A soft patch on a window sill does not automatically mean a new window. On many older houses, the rot is confined to the parts that sit wettest, the sill and the bottom of the frame or sash, and the rest of the window is sound timber that will last for years once the bad section is replaced.
Here is how to judge which way to go, and what to check before you commit to either.
Where windows rot first
Water runs down a window and sits wherever it can. That makes the sill, the bottom rail of each sash or casement, and the lower ends of the frame the first places to go. Joints are another weak point, because once the paint cracks over a joint, water gets in and cannot get out.
Autumn is a good time to look, before the worst of the weather. Press gently with a screwdriver. Sound timber resists. Rotten timber feels soft or spongy and may crumble.
Wet rot or dry rot?
Most rot in windows is wet rot. It needs timber that stays very wet, so it follows the water. Timber specialists describe it as soft, spongy and discoloured, often cracking and crumbling once it dries. Take away the source of the water and it stops spreading.
Dry rot is less common and more serious. It is usually found in damp, unventilated places inside the building, often shows white strands or sheets of growth, and can spread through masonry. If you suspect it, get a specialist survey before any carpentry starts.
When repair makes sense
If the rot is limited to the sill, the bottom rail or the foot of the frame, a repair is usually the better choice. The rotten section is cut out back to sound timber and a new piece is made and jointed in, matched to the original profile.
This is joinery rather than patching. Filler on an exterior frame tends to crack away from the timber around it as the wood moves, letting water back in behind it, so it is a repair you end up doing twice. A properly jointed new section, primed and painted, behaves like the rest of the window.
Repairing also keeps the original glass, mouldings and proportions, which matters on period houses where the windows are a large part of the character. You can read more about matching original profiles in our page on windows and period joinery.
When replacement makes sense
Replacement is worth considering when:
- Rot runs through several joints, or both sides of the frame.
- The sashes or casements are badly distorted and no longer close.
- Earlier repairs have used so much filler that there is little original timber left.
- The window was poorly made in the first place.
Rules worth knowing before you replace
The government's Planning Portal says repairs and maintenance generally do not need planning permission, and nor do new windows of a similar appearance to the originals on a house. A new bay window is treated differently, as an extension.
Building Regulations are a separate matter. The Planning Portal says they normally apply when the whole fixed frame and opening parts are replaced, and that new windows must meet energy efficiency standards. Either the installer is registered with a Competent Person Scheme, or building control approval is needed. Minor repairs, such as replacing broken glass, are normally outside the regulations.
Listed buildings and conservation areas
If your house is listed, the Planning Portal says listed building consent is needed for any significant works, inside or out, and you should speak to the council before starting. Some councils also use Article 4 Directions in conservation areas to remove the usual permitted development rights, so check with Waverley Borough Council, Guildford Borough Council or Horsham District Council, depending on where you are, before changing the look of any window.
Stopping it happening again
Most window rot starts with failed paint. Keeping the paint and putty in good condition, clearing out drip grooves and making sure sills shed water away from the wall all help. Look at the windows each autumn, and deal with small cracks before winter rather than after.
If one window has rotted, check its neighbours on the same elevation. Windows facing the prevailing weather usually go first, and the rest are rarely far behind. The same is true of external doors, which we cover in why doors stick in winter.
Need a carpenter?
LGR Carpentry and Joinery makes and repairs timber windows on older houses across Surrey and Sussex, including Godalming and Guildford. Rotten sections are cut out and replaced with new timber matched to the original, not filled over.
Read more about windows & period joinery, made-to-measure joinery and general carpentry repairs, or see the work Luke does in Godalming and Guildford. You can also ask for a free quote.