When the glass in a rooflight fails, the frame around it is often still sound. The timber is dry, the opener works and the flashing is tight. It is natural to ask whether you can pay for the glass alone. Sometimes you can, and the swap is small and tidy. Sometimes the frame is the real problem, and a new glass unit would be money spent on a window that is on its way out. This guide shows how to tell which you have.
What the glass part of a rooflight is
The glass in a modern skylight is a sealed unit: two or three panes held apart by a spacer, with a gas fill between them and a bonded seal round the edge. It is made to a size, in a factory, and it is not meant to be opened or topped up. Once the seal fails, damp air reaches the gap and the unit mists. We explain that process in our answer on why a rooflight goes misty between the panes.
That sealed unit is a separate component from the frame. In a roof window it sits in a sash, which is the part that opens or pivots. In a flat rooflight or roof lantern it sits in a perimeter frame or between glazing bars, held by gaskets and a pressure cap. Because it is a separate part, it can be ordered on its own and swapped. That is what makes glass-only replacement possible at all.
When glass-only replacement makes sense
We put forward a unit swap when four things line up.
The frame is sound. Timber that is dry, with no soft spots under a thumbnail, and paint or lacquer that has held. A pine frame in a bathroom is the one we look at hardest, because steam works on it from inside.
The opening mechanism works. Hinges, springs, the pivot and the locking handle all do their job. If the window drops or sticks, a new pane on a worn mechanism would not feel like a fix.
The flashing is tight. The flashing is the metal kit that joins the rooflight to the roof covering. If water is coming in at the edge, the glass is not the cause, and our page on telling misted glass from a leaking flashing goes through the signs.
A matching unit can be made. The maker's code on the frame, or the dimensions we take, lets us order a unit that fits the rebate and the gaskets.
When all four hold, a glass swap gives you a clear window, a fresh seal and a unit with a new gas fill, in the frame you already have. It is a modest job, in proportion to the problem.
When replacing the whole rooflight is the better answer
Sometimes the glass is the first thing to give out and not the last. A few signs point towards a full replacement.
- The timber frame has rot at the bottom corners, or the paint is lifting from damp.
- The opener has worn, or the seals round the sash have gone hard and no longer close tight.
- The flashing has lifted, cracked or corroded, or it was never the right kit for the roof covering.
- The maker has stopped producing that model, so no like-for-like unit exists.
- The original window is a type that cannot meet current energy limits with any glass in it.
The last point is the one people overlook. A new glass unit goes into an old frame, and the frame decides how much of the glass's performance the window delivers. A frame with a thin profile and no insulation round it will leak warmth at its edges however good the glass. In that case we say the unit swap is possible but not the best value, and we price the whole rooflight alongside it so you can compare.
The condition of the roof matters as well. If a window is being taken out anyway, the flashing round it is renewed with the new window, and the junction with the slates or tiles is rebuilt. Doing that once, rather than swapping glass now and the whole window in a few years, is often the more economical order.
Pitched roof windows and flat rooflights are different jobs
| Type | How the unit comes out | What it means for the work |
|---|---|---|
| Pitched roof window, centre-pivot or top-hung | The sash is released from the frame and the unit is taken out of the sash | Often done from inside the room, with the covering left as it is |
| Fixed pitched rooflight | The glazing is lifted out from the outside, past the glazing bead or cap | Roof access is needed, with the covering and flashing left in place |
| Flat rooflight on a kerb | The unit is unclamped and lifted from the frame above | Roof access and a crane or hoist for larger sizes |
| Roof lantern | Caps are removed along the bars and each sealed unit is lifted out in turn | Scaffold or tower access, and gaskets are renewed as each unit is bedded back |
On a pitched roof window, there is a real advantage for the owner. When the sash can be removed from inside, the roof covering and flashing round the window are left alone, and a replacement may be done without opening the roof. We think of this when we write the life plan for a new window: it is a reason to favour a model whose glass can be bought and swapped later without disturbing the slates.
A flat rooflight or lantern is different. The units are clamped from above, so access has to be arranged from above. That is not harder, but it asks for a safe working platform and sometimes a lifting device, and it may mean waiting for dry, still conditions so a roof is not left open to the weather.
Getting the right unit made
Ordering glass for a rooflight is not like ordering a pane for a window in a wall. The unit has to match the rebate to a few millimetres, the thickness of the old unit and the gasket system. Most manufactured roof windows carry a code, usually printed on a label inside the frame, and the size codes follow a pattern. Take MK04: M gives the width class and 04 the height class, so the code tells us the family of sizes before we measure.
We still measure. Frames have been painted and refitted, units have been swapped before, and a window that was changed years ago may not match the code on its label. We record the width, height and depth of the old unit, the position of the spacer and the type of glass in it.
Where the original maker's unit is available, we can order it. Where it is not, a unit built to the same dimensions and specification by a glass manufacturer is the like-for-like route. Either way, the unit we fit is ordered for your window, checked on delivery and fitted by us.
Choosing better glass when you are already there
The old unit is out. The frame is open. This is the cheapest moment to change the specification, because the labour of reaching the unit is already paid for. If the frame has room for it, there are three upgrades worth discussing.
A coated, gas-filled unit with a warm-edge spacer. Older units often have no coating and a metal spacer, which is a thermal bridge round the edge. A modern unit reduces heat loss and the cold line at the rim of the glass where condensation tends to settle. Our answer on upgrading the glass when a unit is replaced goes into the choices.
Laminated glass on the inner pane. Above a bed, a stair or a bath, a laminated inner pane holds together if it breaks. The outer pane is toughened to resist impact from hail and debris. It is a sensible safeguard where people sit or sleep below the glass.
A different pane count. Moving from double to triple is only possible if the frame is deep enough, and it adds weight to the opener. Our guide on double or triple glazed rooflights sets out when it is worth it.
A new unit in an existing frame raises the energy rules too. Whether a glass-only swap in a retained frame counts as controlled work is read differently by different building control bodies, so we check it for your job. The figures to aim at are those in Table 4.2 of Approved Document L volume 1: 1.4 W/m²K or WER Band B for a roof window, and 2.2 W/m²K for a rooflight. We say at the survey which way the swap falls.
What happens on the day
A glass-only swap on a pitched window is a short visit. We cover the floor and furniture in the room, release the sash, lift out the old unit and check the rebate and gaskets before the new one goes in. The new unit is bedded on fresh gaskets, the sash is refitted and the window is checked for smooth opening and a full close. On a flat rooflight or lantern the same steps happen from above.
We would rather tell you what we find at the opening than gloss over it. If the timber under the old unit is soft, or the bedding has let water through, the conversation about the whole rooflight happens then, with the frame in front of both of us.
How long a swap takes depends on the type, the size and the access. Our answer on how long a glass unit swap takes breaks that down. For a pitched window you do not usually need to move out of the room for longer than the work itself.
On a glass swap, the workmanship behind our 10-year guarantee is visible: the bedding, the gasket lines and the way the sash closes against its seal. If any of that does not look right, it is ours to put right. The unit itself carries the glass maker's own terms, and we give you that paperwork.
If the replacement counts as controlled work, we deal with Building Control. Whoever buys the house later may ask for that record with the rest of its paperwork, so it goes in your folder beside the quote and the life plan.
What shapes the price, and where we work
We do not quote from a photograph. Four things move the price of a glass swap: the size of the unit, the specification you choose, the access to the roof and whether the swap needs a lifting device. A larger lantern unit with laminated glass sits at a different level from a small roof window pane. The free survey gives a fixed, written quote, and the life plan in it names the unit and how it is replaced.
Our area is Sheffield and the 30 road miles around it, across South Yorkshire, north Derbyshire, north Nottinghamshire and the edges of West Yorkshire and the Peak. Stone terraces on the hills, 1930s semis in the suburbs and cottages at the Peak edge all have different roof structures, and we check the roof as well as the window.
Before you order a new unit
Can just the glass be replaced in a skylight?
Yes, where the frame and opener are sound and a matching sealed unit can be made. The old unit comes out, the rebate is checked and a new one is bedded on fresh gaskets.
How do I know if the frame is still good enough?
We press the timber at the bottom corners, look for lifting paint, test the opener and check the flashing. If any of these are failing, we say so at the survey and price a whole rooflight beside the glass swap.
Do I need scaffolding for a glass swap?
For many pitched roof windows the unit comes out from inside and no scaffold is needed. A flat rooflight, a fixed rooflight or a lantern is reached from above, so access equipment is part of the quote.
Will the new glass match my existing frame?
It is made to the measured size of the old unit, so it fits the rebate. For timber frames the new unit is a standard sealed unit, and its spacer colour and glass type can be chosen at survey.
Can the glass be changed to a better specification?
Yes, if the frame has the depth. A coated unit with a warm-edge spacer, or laminated inner glass over a bed or stair, are the common changes, and both are set out in the quote.