Larger than most rooms need. What sets the practical limit is the span of the glazing bars, because a rafter carrying glass has to resist wind uplift, snow load and its own weight without deflecting enough to stress the seals. Beyond that span the sections get deeper, an intermediate structure appears, or the lantern becomes two.
What the limit actually is
A span figure rather than an area.
Each system states a maximum rafter length for its sections at a given pitch, and it is the distance from the eaves beam to the ridge that matters rather than the plan area.
Which is why pitch and size interact.
Why pitch changes it
Because a steeper lantern has longer rafters.
Raising the pitch on the same plan lengthens every bar, so a pitch chosen for appearance can push a size past what the sections will carry.
The two are decided together, as on what pitch should a roof lantern be.
What the rafter is resisting
Three things, and wind is the one people forget.
Snow load pressing down, the weight of the glass itself, and wind lifting the structure, which on an exposed site is the governing case rather than an afterthought.
Why deflection matters more than strength
Because the glass is stiff and the frame is not.
A bar can be strong enough and still bend far enough to work the sealed units and their gaskets, and it is that movement rather than failure that causes leaks and blown units.
Systems are limited on deflection for exactly this reason.
The ridge beam
The other span, and it is easy to overlook.
A long ridge is itself a beam, and on a lantern where the length greatly exceeds the width it can be the governing element rather than the rafters.
What happens beyond the limit
Deeper sections, and it shows.
The straightforward answer is a heavier bar, which works structurally and reads as chunkier from inside, which is generally the opposite of what somebody wanting a large lantern is after.
An intermediate structure
The other answer, and it is often better.
A structural ridge or a supporting member concealed within the frame lets a large lantern keep slim bars, at the cost of a heavier line where it sits.
Two lanterns instead
Very often the right decision on a long room.
Two lanterns spaced across a kitchen light it more evenly than one large one in the middle, stay within standard sizes and keep the bars slim.
The comparison is on can two lanterns go over one extension.
What the roof structure will take
The limit that comes from below.
Cutting a large opening in a flat roof means trimming joists and carrying their load, and above a certain width that is a designed beam rather than a doubled joist.
Which is a structural engineer’s decision.
What the room can carry visually
The limit nobody quotes and everybody notices.
A lantern that fills most of a small ceiling reads as a conservatory roof rather than as a feature, and the effect in the room is worse rather than better.
Leaving a margin of ceiling around it is what makes it look intended.
A rule of thumb for proportion
Somewhere under half the ceiling, on most rooms.
A lantern occupying a comfortable central portion of the ceiling with plaster around it looks deliberate, and one crowding the walls looks like a glazed roof.
What the glass sizes allow
A separate constraint on very large panes.
Individual sealed units have manufacturing and handling limits, so a very wide lantern is divided into more bays rather than made from fewer larger panes.
Handling and access
The constraint that decides it on a terrace.
A large lantern arrives in sections and large panes, and if those cannot be carried through the house or lifted over the roof, the size is limited by the route rather than by the structure.
Thermal consequences of going large
Worth doing before ordering rather than after.
A large glazed area on a small extension can push the design past what the regulations allow for glazed area, and it is checked as part of the design.
The limits are on can a rooflight be too big for building regulations.
Overheating consequences
The one people live with afterwards. A large lantern over a south facing kitchen collects a great deal of solar gain, and the glass specification has to answer for it.
What to specify is on will a roof lantern make the room too hot.
Cleaning consequences
Practical rather than technical. The middle of a very large lantern is out of reach from any edge, and how it will be cleaned is worth deciding before it is built.
That is on do roof lanterns need cleaning.
Where the plan shape matters
A long thin lantern and a square one of the same area behave differently.
A square plan gives a pyramid with four short rafters and stays comfortably within any system. A long rectangle gives a ridge and two long slopes, and it is the ridge length that starts to govern.
Which is why a proposed size is given as two dimensions rather than as an area.
Standard sizes and where they stop
Useful to know before designing around a number.
Manufacturers hold a range of standard plan sizes that cover most extensions, and going one step beyond the largest standard is often a considerable jump in price rather than a small one.
Designing the opening to a standard size is worth doing where the room allows.
How we size it on a lantern
We size from the room and the structure together and we say when a proposed lantern is larger than the ceiling can carry visually, because that is the mistake that cannot be undone afterwards.
The whole picture is on the roof lantern installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.