Can you retrofit better glass into a rooflight?
Sometimes, and it depends on how the existing unit was built rather than on how good the new glass is. A roof lantern with beaded glazing bars will usually accept a new sealed unit. A factory glazed roof window can often take a manufacturer’s replacement pane for the same model. A bonded flat rooflight, where the glass is structurally adhered to an aluminium carrier frame, will not give the glass up at all.
So the first question is never which glass you want. It is what is holding the current glass in. Get that wrong and a day’s work turns into a new unit anyway, which is why the assessment happens before anything is ordered. The performance side of the decision is set out on energy efficient rooflights.
Three constructions, three different answers
Domestic rooflights are glazed in one of three ways, and identifying which one is looking at you takes a couple of minutes from below.
Beaded systems hold the unit against a rebate with a removable bead and a gasket, which is how nearly every roof lantern and most conservation style frames work. Factory glazed sashes have the unit set into a timber or composite sash with a clip-in cladding profile over it, which is the roof window arrangement. Bonded systems have the sealed unit adhered to an aluminium sub-frame with a structural silicone, so the glass and the frame are a single manufactured component. The first is straightforward, the second is possible with the right parts, the third is not a component you can open up on a roof.
Roof lanterns, where the glass genuinely comes out
A lantern is a framework of rafters and a ridge with individual units set into each facet. Each unit sits on setting blocks in a rebate, is held by a gasket and is retained by a bead that unclips or unscrews from inside.
That makes an upgrade real work but ordinary work. The units are measured facet by facet, ordered with the new make-up, and swapped one at a time with the frame supported and the roof kept covered between panels. The usual reason for doing it is a lantern over a south facing kitchen that was glazed clear and cooks the room from May onwards, where moving to a selective solar control unit changes the room without changing the structure. The constraint is that every facet has to be measured individually, because a lantern built on site rarely has four identical panels.
Roof windows, and the manufacturer pane route
The main manufacturers sell replacement panes for their own sashes, sold by model code and size code rather than by dimension. Where the existing window is a current or recent model and the code is legible on the data plate, a better glazing option for that same sash is often available: a laminated inner pane, an acoustic make-up, a solar control coating or in some ranges a triple unit.
The data plate is behind the top sash cover on most VELUX units and on the sash edge on Fakro and Keylite. It gives the model, the size code and the year. Without it, the pane cannot be ordered with confidence, because sash rebates changed between generations even where the external size did not.
The limit is age. Once a sash is beyond about fifteen years the pane is likely to be obsolete, and the timber and the gaskets are usually near the end of their working life anyway. At that point a whole new window into the existing opening is both cheaper and better, and it is dealt with on skylight replacement.
Bonded flat rooflights, and why they stay whole
A modern flush flat rooflight achieves its slim sightline by bonding the sealed unit directly onto an aluminium carrier with a structural silicone. The adhesive is doing a structural job, carrying wind uplift and the weight of the glass, and it is applied under factory conditions with controlled surface preparation and cure.
That bond cannot be reproduced on a wet roof in Essex in November, and cutting it destroys the carrier frame. The upgrade route for these is a new unit onto the existing kerb, which is frequently a single day’s work because the difficult part, the upstand and the covering termination, is already built. Where the kerb is sound and the height is right, the new unit is measured to the same external size and lands on the same detail.
Weight, thickness and what the rebate will take
| Make-up | Overall thickness | Weight per square metre |
|---|---|---|
| 4-16-4 double, clear | 24mm | About 20kg |
| 6-16-6.8 laminated double | 29mm | About 33kg |
| 8-16-8.8 acoustic laminated | 33mm | About 43kg |
| 4-12-4-12-4 triple, krypton | 36mm | About 30kg |
| Walk-on laminated build-up | 50mm and up | 100kg and up |
Two limits apply at once. The rebate depth has to accept the new thickness with enough edge cover, generally 12 to 15mm all round, and a bead designed for a 24mm unit will not close over a 33mm one. And the frame and the structure below have to carry the additional weight, which on a lantern with a long span or an older timber frame is a genuine question rather than a formality. A hoist or a glass robot may also be needed where a heavier unit has to reach a roof, and that changes the shape of the job.
Applied films, and what they honestly achieve
Solar control film applied to the room side of existing glass does reduce solar gain, and on a lantern that is unbearable in July it can take the edge off for a modest outlay.
The limits are worth stating plainly. Rejecting energy on the inside face means the heat has already entered the cavity, so a film performs well below a coating built into the outer pane. Film costs light transmittance at a worse ratio than a modern coating does, so the room reads darker for less thermal benefit. Applied to the inner pane of a sealed unit it raises the temperature of that pane and can increase thermal stress on the edge seal. And it does nothing at all for the U-value. It is a holding measure on a unit with years left, not an alternative to specification.
Adding a pane inside, and why gravity objects
Secondary glazing works in a wall because a vertical pane can hang in a frame and be sealed by its own weight against a stop. Overhead, the same idea has to resist gravity across its whole area, and any deflection opens the seal at the perimeter.
The practical version at ceiling level is a proprietary insulating panel set into the reveal, which is a lining rather than glazing and blocks the view of the sky. It has a place where a roof void is being converted and an old rooflight is being taken out of use, and almost no place where the point of the opening is daylight.
Comparing an upgrade against a new unit
The glass is rarely the expensive part. On a lantern the labour of removing beads, lifting units off a roof and setting new ones is the bulk of it, and the frame you keep is the part that was already compromising the performance. On a roof window the pane is a large fraction of the price of a complete new window that arrives with fresh gaskets, a current flashing kit and a full manufacturer’s declaration behind it.
The honest test is the age of what you are keeping. Under about eight years, upgrading the glass into a sound frame makes sense. Beyond fifteen, the money goes further into a new unit on the existing opening. Between those, it comes down to the condition of the frame and the gaskets, which is a survey question rather than a catalogue one.
Establishing what is possible on your roof
Four things settle it. The construction of the glazing, beaded, factory glazed or bonded. The model and size code from the data plate. The rebate depth measured at an edge. And the age of the frame.
Photographs of the unit from inside, of the data plate if it can be reached, and of the roof from outside are usually enough for a sensible answer by email. Where an upgrade is viable Sunspire quotes the new make-up with its U-value, its g-value and the plane the figure was declared in, so the improvement is a stated number rather than an expectation.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements