Cracked rooflight glass replacement: the crack is in one pane, the answer is a whole unit

A modern rooflight glass unit is two sheets of glass held apart by a spacer and bonded at the edge. You cannot swap one sheet and keep the other, because the seal that holds the pair together is what keeps the cavity clean and dry.

That is the reason the answer to most questions about cracked skylight glass is the same. A crack in the outer sheet, a crack in the inner sheet, or a crack in both all end with a new sealed unit in the frame. What differs is how quickly you need it, what else has to be checked while the old unit is out, and which glass you choose for the new one.

Start with what you can see from the room. Stand under the rooflight, look up and note three things:

1. Where the crack starts. A line that begins at a corner or along an edge points to a different cause from a star that spreads from a single point in the middle.
2. Whether the crack is on the room side, the weather side or both. Shine a torch across the glass at a shallow angle and the reflections will show you which surface is broken.
3. Whether there is moisture or a grey film between the panes. That tells you the seal has already opened.

A photograph taken from inside, and another from the roof if it can be reached safely from a ladder at the eaves or from a window, is worth more than a description. It lets us check the glass type and the size before anyone climbs onto the roof.

Which sheet has broken decides the urgency. The outer sheet meets the weather. The inner sheet faces the room, and in a correctly built rooflight it is the one that has to hold together if it fails.

An outer crack on its own usually leaves the inner sheet in place, which means the room is still closed off from the weather and from the sky. The cavity will not stay sealed for long, though. Once the outer sheet has a line through it, moisture finds its way into the gap and the unit starts to mist. The pane you can see cracked becomes a pane you cannot see through. We cover the rain question in detail in our answer on whether a cracked outer pane lets rain into a rooflight.

An inner crack deserves more attention than it gets. The inner sheet sits over the people in the room, and a broken sheet that is not laminated can drop pieces. That does not mean panic. It means the job moves up the list and the room under the glass is best kept clear until the new unit is in. Our answer on how urgent a cracked skylight is lays out how to judge it for your own roof.

A crack through both sheets is the case to treat most carefully. Both sheets have given way, so nothing is left to hold the weather out or to carry the load, and the frame should be treated as compromised until it has been looked at.

Glass rarely cracks for no reason. The reason matters because the replacement has to remove it, or the new unit will follow the old one.

The causes fall into a handful of groups:

• Thermal stress. One part of a pane is warm and another is cold, and the difference pulls on the edge. A shadow from a chimney, a blind close to the glass or a dark film applied to the inside can all set this up.
• Impact. A branch, a tile, a stone thrown up by a mower or a ball from next door.
• Edge damage. A tiny chip at the rim of a pane during manufacture or handling can grow over years, then run across the glass on a cold morning.
• Frame movement. A frame that is out of square, or packers that have slipped, puts a point load on the glass.
• Nickel sulphide inclusions, a rare defect in toughened glass that can cause it to shatter without warning.

We set out the full list, with how to tell them apart, in our answer on what causes rooflight glass to crack. For the purposes of replacement, the point is simple. A crack that began at an edge or corner suggests stress from the frame or from heat, and the installer needs to look at the frame and the glazing packers before a new unit goes in. A crack that began at a single point suggests impact, and the frame is more likely to be sound.

The glass type changes both what the crack looks like and how safe the room is afterwards. Two kinds matter.

Toughened glass is heat-treated so that it breaks into many small, blunt pieces. When it fails it often fails completely, and the whole sheet can go at once. A pane that shows a few long lines is unlikely to be toughened.

Laminated glass is two sheets bonded to a plastic interlayer. When it cracks, the pieces stay stuck to the interlayer, so the sheet cracks but hangs in place. A spider web of lines across a pane that is still whole is the typical sign.

That difference is why an overhead inner pane is usually specified as laminated. A cracked laminated sheet is still a barrier. A broken toughened sheet may not be. If you are not sure what you have, look at the edge of the glass. A laminated sheet shows a thin layered line, like a sandwich, and our answer on how to tell whether your rooflight glass is laminated shows what to look for, including the tricks of reflection that work when the edge is hidden in the frame.

Building Regulations care about this as well. Part K covers glazing in places where people could be injured by breakage, and good practice applies the same principle to glass over a room: if it breaks, it should break safely or stay in place. For a replacement unit we specify the inner sheet accordingly. Our page on replacing a failed rooflight unit with laminated glass goes through when that choice makes sense, and when a different build is the better one.

Between the crack and the new unit there is a gap of days, sometimes weeks, and a little care in that gap helps. This is about keeping people and furniture safe, not about working on the glass.

Move anything that sleeps, studies or eats out from under the rooflight. Close the door to a room that cannot be spared and use another one. Do not press on the glass, clean it or try to force a crack to close. Window cleaning fluid will run into a crack and into the edge of the unit. Do not add tape or film in the hope of holding the glass together, because it can hide the crack from the person who comes to measure and can trap moisture against the glass.

Keep a dated record. Take a photograph from the same spot each morning for a few days and note whether the line has grown. A crack that stays put is a different matter from one that lengthens. That record also helps if an insurer is involved, since a policy for glazing or buildings may respond, and a claim is easier when the damage has been documented from the start.

If rain is falling and the outer pane is cracked, put a bowl or a waterproof sheet on the floor beneath, in case damp works its way past the cavity. The inner sheet normally carries the weather when the outer one fails, so a small stain tells you the inner sheet or the frame has also been affected. Mention it when you get in touch. It will change what the survey looks for.

A sealed unit is made to size. It cannot be cut on site, and a unit that is a few millimetres wrong will not bed into its gaskets. The measuring is where the job is won.

The glass is measured in three directions: width, height and overall thickness. The thickness matters as much as the other two, because it decides whether the new unit will sit in the same rebate as the old one. A unit that is thicker than the original may not fit the frame without a change to the glazing bead or the beading gasket.

Beyond size, we record what the old unit is made of:

1. The thickness of each sheet and of the cavity between them.
2. Whether the inner sheet is laminated, and whether the outer sheet is toughened, laminated or plain.
3. The edge detail, which tells us what type of spacer and seal the maker used.
4. Any coating, which can sometimes be detected with a small flame or a light test.
5. The type of frame and the way the glass is held in it, by beads, by pressure plates or by structural bonding.

Our page on how a failed rooflight is measured for its replacement walks through the tolerance and the trade habits in the survey. If you have a stamp, marking or label on the spacer of the old unit, photograph it. It often carries the year and the maker, which saves time.

A lead time follows. Sealed units are built to order, and the time from approved drawing to delivery depends on the glass specification and the season. We set out the usual stages in our answer on how long a replacement glass unit takes to arrive. Ordering is best done only after the measure, because a unit made to the wrong size is a waste of everyone's time.

Most cracked glass ends with a single new unit. Some cases call for more, and the survey decides which.

A fixed rooflight with a sound frame takes a new unit into the existing rebate. The old unit is lifted out, the gaskets and setting blocks are checked, the new unit is bedded in and the beads are refitted. That is a matter of hours for a small rooflight. The frame is left in place, and the work inside is limited to protecting the floor and cleaning up.

A roof lantern is made of several units, one in each panel between the bars. A crack in one panel does not automatically mean a new lantern. Each panel can be taken out in turn, and our guide to replacing units in a roof lantern shows how it is done, with the ridge and hip detail that has to be taken apart first. The same logic answers the question of whether a single pane in a roof lantern can be replaced on its own.

A frame that has moved, rotted or corroded needs more. If the packers have slumped, the glass has been sitting on a point instead of on its edge, and a new unit put into the same frame will crack for the same reason. If the frame itself has twisted, the answer is a new rooflight, not a new pane. Our page on replacing the glass unit or the whole rooflight sets out how we decide between them.

A cracked unit is a chance to specify the glass properly, which is where the quiet in our wedge starts. The unit that cracked may have been the cheapest that would pass. The one that replaces it can be chosen for the room.

The inner sheet is usually the first change. A laminated inner sheet holds together if it breaks, and the plastic interlayer also damps vibration, which takes some of the ring out of thin glass when rain lands on the outside. A thicker outer sheet does the same at the weather end. The cavity between them is the third variable, and a wider gap or a different gas fill changes how the unit behaves for both heat and sound.

The seals and the upstand matter too. A unit set on worn gaskets will tick, buzz and let air past its edge. New gaskets, set in a frame that has been squared and cleaned, are part of the same job. On a flat roof, the kerb or upstand that carries the frame is checked at the same time, because a hollow timber kerb behaves like a drum around the glass.

We set the chosen build down in the quiet spec that comes with every quote, along with what we heard at the survey. For a loft bedroom under the Gatwick approach paths, or a kitchen where rain lands on a lantern, the spec names the laminated inner sheet, the pane thicknesses and the seal. It does not promise a number. It describes the glazing, in plain words that you can compare with any other quote.

Energy comes along with it. Approved Document L sets limits for the heat loss through replacement glazing, and a modern unit with a low-emissivity coating and a warm-edge spacer will meet them more easily than the pane it replaces. A rooflight that has been losing heat through an old unit tends to feel the difference in winter at the edge of the room, where the cold air used to fall. The exact value to meet depends on the type of rooflight and the way the work is classified, and we confirm that at survey.

A glass replacement day is short and clean when the preparation is right. This is how it runs for a typical fixed rooflight, so you know what to expect in the house.

We arrive with the new unit checked against the drawing, the setting blocks, fresh gaskets and sealant. The floor and furniture beneath are covered. Where the glass is in a ceiling above a bed or a table, those are moved first. If the old unit is cracked through, it is taken out in a controlled way, with the broken edges supported, and lowered into a box or a covered bin, not dropped.

The frame is cleaned where the old unit sat and examined for damage, stress marks and rot. Drainage slots are cleared. The setting blocks are placed so that the weight of the glass rests on the correct points and no others. The new unit is lifted in, centred, and held in position with the gaskets and beads. The last step is a check for light leaks and gaps around the edge, inside and out.

The work is tidy and made good inside. The old unit leaves with us. Where the work is notifiable under the Building Regulations, we handle the Building Control notification, and the installation carries our 10-year workmanship guarantee. If you would like the terms, read our guarantee.

Weather does set the date. Glass cannot be lifted safely in strong wind, and wet sealant is a poor bond. A dry, still morning is ideal. If the forecast turns, we move the day and tell you in good time.

These come up most often once someone has found a crack and has not yet called.

Can I swap just the cracked pane of a double glazed rooflight?

No. The two sheets are sealed together at the factory, so a crack in either one means a new unit. The frame, and usually the gaskets, stay in place unless the survey finds a reason to change them.

Is a crack that has not spread still worth acting on?

Yes. A line that has not moved today can open up on the next cold night or after the next gust. It also lets moisture into the cavity, which leads to misting and then to a second job. Plan the replacement while you can choose the date.

Will the new glass match the old in colour and finish?

It can. The tint, the coating and the edge can be matched to the frame and to the other units in a lantern, so a single new panel does not show as a different shade from its neighbours. Tell us if the old glass was tinted or self-cleaning, and we will write that into the order.

Does a new unit make the room noticeably quieter?

It can, when the old glass was thin and the new build is laminated and thicker. The effect depends on the noise the room hears, which is why we listen at the survey. We describe the change by the glass specification, never by a figure.

When you have a photograph of the crack, get a quote and send it in. We will identify the glass, measure the unit, check the frame and write the new build into the quote. You can also read the wider guide to misted and failed rooflight replacement for the other ways a unit can fail.