Soft, rotten window frame: can it still be repaired?

What softness actually means

Wood goes soft when it has held moisture for weeks or months, long enough for fungus to start breaking down the fibres. Paint film often hides this for a long time, so the outside can look intact while the wood underneath has already lost its structure. By the time you can press a finger in, the damage has been developing for a while.

Softness on its own does not tell you the extent. A patch the size of a coin can mean rot that has spread along the whole sill, or it can mean a small pocket where water sat after a paint crack let it in. You need to find the edges of the damage before deciding anything.

The screwdriver test: how to measure the extent of the damage

Take a flat-blade screwdriver or a bradawl and press it into the wood at an angle, starting at the soft spot and working outward in small steps. Sound wood resists and the tool leaves a small dent. Rotten wood lets the blade sink in with little resistance, sometimes crumbling or coming away in fibres rather than a clean shaving.

Map the boundary this way in several directions: along the sill, up the jamb, into the corner joints where sashes meet. Corners and the underside of the sill are where rot usually runs furthest, because water collects there and dries slowest. Note how deep the soft wood goes, not just how wide it is on the surface.

If the blade sinks in cleanly for more than a centimetre or two, or if the soft zone crosses a joint between two pieces of timber, treat that as significant. A shallow, contained patch is a different matter to rot that has travelled into the frame's structure.

• Probe with a screwdriver or bradawl at a low angle, not straight in
• Test every few centimetres outward until you hit sound, resistant wood
• Pay particular attention to corner joints and the underside of the sill
• Record how deep as well as how wide the soft area is
• Check the matching point on the opposite side of the window for comparison

When a localised repair is enough

If the rot is shallow, sits within one section of timber, and the surrounding wood tests sound all the way round, cutting out the damaged section and replacing it with a timber splice or filling it with a two-part epoxy wood filler usually holds up well. This is standard practice on older wooden windows and, done properly, is not a compromise.

The wood needs to be fully dry before any repair goes in, which can mean opening the area up and leaving it to dry for some days first, sometimes with gentle heat. Filling wet rot only traps the moisture and the problem returns within a season.

This kind of repair, along with repainting and resealing the joint, is the sort of job we quote as odd jobs from 39 € an hour, and it is often the more sensible route if the frame is otherwise sound and the joinery still closes properly.

When replacement is the only sound option

If the screwdriver sinks in across a wide area, along more than one side of the frame, or through a structural joint, a local patch will not hold. Filler and splices need sound timber around them to key into; there is no way to anchor a repair inside wood that has no strength left.

The other sign is a sash or frame that has already started to distort, where the window sticks, will not close square, or has visibly dropped. That means the timber has lost enough integrity to move, and no filler restores that.

In these cases a full window fitting is the straightforward answer rather than a repeated round of patching. We quote window fitting and servicing from 90 € per job once we have seen the frame or photos of it, and that figure holds for 30 days.

Where the moisture usually comes from, and why it keeps coming back

The most common source is a failed paint seal, particularly on the sill where rain sits and UV breaks the paint down fastest. Condensation from the inside is the other frequent cause, especially in older flats with single glazing or in rooms with poor ventilation, where moisture collects on the glass and runs down onto the frame.

A third source is a sealant joint between frame and wall that has cracked or pulled away, letting water track in from outside the visible frame surface entirely. This is easy to miss because the rot shows up on the frame while the actual entry point is in the render or cladding nearby.

If rot returns after a repair, the fault is almost always the moisture source, not the filler. Repainting bare wood every few years, keeping window vents or trickle vents clear, and checking exterior sealant annually are what stop the same spot rotting out again.