External doors and doorframes often suffer from damp and rot. Even with meticulous care and maintenance, it is hard to protect them from the wet British weather. Rather than replacing the whole doorframe, it is often easier to cut out and replace only the rotten section.
The first thing you need to do is remove the door. Once this is out the way, check the doorpost carefully to see where the rotten section ends. Mark slightly above the last rotten part and then saw through the post at a 45-degree angle (creating half of a Scarf Joint). If the metal dowels (tying the post to the brickwork and floor) are sound, save them to re-use in the new section.
Measure and cut the new piece of wood with a 45-degree angle at the top that matches the cut in the post. You should use treated timber to help prolong the life of the doorpost. If you can’t get pre-treated timber, apply preservative yourself. Drill a hole in the flat end of the new piece of timber at the same size as the metal dowel (make sure the hole is only deep enough to take half the dowel). If the original dowels are rusted or broken, you can cut new ones from a short length of galvanized steel pipe (available it DIY stores).
Once you have checked the new piece fits, apply primer and then exterior paint that closely matches the rest of the doorframe. Hammer nails along the inside edge of the timber to tie it to the wall. When the paint is dry, mix up some mortar (1 part cement, 3 parts sand) to bed the bottom dowel and nails into. When the post is in place, seal the joint with waterproof mastic sealer. Now you simply need to re-hang the door.