Why lower sections fail first
Sills, lower sash rails and frame corners receive the most rain and hold water longest when paint cracks. End grain at joints absorbs moisture quickly. A small surface blister can therefore sit above a much longer damp zone, while an ugly patch of peeling paint may still have sound timber beneath it.
We open and probe the affected area rather than drawing a neat repair line around the visible defect. The splice boundary belongs in dry, stable material with enough length to form a strong joint.
What a durable local repair needs
Decayed timber is removed, the remaining section is stabilised and a matching timber piece is shaped to restore the sill fall, weathering and moulding. Faces that will be hidden after assembly need protection before the new piece closes them in.
Putty, glazing seals, joints above and adjacent coatings are checked at the same time. Replacing wood without correcting the water entry simply creates a well-made patch in the same wet detail.
Signs that the repair scope is wider
If decay continues into both frame jambs, through structural joints or behind surrounding finishes, a small splice can become false economy. Movement, recurring leaks and previous soft repairs are other warnings. At that point a larger restoration or complete frame renewal should be priced as a genuine alternative.
The quotation should state what happens if opening up reveals additional decay. Clear rates or decision points are better than pretending concealed timber can be known perfectly before work starts.