Why does my exterior woodwork always fail at the bottom?

Timber is a bundle of tubes that once carried sap up a tree. Along the face of a board those tubes run sideways and their walls resist water well, which is why a painted face lasts for years. Cut across them and they are open at the end, and water is drawn in like liquid into a bundle of straws. Every failure of exterior joinery in Britain traces back to that fact and to where gravity puts the water.

Safety

What you'll need

• A scraper, and wet abrasive or a chemical stripper
• A moisture meter, or at least a bradawl to probe for soft timber
• Wood preservative and a suitable primer
• A two-part exterior wood filler, for any decayed sections
• A flexible exterior filler or sealant, for the joint between frame and masonry

Step by step

1. Understand where the end grain is.The bottom of each stile where it meets the sill, both ends of the bottom rail, every mortice-and-tenon or dowelled joint, the cut ends of glazing beads and mouldings, and any hole drilled for a screw or a fixing. Those are the places water gets in, and they are the places to inspect and to over-coat.
2. Probe before you decorate.Push a bradawl gently into the bottom rail, the joints and the base of each stile. Sound timber resists; decayed timber goes in easily and feels soft or spongy. Painting over rot hides it while it spreads, and the repair a year later is far larger.
3. Find out where the water is coming from.Check the drip groove under the sill is clear and not bridged by paint or filler, that the sill actually throws water clear of the frame, that the joint between frame and masonry is sound, and that nothing above — a blocked gutter, a failed seal, a slipped tile — is running water down onto the joinery.
4. Cut out and repair decay properly.Rot has to be removed back to sound timber, not filled over. Small areas take a two-part exterior wood filler after a hardener; larger ones need a spliced-in timber repair. If a bottom rail is largely gone, the honest answer is a new rail or a new sash.
5. Treat and prime the end grain heavily.Apply preservative and let it soak in, then prime — and keep applying primer to the end grain and the joints until it stops being absorbed. It will take several times what the face takes. This is the single most useful thing you can do to extend the life of the joinery.
6. Build the coats and pay attention to the joints.Undercoat and topcoat as your system requires, with an extra coat on every end grain and every joint. Carry the paint film a couple of millimetres onto the glass at the putty line, and onto the sill where the stile lands, so water cannot get into the junction.
7. Do not seal the frame airtight.A frame that has water in it needs a way out. Sealing every gap with a flexible sealant, including the ones designed to drain, traps moisture in the timber. Seal where water gets in from outside; leave the drainage paths open.
8. Set an inspection habit.Every spring, walk the outside and look specifically at the bottom of every frame, every joint and every sill. Touch in any crack or chip in the film immediately, because a break in the coating over end grain is a direct route in.

Tips

• Painting end grain before assembly is why factory-finished joinery lasts. On any new timber you are making up yourself, prime the ends before you put it together.
• South and west elevations get the weather and the UV. Inspect those first and expect to maintain them more often than the sheltered sides.
• A microporous exterior paint system lets a frame dry outwards while shedding rain, which is a genuine advantage on timber that has ever been wet.

Common mistakes

• Painting over soft timber at the bottom of a frame — It hides active decay while it continues. A year later the repair is a new rail rather than a filled patch.
• Filling the drip groove under a sill with paint or filler — The groove exists to make water drop off rather than track back along the underside into the frame. Bridging it defeats the sill.
• Sealing all round a frame with a flexible sealant — Water that does get in then cannot get out. Seal the head and the sides, leave the designed drainage paths clear.

If it doesn't work

Keeping on top of it

• Walk round and inspect the bottom rails, joints and sills of all external joinery (Every spring)
• Touch in any chip or crack in the paint film immediately, especially over end grain (As soon as found)
• Clear drip grooves and check that sills still throw water clear (Annually)
• Full repaint of exposed elevations (As the coating tells you, sooner on south and west faces)

Questions people ask