Wood rot in window frames: spot it, repair it or replace

Wood rot in window frames develops when water can soak into the timber for a long time, usually through damaged paint, an open joint or a poor sealant joint. Small spots can be repaired well: the rotten timber is routed out and filled with a wood repair compound or a new piece of timber. If the rot is deep or in several places, replacing part or all of the frame is often more economical in the long run. The sooner you spot wood rot, the smaller and cheaper the repair.

How do you recognise wood rot in window frames?

Wood rot often starts invisibly under the paint. The first signs are peeling paint, hairline cracks in the paint layer or dark discolouration. Press a screwdriver or bradawl into suspect spots: sound timber barely gives, rotten timber feels soft and spongy.

Look especially at the places where water lingers or drains away slowly.

• Sills and the bottom of the jambs
• Corner joints of frame and casement
• Around the glazing beads and sealant joints
• Spots where the paint has been peeling for a while

What causes wood rot?

Timber rots when it stays damp for a long time and fungi start to grow in it. An intact paint layer keeps water out. As soon as the paint cracks or a sealant joint comes loose, water soaks in and cannot get out again.

The construction plays a part too. A sill without a proper fall, a blocked drainage channel or a glazing bead that sits slightly wrong leaves water standing in exactly the wrong place. That is why we also look at the cause when repairing, otherwise it comes back.

Can wood rot in window frames be repaired?

With local rot, repair is perfectly possible. The affected timber is routed out down to sound wood, then the area is treated and filled with a two-component wood repair compound. After sanding and painting, the repair is barely visible.

If a larger section is affected, a new piece of timber is spliced in, for example part of the sill. That is skilled work, but the original frame is preserved. In older and listed buildings this is often preferred, see also renovating a listed building.

• Small spot: rout out and fill with repair compound
• Larger section: splice in a new piece of timber
• Always remove the cause of the moisture
• Prime and paint immediately after the repair

When is replacing frames better than repairing?

If the rot is in several places, in the joints or deep in the jambs, the repairs start to pile up. It is then often wiser to replace the frame. You immediately gain insulating glass and better draught-proofing.

When replacing, you choose again: timber, uPVC or aluminium. The differences are explained in timber or uPVC frames, and an indication of the cost in the cost of replacing frames. Our way of working is described under window and door frames.

How do you prevent wood rot in window frames?

Regular maintenance is the best protection. Check the frames once a year, for example in spring, and repair small damage to the paint straight away. Timber frames usually need a new coat of paint every six to eight years, sometimes sooner on the sunny and rainy side.

Combine the painting with other exterior work, such as replacing fascia boards, to save on scaffolding costs. Read more about periodic maintenance under maintenance and restoration.

• Check paint and sealant joints every year
• Touch up small damage straight away
• Keep the drainage of sills clear
• Repaint in good time, ideally before it peels

Frequently asked questions