Replacing one misted pane in a roof lantern, on its own

A roof lantern is built as a frame with separate glass units, each held in its own bay. That construction is what makes a single-pane answer possible.

The aluminium or timber bars run from the ridge down to the eaves, and each pair of bars encloses one sealed unit. Along the bars sits a compression gasket on the inside and a cap on the outside that clamps the glass in place. Nothing is bonded across the whole roof of the lantern. If you unscrew the cap on a bay, the unit in it is free, and the neighbours stay where they are, held by their own bars and caps.

The units themselves are flat rectangles or trapezoids for a hipped lantern, and triangles at the end faces of some designs. Every one is a separate sealed unit with its own spacer bar and edge seal, so a failure in one does not spread along the bars to the next. A blown unit next to a clear one is common. Our page on why only one pane of a rooflight is misted explains the pattern, which is usually that the worst-placed unit fails first.

So the mechanical answer is easy: yes, the job can be confined to one bay. Whether it should be is the harder call.

The work is about the order of small steps, and most of it happens on the roof or from a platform inside.

1. Measure the opening, not the old unit. The glass is sized from the gap between the bars, less the clearance the gaskets need. The steps are the same as in how a failed rooflight is measured for its replacement.
2. Match the build. Pane thicknesses, coating, the gas fill, the spacer colour and the laminated or toughened inner pane are all read from the old unit's edge or specified fresh.
3. Have the new unit made. A made-to-size unit takes time to arrive, which is why the wait matters and is set out in how long a replacement glass unit takes to arrive.
4. Lift the capping on the bay. The cap screws come out, the cap lifts off, and the glass can be raised from the bars. Glazing units are heavy, so two people handle it.
5. Clean the seat and fit new gaskets. The rebate is cleaned back, checked for damp or distortion, and lined with new gasket before the new unit goes in.
6. Seat, clamp and close. The unit is set on packers, the cap is refitted and tightened evenly, and the bay is checked for light gaps and rattles before we leave.

The lantern does not need to come down for any of this. The other bays stay glazed, the ridge stays closed, and the room below is open to the sky for only as long as it takes to swap one unit.

A new unit in a tired lantern can look out of place within a year. Four situations push towards more than one.

The first is age. If the lantern is fifteen or twenty years old and one unit has blown, the seals on the rest were made in the same batch and have had the same weather. The second is a mismatch. A new unit has a different coating from the old ones, so it can look a shade bluer or greener in some lights than the glass beside it. That usually matters less than people fear, but in a kitchen with pale walls it can show.

The third is the bars. If the aluminium has corroded at the cap line, or a timber bar has gone soft, the new unit has nothing sound to sit on. The fourth is the lantern system itself. If the maker has stopped supplying sizes, a replacement unit has to be made without the system's own gasket profile, and our page on replacement units for a skylight that is no longer made goes through what that means.

The sensible test is the edge of the neighbours. Look along the spacer bar of each unit for a dark line, a cloudy corner or a stain. If there is none, the single swap is sound. If there is a hint, it is worth pricing the set.

Changing one unit does not mean copying it exactly. If the lantern has been noisy in the rain, the new unit is where you can change that.

A unit with a laminated acoustic inner pane and a different outer thickness will behave differently from its neighbours, and on one bay it can be a deliberate test. More often the choice is to match the existing build so the lantern stays even, and to revisit the specification if the whole lantern is ever renewed. Either way, the decision goes in the written quiet spec with the quote. For the science, read whether a failed rooflight unit should be replaced with laminated glass.

Safety is the other point. Glass over a room needs to hold together if it breaks, and laminated glass does that. Safety glazing sits under Part K of the Building Regulations, and the work is installed to current regulations, with Building Control notified where the job requires it.

The wider rules for lantern panes are in misted roof lantern panels and replacing units in a lantern, and the whole subject sits under misted and failed rooflight replacement in Crawley.

Will the new pane match the others?

The size and the build will. The tint of a coating can differ a little between makers and between production years, so a new unit may show a slight shift in the right light. A full set removes that difference.

Does it need scaffolding?

That depends on the height and the access. A lantern on a single-storey extension can often be reached from a platform and a ladder. A taller or awkward roof needs proper access, and we confirm that at the survey.

How much will it save against a whole lantern?

One unit is a fraction of the price of a new lantern, and the quote will show the gap. For a general view of pricing, see our rooflight and skylight costs page.

Is the lantern glass covered by an old guarantee?

It may be, if the lantern is still in period. Start with whether a misted rooflight is covered by its original guarantee, then send us a photo through the quote form.