material.
Remove the door if rotted wood will need to be removed where the hinges are. Place the door flat on the floor on a blanket somewhere away from the work area to avoid it being damaged. Stack a few books or blocks under the handle side if necessary to prevent damaging the handle. Mark the location of the hinges with tape or a marker and remove them. Also remove the latch plate if needed. Store the hinges and plate with the door so they will not be lost.
Use a saw to cut at the line you made. Use a pry par or hammer to carefully remove the rotted wood. Be sure not to leave any rotted or damaged wood. If you discover more, make a new cut and remove as needed.
Use a tape measure to determine the length of the replacement pieces. Measure twice to ensure the correct measurement is taken. Write the measurement down.
Measure and cut a new piece of frame from the replacement material. Use a square to draw a straight line as a guide for cutting. Use a circular saw to ensure a straight cut.
Use a paintbrush or roller to apply a quality exterior primer to the ends, back and all sides of the cut piece. This will hinder future water penetration of the new wood. Also prime where the old wood was cut on the frame. Allow to dry.
Fit the primed replacement frame into place. Secure with nails or screws. Make sure all ends are straight with the existing frame. Repeat for the other side of the frame if necessary.
Sand and scrape away any peeling or flaking wood on the old part of the frame. Add wood putty to the seams where the new and old wood meet. Let dry, then sand. Spot prime those areas as well as anywhere sanded or raw wood may be visible. Allow the primer to dry, then caulk with a good exterior caulk. Paint the entire frame, both new and old wood. Rehang the hinges, plate and door.
Have help when removing the door. Kilz can be used in lieu of exterior primer if preferred. Plan for at least two days for this project.
Use caution with power tools. Keep children out of the work area. Do not remove any wood until you have suitable replacement material.