The short answer
Replacing an old skylight is worth it when the unit is misted, draughty, stained round the opening or single glazed, because a new one gives back warmth, dryness and a quieter room. It is not worth rushing when the glass is clear, the seals grip and the ceiling is dry. Judge the unit on its condition and on what the room needs, not on its age.
What a new unit gives back
Three things change when an old rooflight comes out: how warm the room is, whether the opening stays dry, and how much of the weather you hear.
Start with warmth. An old single glazed dome or an early sealed unit sits in the roof as the thinnest, coldest surface in the room. In winter it draws heat out and chills the air beneath it, and in summer it lets heat in just as freely. A current unit carries a coated, gas-filled cavity and a better spacer, and it is fitted to meet Building Regulations Part L on thermal performance. The room stops feeling as though a cold window is hanging over the bed. The same change calms the cold spot on the ceiling where damp air meets cold glass.
Next, dryness. A worn gasket or a tired flashing is where water first finds its way in. A replacement renews the unit, the seal round it and the dressing to the roof in one job, so the weak points are replaced together instead of one at a time. If there is a stain on the ceiling by the opening, that is usually the point where replacement stops being optional.
Last, quiet. Rain, hail and aircraft are all heard through the roof first. A thin single pane passes every bit of it. A build with a laminated acoustic inner pane, a thicker outer pane and gaskets that press firmly all the way round changes the room in a way you notice at once. That choice is written into the quiet spec that comes with every quote, so you can see what each part is for before you agree.
When the old unit still has years in it
Replacement is not the answer to every old rooflight. Some are in good order and should be left alone.
A glass unit with clear panes, no mist between them and gaskets that still spring back has not let go. If the ceiling beneath it has never marked, the flashing is doing its job. A unit like that may be a decade or more past its first day and still be sound. Old does not mean failed.
A few questions sort the unit worth keeping from the one that is not:
- Is there mist or film between the panes that does not clear on a dry day?
- Does the frame let in draught, or rattle when the wind rises?
- Is there a stain, bubble or mark on the ceiling or reveal beside the opening?
- Is the pane a single sheet or a plastic dome?
- Is the room colder, louder or duller than it used to be?
A clean run of five noes says leave it. One yes deserves a closer look. Two or more means the unit has done its work. Our page on signs a rooflight has failed explains how to tell each of these apart, and how long a rooflight lasts covers the ageing behind them.
Weighing cost against what you keep paying for
The price of a new unit is easy to see. The cost of keeping a tired one is spread thin, so it is easy to miss.
A misted or draughty rooflight costs a little every winter in heat that leaves the room. A stained ceiling costs a redecoration, and damp left beside a timber upstand can reach the structure. A plastic dome that has crazed leaves a room needing the light on in the afternoon. None of these shows on an invoice, so the old unit looks free.
Timing also affects the sum. If you are already planning to redecorate, extend or re-roof, replacing the rooflight at the same time saves setting up twice. If the roof covering is nearly due to be renewed, the flashing has to be lifted in any case, and the new unit goes in at that point. The general cost drivers are set out in our page on what affects the cost of replacing a skylight, and the rooflight and skylight costs page gives general market ranges.
| Condition | Sensible move |
|---|---|
| Clear glass, dry ceiling | Keep, and look again in a year |
| Mist between panes | Replace the unit |
| Draught or rattle | Replace the unit or whole rooflight |
| Stain beside the opening | Replace unit and flashing together |
| Cloudy plastic dome | Replace with glazed unit |
Like for like, or a better unit
Once replacement makes sense, the next question is what goes in. The answer is not always the same unit again.
A like-for-like swap keeps the opening, the frame and the look, and it is the simplest route if the frame is sound. An upgrade changes the glass build, adds an opening or electric sash, or swaps a dome for a glazed unit. The replacing an old skylight page goes through both options. If the old unit is misted or failed, the work comes under misted and failed rooflight replacement, and if comfort is the aim, energy efficient rooflights cover the glazing choices.
Replacing a unit in its existing opening is also usually a shorter job than a new opening, and it does not call for a new hole in the roof. We confirm what the work involves at the survey.
Smaller questions before you decide
Will a new skylight save money on heating?
A warmer unit loses less heat, so the room needs less to stay comfortable. How much that is worth depends on the room and the heating, so we do not put a figure on it.
Can I replace just one of several rooflights?
Yes. If only one has failed, replace it alone. If they were all fitted together and are all tired, our answer on whether to replace all your rooflights at once helps you choose.
Does a newer rooflight add value to the house?
A dry, bright, quiet room appeals to buyers, but the gain depends on the house. We look at this in do skylights add value to a house.
Is there a wrong time of year to replace one?
Any dry spell will do for the roof side, and the room is covered while the unit is out. Our page on the best time of year to fit a skylight has the detail.
If your rooflight sits in the grey area between keep and replace, get a quote and we will tell you which side it falls on. The full service is on our skylight and rooflight installation page.