The short answer
An orangery roof lantern can usually be repaired when the leak is at a seal, a flashing or a gasket, and needs replacing when the frame has warped or the glazing has failed. Leaks most often start where the lantern meets its upstand, where the bars join and at the ridge. A survey finds out which, before anyone quotes for a new one.
How an orangery lantern is put together
A lantern is a framed, glazed pitched roof that rests on a raised timber or insulated base, called a kerb or upstand, set into the flat roof around it. The frame is made from aluminium, timber or a mix, and the glazing, usually double glazed units, is held in by glazing bars and sealed with gaskets or sealant. A ridge runs along the top and hips run down to the corners.
The flat roof around it is the other half of the system. The waterproof covering (EPDM, GRP or lead) is dressed up the kerb, and a flashing or trim covers the edge of the covering. EPDM is a rubber sheet, while GRP is fibreglass resin that sets hard once laid. Water that gets past any of these layers can run into the room, or into the structure around the lantern, before you see a stain.
Where lanterns leak
The usual weak points are the same on most lanterns. We look at them in a set order.
The junction with the flat roof
Where the covering meets the kerb, the upturn needs to be tall enough and sealed along its top edge. If the covering has pulled away, or a trim has split, water runs behind it. Lead is an option at this junction, and the lead code, meaning its thickness, is chosen for the job.
The glazing bars and gaskets
Rubber gaskets harden and shrink over time, and sealant at the bar ends cracks. A gap of a millimetre is enough to let rain in during a driving shower. Condensation inside a sealed unit is a separate fault: it means the unit's edge seal has failed, and the unit needs replacing.
The ridge, hips and cills
The top of the lantern is a favourite place for trouble, as the ridge cap sits over several bars. Where the lantern has been painted or has timber elements, rot can start at the bottom of a bar and travel.
If the lantern leaks at several of these places, patching each one is a poor use of money. Our guide to conservatory roof leaks covers the equivalent faults on a conservatory.
Repair, or replace the lantern?
Replace the seal, gasket or a single unit and you may have years of use ahead. Replace the whole lantern when the frame itself has failed: aluminium that has bowed so bars no longer meet, timber with rot into the structure, or a design that never drained properly. A replacement is sized to the existing kerb where that is sound, which saves disturbing the roof around it.
The lifespan of glazing depends on what it is. The Eco Experts give 10 to 15 years for glass or polycarbonate conservatory roofing, and put a solid or tiled roof nearer 50. That is a conservatory figure rather than an orangery lantern figure, and individual lanterns vary with materials and care, so it is a guide, not a promise. Glazed lanterns with good frames often outlast their sealants, which is why resealing is often a sensible job on its own.
For a view of the cheaper end of the options, our page on conservatory roof sealants explains what a sealant can and cannot do. Polycarbonate lanterns are covered by the polycarbonate lifespan page.
Glazing choices and the rules that apply
Lantern glazing is normally double glazed, and some lanterns use a self-cleaning coating on the outer pane. Because the lantern is part of the roof, Approved Document L, Table 4.2, gives rooflights their own limit of 2.2 W/m²K. Ask your building control body how it treats a lantern on your project, as the classification depends on the design.
An orangery roof is also a roof that people sit under, so overheating is a practical issue. Solar control glass cuts the heat gain, at the cost of a slightly tinted view. Blinds inside are an alternative that you can change later. Think about ventilation too: a lantern with opening vents lets hot air escape at the top, and a fixed one relies on doors and windows lower down. Vents add more joints, so they need the same checks as the bars.
Where the house is listed or the street is protected, ask the council before you alter how the lantern looks. A replacement that matches the original is judged differently from one that is taller or wider.
What a lantern survey looks at
We start from the outside, with photographs of the lantern, the kerb and the covering around it, and then look from inside for staining on the plaster, the bars and the ceiling lining. A hose test, working from the bottom up, can locate a leak, because water travels along bars and a stain is rarely under the fault.
The quote that follows covers gaskets, sealing and flashing as well as new glazed units or a whole new lantern, and we explain what each option would cure. Scaffold, where it is needed, is included in the quote. The labour on a lantern replacement carries our written ten-year guarantee, and we make good around the kerb and take the waste away. Ask for the lantern's overall size and the kerb height to be written on the quote, so a replacement is ordered to fit.
Read about the wider job on our conservatory roof replacement page.