Why is my rooflight glass rainbow-coloured?

Someone looks up from the sofa or the landing, sees colour swimming across the pane, and assumes the worst: a failed unit, moisture getting in, glass on its way out. Most of the time that is not what is happening. There are three separate effects that all show up as colour on rooflight glass, two of them harmless and one worth checking.

The first is a coating doing exactly what it is meant to do. The second is a fingerprint left by how the glass was made. The third is the one that matters: a sign that the two panes of a sealed unit are touching where they should not be. Telling them apart is mostly about how the colour behaves as you move under it.

A low-emissivity coating is a microscopically thin metallic layer on the glass, applied to keep heat in the room and let daylight through. Thin films like this split light the same way a film of oil on a wet road does: some wavelengths bounce back stronger than others, so you get a shifting rainbow sheen rather than a flat reflection.

It shows up most on double and triple glazed units where a coated surface faces another coated or plain surface across the cavity, and it is strongest in low, raking sun: early morning, late afternoon, or a low winter sun sitting just above the roofline. Walk under the rooflight and the colour moves and changes with you, because it depends on the angle between your eye, the sun and the glass, not on anything happening inside the unit. It is the coating working, not failing.

Toughened glass is heated and then cooled quickly to put the outer surfaces under compression, which is what makes it break into small, blunt pieces instead of long shards. That rapid cooling leaves uneven internal stress spread through the pane in a pattern that is invisible in ordinary light but becomes visible as dark, mottled patches, sometimes called leopard spotting, whenever the light reaching your eye is polarised.

Polarised sunglasses do this. So does light reflected off water, off a car bonnet, or off the pane itself at a shallow angle. You may notice it for the first time on a bright day wearing sunglasses and assume something has changed, when the glass looks exactly as it did the day it was fitted. The pattern is fixed within the glass itself, but you only see it when the light passing through happens to be polarised, so it can seem to appear and disappear at random.

The pattern worth acting on looks different from either of the above: tight, closely spaced rainbow rings or bands, often in a corner, along an edge, or across a pane that has a visible bow to it. Rather than sliding and shifting as you walk beneath it, this pattern stays exactly where it is, in ordinary daylight, regardless of the angle you view it from.

This is an interference pattern known in the glass trade as Newton's rings, and it appears wherever two glass surfaces come into direct contact, whether that is the two panes of a sealed unit that has lost its correct spacing, or the layers of a laminated pane that has not bonded fully at the edge. Contact like this can follow a spacer bar failing, a unit sagging over time, or a manufacturing fault in a laminated pane. It is a sign the unit is not doing its job properly, distinct from the misting covered on our mist between the panes page, and it is a reasonable prompt to get the rooflight looked at.

Neither the coating sheen nor the toughening pattern changes what you actually see through the glass on an ordinary day: they are momentary effects that need a particular light angle to show at all, not a permanent haze sitting across the pane. They do not narrow the sky, dim the room or point to a shorter life for the unit.

A unit with touching panes is a different case. Where the contact point sits inside your regular sightline from the bed, the sofa or the table, you can end up with a patch of colour or distortion that turns up in the same place every time the light catches it, rather than something that only appears for a few minutes at dawn or dusk. That is the practical test: does the mark stay put in your eyeline regardless of the light, or only when the sun sits at a particular angle.

If the colour only shows in low sun or through sunglasses, and is gone at other times of day, there is nothing to book. If a rainbow pattern sits in a fixed place on the glass whatever the light, it is worth a survey rather than a guess from the ground: a rooflight sits well out of easy reach, and the difference between coating sheen and a unit fault is clearer at close range than it is from a landing floor.

Where a unit does need the glass unit replaced, we handle the Building Control notification where the work is notifiable and install to current Building Regulations. Every quote also comes with a sightline sketch, so you can see how the replacement unit sits from where you actually spend time in the room before any glass is ordered. Our misted and failed rooflight replacement page covers the fuller range of unit problems, and the costs page sets out what affects the price of a rooflight generally.

For the coating and toughening effects, no action changes anything: they are built into the glass and the way it was made, and a replacement unit made the same way will do exactly the same thing in the same light. If you are choosing glass for a unit that is being replaced for another reason, our choosing glass for a replacement unit guide covers coating options in plain terms, and our sightline sketch page explains how we plan the view before any glass goes in.

Is this the same as a misted or cloudy rooflight?

No. Misting is moisture inside the cavity fogging the glass, usually a soft, even haze rather than colour, and it does not clear. Our why is my skylight glass cloudy page separates surface haze from the internal fogging that means a failed unit. Rainbow colour is a light effect, not fogging, though a fixed ring pattern can point to the same underlying seal problem.

Will the rainbow pattern get worse over time?

The coating sheen and the toughening pattern will not change: they are fixed properties of the glass, not a fault that develops. Rainbow rings from touching panes can change as the unit moves further, particularly through summer heat and winter cold, which is one reason to have a fixed pattern looked at rather than left.

Does it mean the rooflight is a "blown" unit?

Not on its own. The term blown rooflight usually describes a unit that has lost its seal and let moisture or air in, which shows as misting rather than rainbow colour. Rainbow rings from touching panes are a related but separate sign that something inside the unit has moved.

Can I check which effect I am looking at myself?

Look at the same spot at two very different times of day, once with the sun low and once with it high, and once through ordinary vision rather than sunglasses. A coating sheen or toughening pattern will change or vanish; a fixed ring pattern will still be there in the same place both times. If you are not sure, our is there any way to clear a misted rooflight pane page and a proper survey will settle it either way.