Cracked Dome Rooflight: How Dangerous Is It?

A cracked dome rooflight is genuinely dangerous, in two ways. It is a fall-through risk for anyone who steps on or near it, because a brittle old dome that has already cracked will give way completely under weight, and it is a falling-debris risk, because cracked pieces can drop into the room below. Keep people and pets out of the space directly beneath it, keep everyone off that part of the roof, and arrange a replacement quickly rather than living with it. Do not try to patch or tape it and carry on. A cracked dome has failed structurally as well as visually, and the safe response is replacement, which we treat as a same-week job.

This page covers the risk, how to make the area safe, and what replacement involves. The underlying degradation is on yellowed and brittle dome rooflights, and a storm leak is on a flat rooflight leaking after a storm. The wider job is on flat roof skylight installation.

Why a cracked dome is dangerous

Fall-through risk

A dome is not a floor and was never meant to bear weight, but a sound one has some residual strength. A cracked, brittle old dome has almost none. UV degradation over decades has stripped the toughness out of the polycarbonate, and a crack is the evidence that it has reached the point of failure. Anyone who steps on it, a roofer, a window cleaner, someone retrieving a ball, can go straight through. This is the most serious risk and it is why nobody should be on that part of the roof at all.

Falling-debris risk

Cracked pieces of a brittle shell can drop into the room below, and a large old dome over a kitchen or a hallway is directly above where people stand and sit. A section coming down is a real injury risk, more so if the crack widens in wind or the shell is disturbed. This is why the space underneath is cordoned as well as the roof above.

How to make the area safe

1. Cordon the room below. Keep people and pets out from directly under the dome. Move seating, a dining table or a bed out of the drop zone if the room stays in use.
2. Keep everyone off the roof. No inspecting, no covering, no walking near the dome. Warn anyone who might go up, a cleaner, a gardener, a neighbour, that it is cracked.
3. Do not patch or tape it. Tape does nothing for the structural failure and encourages someone to think it is dealt with. It is not.
4. If it is also leaking, catch the water inside as for any leak, and turn off any circuit where water is near a light. The storm-leak steps are on a flat rooflight leaking after a storm.
5. Photograph it from below and from a window, not from the roof, so a survey can be quoted before the visit.

What a same-week replacement involves

A cracked dome is a replacement, and it is one we prioritise. The sequence is the standard dome replacement, done promptly.

1. Survey. Often from photographs plus a visit, confirming the size, the covering and the likely kerb condition. We flag whether the kerb may need rebuilding.
2. The old dome comes off carefully, containing the brittle shell so nothing drops, and the kerb is exposed and probed. On a unit this age the timber is frequently soft, which we photograph and show you.
3. The kerb is rebuilt where needed, insulated to 150mm, or 200mm on the coast, or the covering detail is built onto a sound existing kerb.
4. The new unit is set, glass on a habitable room or a new UV-stabilised dome on an outbuilding, bedded onto the dressed covering and mechanically fixed. The roof is watertight before we leave.

On a like-for-like swap into a sound kerb it is a one-day job. Prices sit in the published ranges on flat roof rooflight installation cost, with a kerb rebuild as a provisional item confirmed once the old dome is off. Because a cracked dome is a safety matter, we bring it forward rather than putting it in the general queue.

Glass or a new dome

The replacement choice is the usual one. On a habitable room, a flush glass unit is clearer, warmer and safer, with a toughened outer and laminated inner so nothing can fall in, and it does not go brittle. On a garage or outbuilding, a new UV-stabilised dome is a sensible economical swap. The comparison is on dome rooflight or flat glass. Either way, the new unit is not the brittle material that just cracked.

Suffolk and north Essex specifics

Cracked domes turn up across the 1980s and 1990s stock at Kesgrave, Rushmere, Chantry and central Ipswich, often after a hailstorm or a cold snap finishes off a shell that was already brittle. On the coast at Felixstowe and Harwich, driven rain and salt weathering age the plastic and the fixings faster, so coastal domes reach the cracking point sooner, and the replacement takes the marine specification. Wherever you are, the immediate response is the same: make the area safe and keep off the roof. Estuarine areas such as Woodbridge sit between coastal and inland weathering.

Common questions

Get a cracked dome replaced this week

Make the area safe, keep off the roof, and send us photographs from below and from a window along with the rough size and where the property sits. We will survey it quickly, prioritise the work, and give you one written fixed price for glass or a new dome. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them.

Request a fixed quote in Ipswich, Suffolk or north Essex.