Windows, doors and glazing
A timber sub-sill was left in place when the frame above it was replaced in plastic. What can still decay out of sight under the new sill?
The timber sub-sill left under the new plastic frame can still decay out of sight. Replacing the window does not lift out that old timber, so wet end grain or a soft patch can continue under the sill while the plastic looks sound. The survey can record that the sub-sill was left in place. It cannot certify timber it could not reach. A moisture meter, if one was used, is only an aid. The hidden strip is a reason to investigate the junction, not proof of rising damp.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A plastic frame can sit on the old timber rather than in place of it. The decay that matters is then behind the new sill, where a visual visit does not reach. Recording the leftover sub-sill is not the same as calling it dry, and it is not a diagnosis that moisture is rising through the masonry. The junction is simply still unopened.
Ask for that timber to be exposed before anyone treats the window as finished. A reading on the plastic, or on foil-backed board beside it, can mislead a pinless meter and would not prove rising damp. Salts or a bridged course, if they are present, are separate observations and only reasons to investigate the junction further. Do not guess a height for the damp.
What should you ask your surveyor?
- Could you see or probe the timber sub-sill, or only the new plastic frame above it?
- Was any soft timber, staining or open joint visible at the ends of the sill?
- Are you recording a hidden sub-sill, rather than diagnosing rising damp in the wall?
What are the typical next steps?
- Ask for the old sub-sill to be opened up before you accept the window as sound.
- Do not treat a new plastic frame as proof the timber beneath it is dry.
- Keep any meter reading as an aid, not as a diagnosis of the wall.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If the plastic sill looks new, is the timber underneath cleared?
No. A new frame can hide the old sub-sill completely. The survey has not seen that timber, so it cannot call the junction sound.
Does a high reading on the board beside the window prove rising damp?
No. A moisture meter is only an aid, and foil-backed board can mislead a pinless meter. The reading is not a diagnosis of the wall.