Where this starts
Timber takes up water through its END GRAIN many times faster than through its faces — the cut ends are open tubes. So on any piece of external joinery, the places that fail first are the places where end grain meets water, and they are entirely predictable: the bottom rail of a sash or a door, the ends of a cill, the bottom of a mitre, the joints where a rail meets a stile.
SO MAP IT BEFORE ANYTHING ELSE. Walk round and note which parts have failed. If it is the bottoms and the joints, water is getting into the timber and pushing the coating off from behind, and repainting without dealing with that is a one-season repair. If it is the whole face evenly, the coating itself has failed — chalked, embrittled, or applied over something it could not bond to.
PROBE THE TIMBER, do not just look at the paint. A screwdriver or a bradawl pushed gently into a failed area at a bottom rail tells you in a second whether there is sound wood under the paint or soft, punky material. Soft timber is decay and it does not stop because the paint is renewed; it has to be cut out or consolidated.
TWO THINGS ABOUT THE COATING ITSELF that produce the even-failure pattern. A very hard, impermeable film — traditional oil gloss — traps whatever moisture does get in, and the wood then pushes the film off in sheets. A micro-porous or flexible coating lets that moisture out, which is why modern systems on joinery are usually flexible, and why mixing the two is the failure people never connect: flexible paint over old hard gloss moves with the timber and the gloss underneath does not.
THE ORIENTATION MATTERS TOO, and it is free information. South and west elevations take the ultraviolet and the driving rain in most of Britain, and a coating that lasts ten years on a north elevation may last four on a south-west one. Failure on one elevation only is the weather, not the work.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Water into end grain at bottoms and joints
Most commonFailure starting at bottom rails, cill ends and the joints between members, with sound paint elsewhere on the same window, and often open joints visible.
How to check: Where it failed FIRST. Bottoms and joints is end grain drinking, and it is a timber problem before it is a paint one.
Decay already in the timber
Most commonPaint lifting over areas that are soft to a gentle probe, sometimes with the surface intact and the wood behind it gone, and a musty smell at the joint.
How to check: Push a bradawl into a failed area. Soft means decay, and a new coat over decay is a cosmetic delay.
An impermeable coating trapping moisture
CommonPaint lifting in sheets with clean timber behind it, on joinery finished in hard oil gloss, often with the film cracked and curling at the edges.
How to check: Whether the back of a lifted flake is clean. A film pushed off by moisture behind it comes away whole.
Incompatible systems over one another
CommonA newer flexible coating splitting or peeling from an older hard one still bonded beneath it, commonly where a house has been repainted without stripping.
How to check: How many layers are visible in a chip. A flexible film over a rigid one moves against it and loses.
Ultraviolet breakdown on an exposed elevation
CommonChalking, fading and general embrittlement on south and west elevations while other faces of the same building look sound.
How to check: Whether one elevation is far worse. The sun and the driving rain arrive from a direction, and the coating's life follows it.
Painted while the timber was wet
Less commonBlistering and early failure across recently painted work, sometimes with moisture visible inside a blister, often traceable to work done in damp weather or too soon after preparation.
How to check: The dates and the weather when it was done. Timber sealed with water in it pushes the coating off from inside.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Bottoms and joints is water into end grain; an even spread across faces is the coating. That one reading decides whether this is a timber job or a decorating one.
- 2
It separates sound timber under failed paint from decay under intact paint in seconds, and only one of those can be painted over.
- 3
Clean means the film was pushed off by moisture behind it; carrying fibres means the timber surface failed; dusty means it never bonded.
- 4
Failure concentrated on the south or west face is the exposure rather than the workmanship, and it changes what maintenance interval to expect rather than what to do differently.
- 5
A blocked drip groove, a cill without a fall, a putty line that has dropped, or a glazing bead holding water are all small defects that produce exactly this pattern.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a probe finds soft timber, since decay has to be cut out or consolidated and a new coating over it hides the extent rather than stopping it.
- Where the joinery is original to a period building, because replacement sections may need to match a profile and the repair is joinery rather than decorating.
- Where failure is at the glazing line on old windows and the putty has gone, which lets water straight into the rebate and rots the sash from inside.
- Where the work is at height on a sash window that has to come out, which is a different job from painting what can be reached from a ladder.
- Where the paint being disturbed is on a building old enough that earlier coatings may contain lead, in which case disturbing it is a controlled activity.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.