A cracked outer pane in a rooflight, and the rain

A sealed unit is built in layers, and each one has a job. The crack matters because of which layer it is in.

The outer pane faces the sky. It takes the rain, the hail, the frost, the grit and the heat of the sun. Its edge is bedded in the frame with gaskets and a cap so that water running over it is carried away down the roof. It is also the first layer that stops debris from reaching the cavity.

Behind it sits the cavity, sealed at the edge and dry. Behind that is the inner pane, which faces the room and is, in a good build, the laminated one. The cavity is a closed space, and it stays useful only for as long as it stays closed.

A crack through the outer pane opens the cavity to the weather. The opening may be a hairline that you only see in low sun or a clear line across the glass. Water is drawn into a fine gap by capillary action, so even a crack too thin to slip a fingernail into can take in rain during a storm and breathe out vapour in the dry.

That is the answer to the question as asked. Rain does get into the unit, but it gets into the cavity, not into the room, as long as the inner pane is intact and the frame is sound.

A cracked outer pane rarely shows you a dramatic failure. It shows you a slow one.

The first sign is usually mist or droplets between the panes after rain. The second is a tide mark on the inside face of the outer pane as the water dries. The third, if the crack is at an edge, is a dark band where water has reached the edge seal and the spacer bar. Damp works at the sealant and the desiccant, and the unit ends up behaving as a blown one, as described on failed seals on rooflights.

The crack can also grow. Glass expands in sun and contracts in frost, and a crack behaves like a stress line. A line that starts at a corner can run across the pane in a cold snap. Toughened glass behaves differently from annealed glass. A toughened pane that breaks tends to crumble into small blunt pieces, whereas an annealed pane breaks into longer, sharper shards and cracks along a line. In a rooflight over a room, neither is a thing you want to leave.

The things to watch between the crack and the replacement are:

• mist or water between the panes that was not there before;
• a crack that lengthens, branches or opens at the edge;
• a stain on the ceiling or reveal below, which means water is reaching the frame;
• any rattle or movement in the glass, which can mean it has loosened in its gaskets.

Two rooflights with the same crack can need different answers, depending on how they are built.

In a unit with a laminated inner pane, a crack in the outer pane is a weather problem and not a safety one. The inner pane is two sheets bonded together, so it holds even if it is hit. In a unit with a plain toughened inner pane, the same crack in the outer is less reassuring, because if the inner pane went too it would fall as granules. That is why laminated glass is the usual specification overhead, and Part K of the Building Regulations is the part that deals with safety glazing.

It is also worth knowing how the crack began. Thermal stress from a dark blind or a shaded edge can do it. So can an impact from a branch or a thrown ball, a fixing that has been overtightened, or glass that has been sitting unevenly in the frame. Our page on what causes rooflight glass to crack runs through each cause, because a replacement unit set into the same stress will fail again.

If you want to know whether your inner pane is laminated, the guide on how to tell if rooflight glass is laminated shows you how to check the edge.

The solution is a new sealed unit, made to the size of the opening and fitted into clean gaskets. There is no way to make the old one sound again.

A cracked unit cannot be made to keep its seal, since the crack is the breach. So the plan is to measure, order the new unit, and fit it. Our page on cracked rooflight glass and what replacing it involves sets out the steps from survey to fit. The timing is covered in whether a cracked skylight is an urgent job.

A replacement is also the point at which you choose the glazing build for the new unit. A thicker outer pane resists knocks better. A laminated inner pane keeps the room safe and quieter in rain. A warm-edge spacer improves the edge. These go into the written quiet spec that comes with the quote, so you can see what each change is for. If the frame is damp or soft where the water ran, the better answer may be a new rooflight, and when the frame has failed too covers that case.

The parent page, misted and failed rooflight replacement in Crawley, brings the whole process together. To begin, send a photo and the rough size through the quote form or by WhatsApp.

Will rain come through into the room?

Not usually at first. The inner pane is a separate barrier, and the frame drains water away. If the crack has reached the frame, or the inner pane is also damaged, water can reach the room, so watch the ceiling below.

Is it safe to leave it over a bedroom?

A crack in the outer pane alone, with a laminated inner pane, is a slower problem. With any doubt about the inner pane, or if the crack is growing, keep the area clear and arrange the replacement soon.

Could the crack have come from the frost?

It could. Water in the cavity that freezes expands, and a fine crack can grow as a result. That is one more reason a cracked unit does not improve by waiting through the winter.

Does the crack affect heat loss?

Yes. Once the cavity takes in damp air it stops insulating as it did. The effect is similar to the one described on what a misted rooflight means for heat loss.