Why Does My Roof Lantern Make Cracking Noises?

A lantern's frame runs the full width and length of the opening, so even a small change in temperature becomes real movement across a run of two or three metres. Aluminium expands and contracts more for each degree than the timber or masonry it sits on, which is exactly why slim aluminium rafters can be built narrower than older timber ones. That strength comes with a trade-off: the metal has to be free to move a little, and where it meets something rigid, a cracking or ticking sound is how you hear it doing so.

The glass moves too, though less than the frame. A big pane heats unevenly, warmer at the centre than at the edge where the frame shades it, and that uneven heating flexes the pane very slightly. On its own that flex is silent. It is only once the glass edge, the gasket and the frame rub or slip against each other that you hear anything.

The sound clusters around the moments when temperature is changing fastest, not when it is hottest. First direct sun on a cold frame at sunrise, the frame cooling quickly once the sun drops behind the roofline at dusk, and a cloud passing in front of the sun on a bright day all produce a faster swing than a steady warm afternoon. That is why the same lantern can tick through a whole clear morning and stay silent through a warmer, duller one: it is the rate of change that matters, not the top temperature reached.

A south facing lantern over a kitchen extension will do this more than one facing north, simply because it sees a bigger daily swing between direct sun and shade. Choosing solar control glass reduces how hot the inner pane gets on a sunny day, which softens the swing and the noise with it, though it will not remove thermal movement altogether. That is physics, not a product fault.

If the sound is a sharper, more frequent crackle rather than an occasional tick, and the roof is an older polycarbonate lantern rather than glass, that fits the material. Multiwall polycarbonate expands more than glass for the same temperature change, and the plastic itself flexes audibly as it does, which is part of why a polycarbonate roof lantern often sounds livelier through a hot afternoon than a glazed one on the house next door. The glazing bars on these older units also tend to be thinner aluminium sections holding thinner sheet, so there is less mass to damp the sound.

Sun yellowing, a chalky surface and a lantern that has become noticeably noisier over the years it has been up are all signs the unit itself, not just the weather, is behind the sound. That is a question of the product's age rather than anything wrong with how it was fitted.

A well specified lantern has several places designed to absorb thermal movement quietly:

• Sliding or expansion joints in the bar and eaves sections, which let a length of frame grow without pushing on the next section.
• Rubber or EPDM gaskets between glass and frame, which flex rather than let glass meet metal directly.
• A weathertight but not rigid seal where the frame meets the upstand, so the frame can move independently of the kerb it sits on.

When one of these has gone slightly stiff, most often an older gasket that has hardened with age, the movement that would normally be silent starts to transfer into a click or a crack instead. That is a reason a noise can appear on a lantern that was quiet for its first few winters and has only started ticking as it has aged, rather than being loud from day one.

It helps to notice roughly where a sound comes from. A tick or crack that seems to travel along a glazing bar, especially near a corner or a joint between bar sections, points to the frame itself. A duller knock that seems to come from lower down, at the base of the lantern where it meets the flat roof, points instead to the upstand or kerb. Timber kerbs move with moisture as well as temperature, so a timber upstand that has dried out over a hot summer can develop its own small creak as the frame sits on it. An insulated kerb built from a stable material moves less and tends to stay quieter over the years.

Most cracking and ticking from a lantern is ordinary thermal movement and needs nothing done about it. A few patterns are worth mentioning when you book a home visit, or message us on WhatsApp, so the lantern can be looked at properly rather than guessed at over the phone:

1. A single loud crack followed by a visible change, a hairline mark in the glass, a gap opening at a joint, or a bar that now sits slightly out of line.
2. Noise that has grown steadily worse over several years and is now accompanied by draughts, staining, or water finding its way past a seal, since that points to a seal or gasket that has failed rather than simple expansion.
3. Any sound from the kerb or roof structure itself rather than the glazing above it, particularly if the flat roof around the lantern has ever felt soft underfoot.

None of these are urgent in the sense of stopping using the room, but they are the difference between a lantern doing what aluminium and glass always do, and one where a fixing, gasket or the structure it sits in needs attention. If in doubt, a look from someone who fitted the last one is worth more than guessing from the sound alone.

Does a bigger roof lantern make more noise than a small one?

Generally yes, because a longer run of frame and a larger pane both move further for the same change in temperature. A lantern sized well for its room, as covered on sizing a roof lantern for your extension, is not chosen to avoid noise, but a large lantern is more likely to be audible than a small rooflight of the same design.

Will a new roof lantern be completely silent?

No frame and glass combination removes thermal movement altogether, since it is the material behaving normally rather than a fault to design out. A well detailed frame with working expansion joints and fresh gaskets will usually be quieter than an older or lower specification one, but some faint sound on a fast temperature swing is normal even on a lantern fitted this year.

Does the colour of the frame change how much it ticks?

A darker frame absorbs more heat from direct sun and so warms up faster and further than a pale one, which can make the swing, and the sound with it, slightly more noticeable. It is a minor factor next to the size of the lantern and its aspect, and is not usually a reason on its own to choose one frame colour over another.

Is cracking glass the same thing as cracking noise?

No. The noises covered here are the frame, gaskets and glass edge moving against each other, not the glass itself cracking. A genuine crack in the pane is a visible line in the glass and is a different situation entirely, one to have looked at rather than left.