How do I repair a rotten door frame at the bottom?

Frame legs rot from the bottom because that is where water sits and where end grain is exposed. The repair is straightforward; the mistake is repairing it without stopping the water.

Safety

What you'll need

• A bradawl or screwdriver for probing
• A handsaw or multi-tool
• Treated timber to match the section
• Exterior wood adhesive and stainless screws
• Wood preservative and primer
• Two-part wood repair resin, for smaller areas

Step by step

1. Find the extent of the decay.Push a bradawl into the timber every 25mm up the leg. Sound timber resists; rotten timber takes the point with no effort. Mark where it becomes firm and go 50mm beyond it.
2. Find the water source first.A blocked gutter, a raised path bridging the damp course, a failed threshold, splashback from a hard surface, or a sill that no longer throws water clear. Repairing without fixing this is temporary.
3. Decide between resin and a splice.Up to about 50mm deep and a small area, a two-part epoxy repair with a hardener soaked in first works well. Anything larger, or any loss of structural section, needs new timber.
4. Cut out square.A splayed scarf joint sheds water better than a square butt, but a square cut is easier and acceptable if the joint is well sealed. Cut into sound timber, not to the edge of the rot.
5. Support the frame while you cut.The leg may be carrying nothing, or it may be helping hold the frame square. Wedge the head and check the door still fits before you commit.
6. Cut the new piece slightly oversize.Treated timber, planed to the frame section, and long enough to trim flush. Prime or preserve all faces of the new piece, especially the end grain, before it goes in.
7. Glue and screw the splice.Exterior-grade adhesive on the joint faces, stainless steel screws set below the surface. Never plain steel — it stains through paint and corrodes at the joint.
8. Keep the new timber clear of the ground.If the leg meets a path or step, seal the end grain and, if possible, leave a small gap or set the leg on a damp-proof pad rather than direct on wet masonry.
9. Fill, sand and prime.Two-part filler for the screw heads and the joint line, sanded flush. Prime the whole repair including the joint, then undercoat and topcoat.
10. Reseal the junction with the wall.A flexible exterior sealant between frame and masonry, not rigid filler, so seasonal movement does not open the joint again.

Tips

• Wood hardener before filler is what makes an epoxy repair last. It soaks into the softened fibres and gives the filler something solid to bond to.
• Take the door off before you cut. It gives you room, and it stops the leaf being damaged if the frame moves.
• Paint the end grain of the new piece three times before fitting. End grain absorbs many times faster than the face, and that is where the next rot starts.

Common mistakes

• Filling rotten timber rather than removing it — Filler over decayed wood traps moisture against it and speeds the decay up. The rot continues under a hard shell, and by the time it shows the repairable section has gone.
• Painting the repair before it has dried — Paint over damp timber blisters and peels within a season, and seals the moisture in. The frame needs a spell of dry weather, or a fan heater, before priming.

If it doesn't work

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