Can a Roof Lantern Go on an Existing Flat Roof?

A lantern going onto a new orangery or extension is designed in from the start, with the roof built around the opening. Fitting one into a flat roof that already covers a kitchen, garage or older extension is a different job: the joists, decking and covering are already fixed in place, and the opening has to be formed without weakening what is left standing around it.

On a flat roof over a Fletton-brick kitchen extension, or one of the flat-roofed additions common on Peterborough's new-town streets in Bretton, Orton and Werrington, the timbers were sized for a plain roof, not a hole cut through the middle of it. Forming the opening means checking what is actually there before agreeing a size or position, then cutting, trimming and rebuilding that section so the finished roof carries its load at least as well as before.

The span, spacing and depth of the existing joists decide how much roof can open up in one place, and whether extra support is needed either side of it.

A survey looks at joist direction and centres, the span between supporting walls, and any sag or softness in the deck that a new opening would only make worse. Can my flat roof take the weight of a roof lantern? covers how those joists are trimmed and doubled once a size is agreed, and when the span calls for an engineer's input rather than a standard trimmed opening.

• Which way the joists run, and how far they span between walls
• Centres and depth, checked against the proposed opening width
• Condition of the decking and any existing sag near the new opening
• What the roof already carries: insulation, decking, ballast or covering

Felt, GRP and single-ply coverings all get cut back further than the new kerb line, not stopped at it, so the finished lap sits on decking that is dry and firm rather than on a covering edge that could work loose.

Older felt roofs, in particular, often turn out softer near the edge of a proposed opening than they look from above. Where the covering is old enough that this is likely across the whole roof rather than just at the opening, Replacing an Old Polycarbonate Roof Lantern sets out how an existing lantern and the roof around it get assessed together, which is the same check applied to a first-time opening.

Once the opening is marked out, the joists it crosses are cut and the load carried around it instead, using trimmer joists doubled up on each side and a header at each end.

This is standard flat-roof carpentry, but the sizes are set by the span and the load, not by habit, which is why the survey and the trimming go together rather than being agreed separately. A wider opening, or one running close to a wall plate, changes the trimmer sizes and sometimes the fixings used to hang them.

A warm flat roof has its insulation above the deck, and that layer has to run up and over the new kerb rather than stopping at the opening edge, or the kerb becomes a cold bridge.

A cold bridge at the kerb shows up later as a cold line around the inside of the lantern lining, and in the worst cases as damp patches on the plasterboard rather than on the glass itself. Getting the insulation continuous at this one junction is a small detail with a long payoff, since it is much harder to correct once the ceiling is closed up.

The kerb the lantern sits on is built onto the trimmed opening, not simply screwed to whatever covering happens to be there, and the roof's waterproofing is lapped up its sides and dressed under the lantern's own flashing.

Roof Lantern Upstands: Timber, Kerb or Insulated compares the kerb options in more depth; on an existing roof, the choice also depends on how much upstand height the existing ceiling below can absorb without the room feeling boxed in.

Flat roofs need a fall to shed water, commonly around 1:80, and an older roof does not always have one that is still true by the time a lantern goes in.

Forming a new opening is a natural point to correct falls that have settled or were never quite right, using firring pieces or tapered insulation around the kerb so water runs away from the upstand on every side rather than pooling against it. Getting this right at the same time as cutting the opening avoids going back into a finished roof later.

Below the kerb, the ceiling opening is trimmed to match, and the reveal is lined and plastered to whatever depth the kerb height and existing ceiling leave.

That reveal shape is worth planning rather than leaving to whatever the structure happens to produce, since a deep, square reveal narrows the sky you see from below far more than a shallow, splayed one of the same opening size. A sightline sketch drawn from where you actually sit under the new lantern shows how much that reveal depth changes the view before the ceiling is closed up.

Occasionally the survey finds a roof that needs work beyond the area of the new opening, most often where the covering is at the end of its life across the board, or where joist sizes were marginal even for a plain roof.

Where that is the case, it changes the programme and the order of work rather than ruling out a lantern; the opening is still formed the same way, just as part of a larger re-cover. Roof Lantern Building Regulations covers what Building Control expects to see for both a straightforward opening and a wider re-cover, and Sizing a Roof Lantern for Your Extension covers how the opening size itself gets agreed against the room below.

Every quote for a lantern in an existing roof starts with that structural check, so the size and position agreed on paper are ones the roof can actually take, and the costs page sets out what affects the price of a rooflight or lantern once that survey is in hand. The roof lantern installation page covers the full process from survey to finished ceiling.

Does the extension need to be finished before the lantern goes in?

The roof structure, including the trimmed opening and kerb, needs to be in place and weathertight before the lantern itself is glazed, but the room below can still be at an earlier stage of its finish. Coordinating the kerb height with the ceiling and plaster below is usually agreed at survey stage.

Can a lantern go into a roof over an existing garage or outbuilding?

Yes, provided the joists and covering are checked in the same way as for a house extension. Garage roofs are sometimes built to a lighter specification than a habitable room, so the survey pays particular attention to span and joist size before an opening is agreed.

Does an existing flat roof always need a structural engineer involved?

Not always. Many openings sit within joist sizes and spans that a standard trimmed opening can carry safely. An engineer becomes involved where the opening is wide, close to a supporting wall, or where the existing joists are already close to their limit.

How much of the existing roof gets disturbed to fit one lantern?

Usually just the area around the new opening, cut back far enough to reach sound decking and a firm lap for the covering. The rest of the roof is left alone unless the survey finds the covering or joists need attention beyond that area.