Windows rarely fail all at once. They deteriorate quietly over years, and by the time you notice the draught or the damp sill you have usually been living with a problem for a while. The difficult part is judging whether the answer is a £90 repair or a £6,000 house.
This guide gives you a checklist you can work through yourself, and it is deliberately honest about the symptoms that do not justify replacement.
Ten signs replacement is genuinely warranted
- Timber frames that are soft when you push a key into the bottom rail — that is rot, and it spreads.
- uPVC frames where the welded corners have opened up or the profile has bowed.
- Windows you cannot open at all, or that need force to close.
- Single glazing, where the cold radiating off the glass makes the room by the window unusable in winter.
- Frames that leak water inside during driving rain despite sound external sealant.
- Locks that no longer engage and for which no parts are available.
- Damp or mould consistently around the reveal in more than one room.
- Condensation between the panes across most of the house rather than one or two units.
- Frames so distorted that a sash cannot be adjusted square.
- Obsolete profile systems where hinges, stays and gaskets can no longer be sourced.
Five signs that do not justify replacement
- A single misted unit — replace the glass, keep the frame.
- One draughty window — usually a perished gasket or a dropped sash.
- Grubby or discoloured white uPVC — a uPVC cream cleaner will surprise you.
- Condensation on the inside of the glass — that is humidity, not window failure.
- Simply looking dated. Sound windows that you dislike are a decorating decision, not a repair.
Repair or replace, by cost
| Problem | Repair cost | Replacement cost | Usually best |
|---|---|---|---|
| Misted sealed unit | £95 – £260 | £450 – £900 | Repair |
| Perished gasket | £40 – £80 | £450 – £900 | Repair |
| Broken friction stay | £70 – £130 | £450 – £900 | Repair |
| Failed window lock | £70 – £150 | £450 – £900 | Repair |
| Rotten timber sub-frame | £200 – £600 patch | £550 – £1,000 | Usually replace |
| Bowed or split uPVC frame | Not economic | £450 – £900 | Replace |
| Whole house of 1980s units | £1,200+ in patches | £4,400 – £7,500 | Replace |
A room-by-room inspection
From inside
Open and close every sash. Check the handle operates with light pressure. Press each gasket for softness. Look at the corners of the reveal for mould, and run your hand around the perimeter on a windy day for draughts. Note any glass with misting or a permanent tide mark.
From outside
Look along the frames for bowing, check the sealant at the reveals, and look for green algae growth sitting in the drainage slots. On timber, check the bottom rails and the joints at each corner. On stone or render, check the lintel above for cracking.
The order to do things in
If the survey throws up a mixture, deal with anything affecting security or water ingress first, then anything affecting the fabric of the building, then comfort, then appearance. That sequence keeps money going to the things that would otherwise get more expensive.
It is also perfectly reasonable to replace the worst elevation now and the rest in a year or two. Just record the profile system and colour so the second phase matches.
When to get a professional opinion
Get someone out if you find rot, if water is getting in, if a lintel looks cracked, or if you simply cannot tell whether a frame is sound. A survey is free from most installers and takes twenty minutes.
We are based in Cumbernauld and cover Central Scotland, and we will tell you plainly if the answer is a glass unit rather than a window — that is where 'replace the pane, not the frame' comes from.
Common questions
Frames typically 25–35 years in uPVC; sealed glass units 15–20 years. Units usually fail before frames, which is why glass-only replacement is so common.
Want a straight answer on your own windows or doors?
DM Installations fits and repairs windows and doors across Central Scotland — Cumbernauld, Glasgow, Falkirk, Stirling and the surrounding towns. Free survey, written quote, and an honest answer on whether you need a repair or a replacement.
Related guides
Misted Double Glazing Repair Cost
Failed units are usually a glass problem, not a window problem. Here is what replacing the pane costs against replacing the whole thing.
How-toHow to Fix a Draughty Window
Track down where the cold air is really coming from and fix it for under £30 in most cases.
Cost guidesBudgeting for a Whole House of New Windows
A practical method for costing a full re-glaze, deciding whether to phase it, and avoiding the finance traps that make it cost more.