Replacing a Cracked Rooflight

Chapter 1

First, work out which pane has cracked

A cracked rooflight is rarely one piece of glass. Knowing whether the crack is in the outer pane, the inner pane or a single sheet of older glass tells you how the unit will behave while you wait, and what cracked rooflight glass replacement will involve.

A modern double glazed flat rooflight or roof window is a sealed unit of two panes with a spacer and a gas-filled cavity between them. The outer pane, the one the weather sees, is usually toughened. Toughened glass is heat treated so that it is several times stronger than ordinary float glass, and when it does fail it breaks into small, blunt, cube-like pieces rather than long shards. The inner pane, the one over your head, is usually laminated: two thin sheets bonded to a plastic interlayer, so that if it cracks the pieces stay stuck to the interlayer and the pane stays in the frame. Approved Document K expects overhead glazing to behave safely if it breaks, and that laminated inner pane is how quality units meet it.

You can often tell which pane has gone from inside the room. Shine a torch across the glass at a low angle. A crack in the inner pane catches the light at the surface you can touch, and you may feel a faint ridge along it. A crack in the outer pane sits behind the cavity, so it looks slightly deeper, and toughened glass that has broken shows a fine all-over crazing, like frosted ice, rather than a single line.

Older rooflights change the picture. Single glazed wired glass in 1960s and 1970s extensions cracks in a line and is held loosely by the mesh. Polycarbonate domes and sheets do not crack in the same way; they craze, yellow and go brittle. If yours is wired glass, the guide to replacing wired glass rooflights covers that upgrade in more detail. This guide is about glass skylights that have cracked.

Chapter 2

Keeping the room below sensible while you wait

A cracked sealed unit with a laminated inner pane is designed to stay in place, so the room below does not need to be abandoned. A few plain precautions keep everyone comfortable until the new glass or unit is fitted.

• Keep the area directly under the skylight clear of seating, a cot or a desk that someone uses for hours at a time. This is caution rather than alarm.
• Leave the glass alone. Do not press on a crazed outer pane from a ladder on the roof, and do not tap an inner crack to see if it spreads.
• Stay off the roof. Flat roofs over kitchen extensions look easy to walk on, but the rooflight kerb, the upstand and the roof edge are where people slip.
• Lift any blind or film away from the glass if it presses against it, so the pane is not heated unevenly on the next sunny day.
• Take clear photographs from inside, and from a safe window if you can see the roof, before anything changes.

If the crack is in an older single sheet of annealed glass, or pieces have already come away, treat it differently: keep the room below closed off and say so when you get in touch, so the survey can be arranged around getting that glass out and a new unit in. Our answer on whether misted or cracked rooflight glass is dangerous sets out how each glass type behaves.

A cracked outer pane also means the seal has lost its integrity. Within weeks the cavity usually starts to mist, and the unit loses much of its insulating value. That does not make it unsafe, but it is a clear sign that a like-for-like wait of many months is not worth it.

Chapter 3

What cracks rooflight glass in the first place

Knowing the cause matters because the replacement should not repeat it. There are five usual culprits, and each points to a slightly different specification for the new glass.

Impact. Hail, a falling branch from a garden tree, a dropped tool during other roof work or a foot on a unit that was never meant to be walked on. Flat rooflights over single storey extensions in gardens around Chesterton and Newnham, where mature trees overhang the roof, are more exposed to this than a roof window high on a main roof.

Thermal stress. Glass cracks when one part of a pane is much hotter than another. The centre of a pane in full sun expands; the edge, shaded by the frame, stays cooler. A dark blind or tinted film fixed tight against the glass makes this far worse, and so does partial shade from a chimney or a parapet that falls across the pane through the afternoon. A thermal crack usually starts at the edge, runs in at right angles to it and then wanders. Annealed and laminated glass are more vulnerable than toughened glass.

Spontaneous breakage of toughened glass. Toughened glass can occasionally shatter with no obvious cause because of a tiny nickel sulphide inclusion formed when the glass was made. The inclusion slowly changes size and, years later, can break the pane. Heat soak testing during manufacture reduces the risk, and it is worth asking for on a large pane.

Frame and building movement. An old timber kerb that has softened, a frame screwed down unevenly, or glass seated without the right setting blocks can put a point load on the edge of a pane. That shows up as a crack that starts at a corner.

Age. Older units were often glazed with ordinary annealed glass on both sides. It is weaker, it breaks into sharp pieces, and it does not meet current expectations for glass overhead.

Chapter 4

New glass in the old frame, or a whole new unit

A cracked pane does not automatically mean a whole new rooflight. The decision turns on the frame, its age and whether a correct replacement glass unit can still be made for it.

If the frame is modern, sound and thermally broken, and the manufacturer or a glass processor can supply a sealed unit of the right size, thickness and edge detail, then replacing the glass alone can be the sensible choice. A pane in a lantern can often be changed on its own too, which is covered in our answer on replacing a single roof lantern glass panel. On a frameless flat rooflight, where the glass is bonded to the kerb with structural silicone, the glass and the unit are essentially the same thing, so the whole glazed top is renewed.

A new unit makes more sense when the frame has no thermal break, when the model is discontinued, when the old glass was annealed, or when the rooflight is old enough that the seal on the uncracked pane is also near the end of its life. The fuller comparison sits in glass only or full unit replacement, and the short version of that question is answered in can you replace just the glass in a rooflight?

One more thing to weigh: a crack is a forced moment to decide. If the skylight was never in quite the right place, too small for the room or a source of summer glare, replacing it rather than reglazing it lets you change that at the same time.

Chapter 5

Choosing the glass so it suits the slope

The replacement glass is a chance to specify properly for the light, heat and safety you want in the room, rather than simply matching what was there.

The safety layers are the fixed part of the specification. A toughened outer pane, heat soaked on a large unit, and a laminated inner pane are the baseline for any glass above people. Current Building Regulations under Part L set a limiting U-value of 2.2 W/m²K for rooflights, now measured in the horizontal plane, so even a like-for-like replacement should come out warmer than a unit fitted fifteen or twenty years ago. A low-emissivity coating, an argon-filled cavity and a warm edge spacer are how a double glazed unit gets there; triple glazing goes further on a large pane or a north slope where winter heat loss is the bigger concern.

The rest of the choice follows the orientation. Cambridge sits on flat land under a wide sky, so there is little to shade a roof, and the direction the glass faces decides how the room will feel.

If the old pane cracked from thermal stress on a south or west slope, the replacement should reduce the heat the glass absorbs rather than simply copy it. A solar control outer pane rejects more of the sun's energy at the glass surface, and an external awning or a blind with a gap behind it keeps the inner pane cooler. An acoustic laminated interlayer quietens rain noise on a flat roof, which is worth having over a bedroom or a study. A self-cleaning coating on the outer pane helps on a low-pitch flat unit where rainwater runs off slowly.

Chapter 6

Upgrading the unit, not only the glass

When the whole rooflight is being renewed, the choice of unit matters as much as the glass. A cracked pane is a good reason to fit a better skylight rather than a copy of the old one.

On a flat roof, the modern choice is between a framed unit with a thermally broken aluminium frame and a drained glazing detail, and a frameless unit with the glass bonded to the kerb for the cleanest view of the sky. Both sit on an upstand at least 150mm above the finished roof surface, and the glass should fall at around five degrees so that rain runs off. Many older flat rooflights were set on low timber kerbs with barely any fall, which is one reason water sat on the glass and dirt dried in lines. Replacing the unit is the moment to raise the upstand and build in that fall.

On a pitched roof, a new roof window can usually go back into the same opening, and on older VELUX windows a replacement flashing kit is made to keep the install depth without disturbing the lining. A centre pivot window lets furniture sit beneath it; a top hung one gives a clearer view out and suits a loft room where you want to lean out.

Opening rooflights deserve a thought too. If the old unit was fixed and the room below is a kitchen or a bathroom, an electric opening unit with a rain sensor lets warm, damp air escape from the highest point of the ceiling. If the old unit was an opener that nobody used, a fixed unit is simpler and slightly better insulated.

Size and position can change at the same time, within limits set by the roof structure and, where it applies, by planning. Our answer on whether a replacement rooflight has to be the same size explains what each option involves.

Chapter 7

Warranty, insurance and Building Control

Three pieces of paperwork are worth checking before the replacement is ordered. None of them should hold the work up for long, but each can change who pays or what the new unit has to achieve.

The manufacturer's warranty. If the rooflight is only a few years old, look for its product warranty. Glass warranties usually cover manufacturing faults such as seal failure within the stated period; they rarely cover impact, and they often exclude glass that has had film or tint applied afterwards. Spontaneous breakage of toughened glass is sometimes treated as a manufacturing fault. Our answer on manufacturer warranties for rooflights goes through what to look for.

Home insurance. Accidental damage to glass, including a pane cracked by hail or a falling branch, is often covered under buildings insurance, sometimes only with an accidental damage add-on. Gradual seal failure usually is not. Photograph the crack, note when you first saw it and call your insurer before the old glass is removed. The answer on whether home insurance covers rooflights has more on how policies tend to treat each case.

Building Control. Replacing only the glass in an existing frame is generally not notifiable. Replacing the whole rooflight usually is, because the new unit has to meet current Part L thermal standards. Where the work is notifiable, we handle the Building Control notification as part of the job, and the paperwork for the house comes to you at the end. The details are in Building Control for replacement rooflights. In a conservation area or on a listed building, a like-for-like replacement is usually straightforward, but any change in size, profile or position should be checked with Greater Cambridge Shared Planning or the relevant district council first.

Chapter 8

How the replacement goes on the day

Most of the thinking happens at the survey. The day itself is about getting the cracked glass out cleanly and the new glass or unit in without disturbing more of the room than necessary.

1. Survey and light plan. We measure the opening, identify the frame and glass, look at the kerb or flashing, check the slope and the orientation, and see how the room is used. The written quote comes with a short light plan: the glass, the unit, any change of size or position, and the reason for each.
2. Ordering. Sealed units and rooflights are made to order, so there is a lead time between the survey and the fitting date. A standard roof window size is usually quicker than a custom flat unit or a lantern.
3. Access. A roof window on a two storey house or a large flat unit may need a tower or scaffolding; a rooflight on a single storey extension often does not. Our answer on whether replacing a rooflight needs scaffolding explains how that is decided.
4. Removal. The room below is sheeted, and the cracked pane or the whole unit is taken out from the roof side where possible, with a laminated inner pane held on its interlayer until it is lifted clear.
5. Fitting. The new glass or unit goes in, the kerb or flashing is made weathertight to the roof covering, and the internal lining is finished and made good.
6. Clearing up. The old glass and frame are taken away for disposal, with glass and aluminium sent for recycling where the local facilities take them.

A glass-only swap on a single unit is often a few hours' work. A whole new flat rooflight on a new upstand, with the roof covering dressed up to it and the lining finished inside, usually takes a day, sometimes a little longer if the kerb has to be rebuilt. The whole job is covered by our 10-year workmanship guarantee, alongside the manufacturer's own product warranty on the new glass or unit.

The wider picture for tired, misted and failed units, and how we approach them across Cambridge and the villages around it, is on our page for misted and failed rooflight replacement. When you are ready, book a light survey or message us on WhatsApp with a photograph of the crack.

Chapter 9

Questions people ask

Can a cracked skylight get worse?

It can. A crack in annealed or laminated glass can lengthen with temperature swings, particularly on a sunny slope where the pane heats and cools each day. Toughened glass that has shattered will not spread further, but the crazed pane is weaker and the seal behind it has gone, so the unit will start to mist.

Is cracked rooflight glass replacement the same as for a misted unit?

The route is often the same: a new sealed unit in a sound frame, or a new rooflight where the frame is old. The difference is the cause. A crack from impact or heat may call for a tougher or cooler specification, where a misted unit has simply reached the end of its seal life.

Will the new glass match the rest of my roof glazing?

It can be specified to. If other panes in a lantern or other rooflights in the same room have a particular tint or coating, the new glass can be ordered to match. If the whole group is ageing, replacing them together gives an even colour and the same performance across the ceiling.